For list of authors, see Credits (Chapter 18).
This is a DRAFT proposal for voting purposes. Depending on the outcome of voting, some version of this document will become version 0.98.
Comments on this document can be sent to the PNG specification maintainers at one of the following addresses:
Distribution of this memo is unlimited.
At present, the latest version of this document is available on the World Wide Web from
ftp://swrinde.nde.swri.edu/pub/mng/documents/.
image_sample_depth != 20
image/jng and file extension
".jng" for JNG files.
This document defines the MNG (Multiple-image Network Graphics) format. It also defines the MNG-LC (Low Complexity), MNG-VLC (Very Low Complexity), and JNG (JPEG Network Graphics) formats, which are proper subsets of MNG.
MNG is a multiple-image member of the PNG (Portable Network Graphics) format family that can contain animations (slide shows) or complex still frames, comprised of multiple PNG or JNG single-image datastreams.
The MNG and JNG formats use the same chunk structure that is defined in the PNG specification, and they share other features of the PNG format. Any MNG decoder must be able to decode PNG or JNG datastreams.
The MNG format provides a mechanism for reusing image data without having to retransmit it. Multiple images can be composed into a "frame" and a group of images can be used as an animated "sprite" that moves from one location to another in subsequent frames. "Palette animations" are also possible. MNG (but not MNG-LC or MNG-VLC) can also store images in a highly compressible "Delta-PNG" format, defined herein.
A MNG frame normally contains a two-dimensional image or a two-dimensional layout of smaller images. It could also contain three-dimensional "voxel" data arranged as a series of two-dimensional planes (or tomographic slices), each plane being represented by a PNG or Delta-PNG datastream.
A Delta-PNG datastream defines an image in terms of a parent PNG or Delta-PNG image and the differences from that image. This has been demonstrated to provide a much more compact way of representing subsequent images than using a complete PNG datastream for each.
This document includes examples that demonstrate various capabilities of MNG including simple movies, composite frames, loops, fades, tiling, scrolling, storage of voxel data, and converting GIF animations to MNG format.
If "231" looks like
the number "231"
instead of 2 raised to the power
31, your viewer is not
recognizing the HTML 4.0 <SUP> tag; you need
to look at the HTML 2.0, ASCII text, or PostScript
version of this document instead.
This specification defines the format of a MNG (Multiple-image Network Graphics) format. It also defines low-complexity and very-low-complexity versions (MNG-LC and MNG-VLC), which are proper subsets of MNG.
Note: This specification depends on the PNG (Portable Network Graphics) [PNG] and the JPEG (Joint Photographic Experts Group) specifications. The PNG specification is available at the PNG web site,
http://www.libpng.org/pub/png/
A MNG datastream describes a sequence of zero or more single frames, each of which can be composed of zero or more embedded images or directives to show previously defined images.
The embedded images can be PNG, JNG, or Delta-PNG datastreams. MNG-LC and MNG-VLC datastreams do not contain JNG datastreams, but MNG-LC and MNG-VLC applications can be enhanced to recognize and process those as well.
A typical MNG datastream consists of:
MNG is fundamentally declarative; it describes the elements that go into an individual frame. It is up to the decoder to work out an efficient way of making the screen match the desired composition whenever a nonzero interframe delay occurs. Simple decoders can handle it as if it were procedural, compositing the images into the frame buffer in the order that they appear, but efficient decoders might do something different, as long as the final appearance of the frame is the same.
Images can be "concrete" or "abstract". The distinction allows decoders to use more efficient ways of manipulating images when it is not necessary to retain the image data in its original form or equivalent in order to show it properly on the target display system.
MNG is pronounced "Ming."
When a MNG datastream is stored in a file, it is recommended that ".mng" be used as the file suffix. In network applications, the Media Type "video/x-mng" can be used. Registration of the media type "video/mng" might be pursued at some future date.
The first eight bytes of a MNG datastream are
138 77 78 71 13 10 26 10
(decimal) which is similar to the PNG signature with "\212 M N G" instead of "\211 P N G" in bytes 0-3.
MNG does not yet accommodate sound or complex sequencing information, but these capabilities might be added at a later date, in a backward-compatible manner. These issues are being discussed in the mng-list@ccrc.wustl.edu mailing list.
Chunk structure (length, name, data, CRC) and the chunk-naming system are identical to those defined in the PNG specification. As in PNG, all integers that require more than one byte must be in network byte order.
The chunk copying rules for MNG employ the same mechanism as PNG, but with rules that are explained more fully (see below, Chapter 7). A MNG editor is not permitted to move unknown chunks across the SAVE and SEEK chunks, across any chunks that can cause images to be displayed, or into or out of a IHDR-IEND or similar sequence.
Note that decoders are not required to follow any decoding models described in this specification nor to follow the instructions in this specification, as long as they produce results identical to those that could be produced by a decoder that did use this model and did follow the instructions.
Each chunk of the MNG datastream or of any embedded object is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk. MNG-compliant decoders are required to recognize and decode independent PNG or JNG datastreams.
Because the embedded objects making up a MNG are normally in PNG format, MNG shares the good features of PNG:
In addition:
See also the glossary in the PNG specification.
When the layers of a frame do not cover the entire area defined by the width and height fields from the MHDR chunk, the layers are composited over the previous frame to obtain the new frame.
`object_id=N'".
In MNG-LC and MNG-VLC, only image 0 is permitted.
Note that a layer can be completely empty if the image is entirely outside the clipping boundaries.
A layer can be thought of as a transparent rectangle with the same dimensions as the frame, with an image composited into it, or it can be thought of as a rectangle having the same dimensions (possibly zero) and location as those of the object after it has been located and clipped.
An embedded visible PNG or JNG datastream generates a single layer, even though it might be interlaced or progressive. If the background consists of both a background color and a background image, these are combined into a single layer.
object_id is
an unsigned sixteen-bit number that serves as the identifier of a set of
object attributes. In MNG-LC
and MNG-VLC
only object 0 is permitted.
do_not_show flag to zero.
do_not_show flag to zero.
An "object", which is identified by
an object_id, is an
image or it is a nonviewable entity that is created by the BASI
chunk. The object_id is an unsigned sixteen-bit number that
serves as the identifier of a set of object attributes.
An "image" is a viewable object.
An embedded object is:
Objects have object attributes that can be defined and modified by the contents of various MNG chunks. Decoders are responsible for keeping track of them. The simplest decoder might establish a 65,536-element array for each attribute, but real applications will undoubtedly use a more memory-efficient method. Object attributes include:
object_id replaces it
without an intervening DEFI chunk. In this case, the new object
inherits the object attribute set from the previous object with the same
object_id.
do_not_show byte of the DEFI or CLON chunk
that introduced it. The "potential visibility" of viewable objects
can be changed by the SHOW chunk. When an embedded object is
"potentially visible," it can be displayed "on-the-fly"
as it is being
decoded. Later, the SHOW chunk can direct that a "potentially
visible" viewable object be displayed.
It is permitted to change the potential visibility of "frozen"
objects; if this is done, the potential visibility must be restored by
the encoder prior to the end of the segment.
A nonviewable object becomes viewable immediately when the decoder receives a viewable object buffer or when an image delta makes it viewable, and if the object is potentially visible it can be displayed "on-the-fly" while the object buffer is being decoded or updated. Note that object 0 is only viewable while its embedded image is being decoded and displayed on-the-fly, after which it becomes nonviewable again because no object buffer is ever created for object 0.
An object buffer is created by the appearance of an embedded object
in the datastream, with a nonzero object_id, or by the
appearance of a CLON chunk that specifies a "full clone".
The contents of an object buffer can be modified by processing an image
delta or a PAST chunk.
Object buffers contain a 2D array of pixel data and can contain additional information. In addition, decoders are responsible for keeping track of some properties of the data in the object buffer:
Object 0 conceptually never has an object buffer. Decoding applications can create one for their own convenience, but such an object buffer must never be made available to the rest of the MNG datastream or be considered viewable after it has been processed.
When the "stored objects" flag (bit 9 of the simplicity profile) is 0
and valid (i.e., bit 6 is 1 and bit 9 is 0), an object buffer need not be
created even when an embedded object with a nonzero object_id
appears, since the flag promises that the object buffer will never be used.
There is no requirement not to create an object buffer; no harm
will be done except for some unnecessary memory consumption.
When a partial clone is made of an object via the CLON chunk, the reference count for the object buffer is incremented, and no new object buffer is created.
When an object is discarded, and it points to an object buffer that has a nonzero reference count, that reference count is decremented, and the object buffer is also discarded if the resulting reference count is zero.
This chapter describes chunks that can appear at the top level of a MNG datastream. Unless otherwise specified in the Delta-PNG chapter of this specification, they need not be recognized within a Delta-PNG datastream.
This section describes critical MNG control chunks that MNG-compliant decoders must recognize and process ("processing" a chunk sometimes can consist of simply recognizing it and ignoring it; some chunks have been declared to be critical only to prevent them from being relocated by MNG editors).
The MHDR chunk is always first in all MNG datastreams except for those that consist of a single PNG or JNG datastream with a PNG or JNG signature.
The MHDR chunk contains exactly 28 bytes:
Frame width: 4 bytes (unsigned integer).
Frame height: 4 bytes (unsigned integer).
Ticks per
second: 4 bytes (unsigned integer).
Nominal layer
count: 4 bytes (unsigned integer).
Nominal frame
count: 4 bytes (unsigned integer).
Nominal play
time: 4 bytes (unsigned integer).
Simplicity
profile: 4 bytes:(unsigned integer).
bit 0:
0: Presence or absence of any
features is unspecified. All
other bits of the simplicity
profile must be zero (i.e, all
other even numbers are invalid).
1: Presence or absence of certain
features is specified by the
remaining bits of the simplicity
profile.
(must be 1 in MNG-LC and MNG-VLC
datastreams)
bit 1:
0: Simple MNG features are absent.
1: Simple MNG features are present.
(must be 0 in MNG-VLC datastreams)
bit 2:
0: Complex MNG features are absent.
1: Complex MNG features are present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 3:
0: Transparency is absent. No
image in the datastream has
transparency, or its transparency
can be ignored and all images
can be rendered as opaque.
1: Transparency is present and is
essential (critical) for proper
display of the images.
bit 4:
0: JNG is absent.
1: JNG is present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 5:
0: Delta-PNG is absent.
1: Delta-PNG is present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bit 6: Validity flag for bits 7, 8, and 9
0: The presence or absence of
background transparency, stored
objects, and semitransparency is
unspecified.
1: Background transparency is
expressed by bit 7, semitransparency
by bit 8, and the presence of stored
objects by bit 9.
bit 7:
0: Background transparency is absent
(i.e., the first layer fills the
entire MNG frame with opaque pixels)
or can be ignored.
1: Background transparency is present
and is essential (critical) for
proper display of the images.
bit 8:
0: Semitransparency (i.e., an image
with an alpha channel that has
values that are neither 0 nor the
maximum value) is present or can
be dithered down to binary
transparency.
1: Semitransparency is present and is
essential (critical) for proper
display of the images. Full alpha
composition rather than dithering is
required.
bit 9:
0: Stored objects are not present.
1: Stored objects are present.
(must be 0 in MNG-LC and MNG-VLC
datastreams)
bits 10-15:
Reserved for public expansion. Must
be zero in this version.
bits 16-30:
Available for private or experimental
expansion. Undefined in this version
and can be ignored.
bit 31:
Must be zero.
Decoders can ignore the "informative" frame-count, layer-count, play-time, and simplicity-profile fields.
The frame_width and frame_height fields
give the intended display size (measured in
pixels) and provide
default clipping boundaries
(see Recommendations for encoders, below).
It is strongly recommended that these be set to zero if
the MNG datastream contains no visible images.
The ticks_per_second field gives the
unit used by the FRAM chunk to specify interframe delay and timeout.
In MNG-VLC datastreams, it gives the framing rate.
It must be nonzero if the datastream contains a sequence of images.
When the datastream contains exactly one frame,
this field should be set to zero.
When this field is zero, the length of a tick is infinite, and
decoders will ignore any
attempt to define interframe delay, timeout, or any other variable that
depends on the length of a tick. If the frames are intended to be
displayed one at a time under user control, such as a slide show or
a multi-page FAX, the tick length can be set to any positive number
and a FRAM chunk can be used to set an infinite interframe
delay and a zero timeout. Unless the user intervenes, viewers will only
display the first frame in the datastream.
When ticks_per_second is nonzero,
and there is no other
information available about interframe delay,
viewers should display the sequence of frames
at the rate of one frame per tick.
If the frame-count field contains a zero, the frame
count is unspecified. If it is nonzero, it contains the number
of frames that would be displayed, ignoring the
fPRI chunks and the
TERM chunk. If the frame count is greater
than 231-1,
encoders should write 231-1, representing an infinite
frame count.
In MNG-VLC datastreams, the frame count is the same as the number of
embedded images in the datastream (or one, the background layer, if there are
no embedded images).
If the layer-count field contains a zero, the layer
count is unspecified. If it is nonzero, it contains the number of
layers in the datastream, ignoring the
fPRI chunks and the
TERM chunk.
If the layer count is greater than 231-1, encoders
should
write 231-1, representing an infinite layer count.
In MNG-VLC datastreams, the layer count is the number of embedded images,
plus one (for the background layer).
If the nominal-play-time field contains a zero, the
nominal play time is unspecified. Otherwise, it gives the play time,
in ticks, when the file is displayed ignoring the
fPRI chunks and the
TERM chunk.
Authors who write this field should choose a
value of "ticks_per_second" that will allow the nominal play time
to be expressed in a four-bit integer. If the nominal play time is greater
than 231-1 ticks, encoders should write 231-1,
representing an infinite nominal play time.
In MNG-VLC datastreams, the nominal play time is the same as the frame count,
except when the ticks_per_second is zero, in which case the nominal play
time is also zero.
When bit 0 of the simplicity profile is zero, the simplicity (or complexity) of the MNG datastream is unspecified, and all bits of the simplicity profile must be zero. The simplicity profile must be nonzero in MNG-LC and MNG-VLC datastreams.
If the simplicity profile is nonzero, it can be regarded as a 32-bit profile, with bit 0 (the least significant bit) being a "profile-validity" flag, bit 1 being a "simple MNG" flag, bit 2 being a "complex MNG" flag, bits 3, 7, and 8 being a "transparency" flags, bit 4 being a "JNG" flag, bit 5 being a "Delta-PNG" flag, and bit 9 being a "stored objects" flag. The upper 15 bits (except for the most significant bit, which must be zero) are available for private test or experimental versions, and the remaining bits are reserved for future MNG versions, and must be zero in this version. If a bit is zero, the corresponding feature is guaranteed to be absent, and if a bit is one, the corresponding feature may be present in the MNG datastream.
When bit 1 is zero ("simple" MNG features are absent), the datastream does not contain the DEFI, FRAM, or global PLTE and tRNS chunks.
When bit 2 is zero, the datastream does not contain any "complex MNG
features". These are the BASI,
CLON, DHDR/IEND, PAST, DISC,
MOVE, CLIP, and SHOW
chunks. If the DEFI chunk is present, it only defines object 0.
If the BACK chunk is present, it does not define
a background image.
If the LOOP chunk is present, it has iteration_min=1.
A MNG with a "complex MNG feature" (simplicity
profile that has bit 2 set to 1) may contain
at least one of these chunks. A simple
decoder can display "simple" MNGs without having to store any
objects or dealing with the SAVE/SEEK mechanism, and it can ignore
the LOOP and ENDL chunks and execute all loops
exactly once.
"Transparency is absent or can be ignored" means that the MNG or PNG tRNS chunk is not present and no PNG or JNG image has an alpha channel, or that if they are present they are not essential (or critical) for displaying the images.
"Semitransparency is absent or can be dithered" means that if the MNG or PNG tRNS chunk is present and any PNG or JNG image has an alpha channel, they only contain the values 0 and the maximum (opaque) value, or that if other values are present the decoding application is allowed to dither them down to binary transparency. The "semitransparency" flag means nothing and must be 0 if bit 6 is 0.
"Background transparency is absent or can be ignored" means that the first layer fills the entire frame with opaque pixels, or if any part of the first layer is not filled with an opaque pixel, the application is free to fill it with a color of its own choosing. Whatever is behind the first layer need not show through, or it is not essential (or critical) that it do so.
When "Background transparency" is present, the application is responsible for supplying a background color or image against which the MNG background layer is composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes. The "background transparency" flag means nothing and must be 0 if bit 6 is 0.
The "stored objects" flag is only useful when bit 2 is nonzero (i.e., "complex MNG features" are present). This flag promises that even though such features are present, it is never necessary to store an object buffer for any object. An object attributes set is necessary for each object, however. Therefore, the MOVE, CLIP, DISC, deterministic LOOP, partial CLON, and immediately-displayed BASI chunk are permissible. The "stored objects" flag means nothing and must be 0 if bit 6 is 0.
A MNG-LC (i.e., a "low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 1, 3, 6, and 7 ("simple MNG" MNG features and transparency) equal to zero. If bit 4 (JNG) is 1, the datastream is a "MNG-LC with JNG" datastream that carries an image or an alpha channel. MNG-LC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
A MNG-VLC (i.e., a "very low-complexity MNG") datastream must have a simplicity profile with bit 0 equal to 1 and all other bits except possibly for bits 3, 6, and 7 (transparency) equal to 0. If bit 4 (JNG) is 1, the datastream is a "MNG-VLC with JNG" datastream. The JNG datastream might carry an image or an alpha channel. MNG-VLC decoders are allowed to reject such datastreams unless they have been enhanced with JNG capability.
Encoders that write a nonzero simplicity profile should endeavor to be accurate, so that decoders that process it will not unnecessarily reject datastreams. For example, the simplicity profile 351 (0x15f) indicates that JNG, critical transparency, semitransparency, stored objects, and at least one "complex" MNG feature are all present, but Delta-PNG and background transparency are not. This example would not qualify as a MNG-LC or a MNG-VLC datastream because a "complex" MNG feature might be present. If the simplicity profile promises that certain features are absent, but they are actually present in the MNG datastream, the datastream is invalid.
The MEND chunk's data length is zero. It signifies the end of a MNG datastream.
The LOOP chunk provides a "shorthand" notation that can be used to avoid having to repeat identical chunks in a MNG datastream. The LOOP chunk can be ignored by MNG-LC and MNG-VLC decoders, along with the ENDL chunk. Its contents are the first two or more of the following fields:
Nest level: 1 byte (unsigned integer).
Iteration count: 4 bytes (unsigned integer),
range [0..2^31-1].
Termination
condition: 1 byte (unsigned integer).
Must be omitted if termination_condition=0,
which means Deterministic, not cacheable,
or if iteration_count=0.
1: Decoder discretion, not cacheable.
2: User discretion, not cacheable.
3: External signal, not cacheable.
4: Deterministic, cacheable.
5: Decoder discretion, cacheable.
6: User discretion, cacheable.
7: External signal, cacheable.
Iteration min: 4 bytes(unsigned integer). Must be
present if termination_condition
is 3 or 7. If omitted, the default
value is 1.
Iteration max: 4 bytes (unsigned integer). Must be
present if termination_condition is
3 or 7; must be omitted if iteration_min
is omitted; if omitted, the default value
is infinity.
Signal number: 4 bytes (unsigned integer). Must be
present if termination_condition is 3 or 7.
Additional
signal number: 4 bytes. May be present if
termination_condition is 3 or 7.
...etc...
Decoders must treat the chunks enclosed in a loop exactly as if they
had been repeatedly spelled out. Therefore, during the first iteration
of the loop, the parent objects for any Delta-PNG datastreams in the
loop are the images in existence prior to entering the LOOP
chunk, but in subsequent iterations these parent objects might have been
modified. The termination_condition field can be used to
inform decoders that it is safe to change the number of loop iterations.
Simple decoders can ignore all fields except for the
iteration_count.
When the LOOP chunk is present, an ENDL chunk
with the same nest_level must be present later in the MNG
datastream. Loops can be nested. Each inner loop must have a higher
value of nest_level than the loop that encloses it, though
not necessarily exactly one greater.
The termination condition specifies how the actual number of iterations is determined. It is very similar to the termination condition field of the FRAM chunk, and can take the same values:
iteration_min nor more than
iteration_max. If the decoder has no reason to choose its
own value, it should use the iteration count. One example of a decoder
wishing to choose its own value is a real-time streaming decoder
hovering at a loop while waiting for its input buffer to fill to a
comfortable level.
iteration_min and iteration_max limits. Some
decoders might not be able to interact with the user, and many decoders
will find that nested user-discretion loops present too great of a
user-interface challenge, so the <user-discretion> condition
will probably usually degenerate into the <decoder-discretion>
condition.
iteration_min nor more than iteration_max. The
exact number can be determined by the arrival of a signal whose number
matches one of the signal_number fields.
When the value of the termination condition flag is 4 or more, the loop is guaranteed to be "cacheable", which means that every iteration of the loop produces the same sequence of frames, and that all objects and object buffers are left in the same condition at the end of each iteration. Decoders can use this information to select a different strategy for handling the loop, such as storing the composited frames in a cache and replaying them rather than decoding them repeatedly.
The iteration_min and iteration_max
can be omitted. If the condition is <deterministic> the
values are not used. Otherwise,
defaults of 1 and <infinity> are used. The iteration_count,
iteration_min, and iteration_max can be any
non-negative integers or <infinity>, but they must satisfy
iteration_min <= iteration_count <= iteration_max.
Infinity is represented by 0x7fffffff. If all of the loops in
a MNG datastream have iteration_min=1, the datastream
can qualify as a "simple" MNG for the purpose of setting bits
1 and 2
of the "simplicity profile" to zero, unless there are other
reasons for setting them to one.
If iteration_count is zero,
the termination_condition, and the remaining fields must be
omitted, and the loop is done zero times.
Upon encountering a LOOP chunk with iteration_count=0,
decoders simply skip chunks until the matching ENDL chunk is
found, and resume processing with the chunk immediately following it.
The signal_number can be omitted only if the termination
condition is not <external-signal>. There can be any number
of signal_number fields. Signal_number = 0 is
reserved to represent any input from a keyboard or pointing device,
and 1-255 are reserved to represent the corresponding character code,
received from a keyboard or simulated keyboard, and values 256-1023 are
reserved for future definition by this specification.
An infinite or just overly long loop could give the appearance of having locked up the machine. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by abandoning the MNG entirely or just proceeding to the next SEEK chunk (the SEEK chunk makes it safe for a viewer to resume processing after it has jumped out of the interior of a segment).
MNG editors that extract a series of PNG or JNG files from a MNG datastream
are expected to execute the loop only iteration_min times, when
the termination condition is not <deterministic>.
The ENDL chunk ends a loop that begins with the LOOP chunk. It contains a single one-byte field:
Nest_level: 1 byte (unsigned integer),
range [0..255].
When the ENDL chunk is encountered, the loop
iteration_count is decremented, if it is not already zero. If
the result is nonzero, processing resumes at the beginning of the loop.
Otherwise processing resumes with the chunk immediately following the
ENDL chunk.
When the ENDL chunk is present, a LOOP chunk with
the same nest_level must be present earlier in the MNG
datastream. See below.
Loops must be properly
nested: if a LOOP chunk with higher nest_level
appears inside a LOOP/ENDL pair, a matching ENDL chunk
must also appear to close it.
The SAVE and SEEK chunks are not permitted inside a LOOP-ENDL pair. To rerun an entire datastream that includes these chunks, use the TERM chunk instead. See below (Paragraph 4.3.6).
The DEFI chunk sets the default object attribute
set (object_id, potential_visibility,
concrete_flag, location, and clipping boundaries) for
any subsequent images that are defined with IHDR-IEND,
BASI-IEND,
or JHDR-IEND datastreams.
Bit 1 of the MHDR simplicity profile can be used to promise that the DEFI chunk is not present.
The DEFI chunk contains 2, 3, 4, 12, or 28 bytes:
Object id: 2 bytes (unsigned integer) identifier
to be given to the objects that
follow the DEFI chunk.
This field must be zero in MNG-LC
files.
Do_not_show
flag: 1 byte (unsigned integer)
0: Make the objects potentially
visible.
1: Do not make the objects potentially
visible. This field can be omitted
if the concrete_flag, location, and
clipping boundary fields are also
omitted.
Concrete flag: 1 byte (unsigned integer)
0: Make the objects "abstract" (image
cannot be the source for a
Delta-PNG)
1: Make the objects "concrete" (object
can be the source for a Delta-PNG).
This field can be omitted if the
location and clipping boundary fields
are also omitted.
MNG-LC decoders can ignore this flag.
X_location: 4 bytes (signed integer).
The X_location and Y_location fields can
be omitted if the clipping boundaries
are also omitted.
Y_location: 4 bytes (signed integer).
Left_cb: 4 bytes (signed integer). Left clipping
boundary. The left_cb, right_cb,
top_cb, and bottom_cb fields can be
omitted as a group.
Right_cb: 4 bytes (signed integer).
Top_cb: 4 bytes (signed integer).
Bottom_cb: 4 bytes (signed integer).
If the object number for an object is nonzero, subsequent chunks can use this number to identify it.
When the object number for an object is zero, its object buffer
can be discarded immediately after it has been processed, and it can
be treated as an "abstract" image, regardless of the contents of the
concrete_flag field.
Negative values are permitted for the X and Y location and clipping boundaries. The positive directions are downward and rightward from the frame origin (see Recommendations for encoders, below).
Multiple IHDR-IEND, JHDR-IEND, and
BASI-IEND objects can follow a single DEFI chunk.
When object_id is nonzero, the DEFI chunk values
remain in effect until another DEFI chunk or a SEEK
chunk appears, unless they are modified by SHOW, MOVE,
or CLIP chunks.
The object_id and concrete_flag
can only be changed by using another DEFI chunk. If no
DEFI chunk is in effect (either because there is none in
the datastream, or because a DISC or SEEK chunk has
caused it to be discarded),
the decoder must use the following default values:
Object_id = 0
Do_not_show = 0
Concrete_flag = 0
X_location = 0
Y_location = 0
Left_cb = 0
Right_cb = frame_width
Top_cb = 0
Bottom_cb = frame_height
These default values are also used to fill any fields that were omitted from
the DEFI chunk, when an object with the same object_id
has not been previously defined or a DISC or SEEK chunk has
caused it to be discarded.
The object attributes for all unfrozen objects, including object 0, become undefined when a SEEK chunk is encountered. Encoders must ensure that the first DEFI chunk for every object_id after any SEEK chunk does not omit any fields that were set to values other than these defaults in the previous DEFI chunk for that object_id, because simple decoders are allowed to ignore the SEEK chunk. Decoders that jump or skip to SEEK chunks must assume these defaults for any omitted fields in the first DEFI chunk in a segment for each object_id.
If object_id is an identifier that already exists when
a DEFI chunk appears,
the object attribute set (except for
the pointer to the object buffer) is immediately replaced, but the
contents of the object buffer do not change until an IHDR,
JHDR, or BASI chunk is encountered; then the contents
of the object buffer previously associated with the identifier are
replaced with the new embedded object data.
Note that if the object has partial clones, the object buffer of the clones is naturally affected because it is shared, but the object attribute sets of the clones are not affected.
The PLTE chunk has the same format as a PNG PLTE chunk. It provides a global palette that is inherited by PNG datastreams that contain an empty PLTE chunk.
The tRNS chunk has the same format as a PNG tRNS chunk. It provides a global transparency array that is inherited along with the global palette by PNG datastreams that contain an empty PLTE chunk.
If a PNG datastream is present that does not contain an empty PLTE chunk, neither the global PLTE nor the global tRNS data is inherited by that datastream.
If the global PLTE chunk is not present, each indexed-color PNG in the datastream must supply its own PLTE (and tRNS, if it has transparency) chunks.
A PNG (Portable Network Graphics) datastream.
See the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT] for the format of the PNG chunks.
The IHDR and IEND chunks and any chunks between them are written and decoded according to the PNG specification, except as extended in this section. These extensions do not apply to standalone PNG datastreams that have the PNG signature, but only to PNG datastreams that are embedded in a MNG datastream that begins with a MNG signature and the MHDR chunk.
1: Implicit "none" filtering of every scanline;
filter bytes are omitted from the compressed
datastream in the IDAT chunk(s).
64: Adaptive filtering with five basic types,
intrapixel differencing, and leveling.
A set of level samples appears immediately
prior to the filter byte for the first
scanline in the IDAT chunk.
65: Intrapixel differencing and pixel leveling,
with implicit "none" scanline filtering.
This transformation, which is a modification of a method previously used in the LOCO image format [LOCO], is
S0 = Red - Green - L0 (when color_type 2 or 6)
or
S0 = Index - L0 (when color_type 3)
or
S0 = Gray - L0 (when color_type 0 or 4)
S1 = Green - L1 (when color_type 2 or 6)
or
S1 = Alpha - L1 (when color_type 4)
S2 = Blue - Green - L2 (when color_type 2 or 6)
S3 = Alpha - L3 (when color_type 6)
The transformation is done in integer arithmetic in sufficient precision
to hold intermediate results, and the result is stored modulo the
sample depth. Intrapixel differencing (subtracting the green sample)
is only done for color types 2 and 6, and only when the filter method
is 64 or 65.
When the filter method is 64 or 65, the level values are inserted
at the beginning of the
compressed datastream contained in the IDAT chunk, immediately
ahead of the first filter byte for the first row of pixels.
[Experiments with putting a separate level set ahead of each filter byte have not been promising; the savings in filesize was offset by the extra storage for the level sets.]
The filter method can be understood and implemented as two separate bit fields:
bit 0: Scanline filtering method
0: Adaptive filtering with five basic types,
with a filter_type byte for each scanline.
1: No scanline filtering, and no filter_type
bytes (implicit filter_type "none").
bit 6: Intrapixel methods
0: No pixel leveling or intrapixel filtering.
1: Pixel leveling and intrapixel differencing.
The level
set contains 16-bit samples if the image sample depth is 16, otherwise
it contains 8-bit samples. In indexed-color images, the level is
applied to the raw palette index, not to the decoded colors.
When the sample depth is less than 8, it is not an error for the
level to be greater than 15; the modulo arithmetic will simply ignore
the most significant bits.
In tests, the set (132, 0, 132) has worked well with a large set of truecolor images. It has been demonstrated to improve the compressibility of natural images by 10 to 15 percent of the compressibility of RGB data. This seems to be due to the intrapixel correlation between color samples, which is ignored by the PNG filters.
The RGB samples are recovered, after undoing the basic filtering, by the inverse of the transformation, which for RGBA images is
Green = S1 + L1
Red = S0 + L0 + Green
Blue = S2 + L2 + Green
Alpha = S3 + L3
As in the forward transformation, the inverse transformation is done in
integer arithmetic in sufficient precision to hold intermediate results,
and the result taken modulo the sample depth.
Applications that convert a MNG datastream to a series of PNG datastreams must convert any PNG datastream with one of the additional filter methods to a standard PNG datastream with a PNG filter method (currently 0 is the only valid filter method).
These extra filter methods can also be used in PNG datastreams that are embedded in Delta-PNG and BASI datastreams and in the JNG IDAT chunks that carry the JNG alpha channel.
It is suggested that encoders write a "nEED MNG-1.0" chunk if they use this feature, for the benefit of pre-MNG-1.0 decoders.
Applications must not write independent PNG datastreams (with either the .png or .mng file extension) with these new filter methods, unless they should become officially approved for use in PNG datastreams. For experimenting and testing, applications can use a value greater than 128 for this purpose, as permitted in the PNG specification.
Note that the top-level color space chunks are used only to supply missing color space information to PNG or JNG datastreams when the datastreams are being decoded. They do not have any effect on already-decoded objects.
If do_not_show=0 for the image when the IHDR
chunk is encountered, a viewer can choose to display the image while
it is being decoded, perhaps taking advantage of the PNG interlacing
method, or to display it after decoding is complete.
If object_id=0, there is no need to store the
pixel data after decoding it and perhaps displaying it.
If concrete_flag=1 and object_id != 0,
the decoder must store the original pixel data losslessly, along
with data from other recognized PNG chunks, because it is possible
that a subsequent Delta-PNG datastream might want to modify it. If
concrete_flag=0, the decoder can store the pixel data in any
form that it chooses.
If the "stored objects" flag in the simplicity profile is
valid and zero, there is no need to store the pixel data and other chunk
data after decoding and perhaps displaying the image.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
A JNG (JPEG Network Graphics) datastream.
See the JNG specification below (Chapter 5) for the format of the JNG datastream.
The JHDR and IEND chunks and any chunks between them are written and decoded according to the JNG specification.
The remaining discussion in the previous paragraph about PNG datastreams also applies to JNG datastreams.
MNG-LC and MNG-VLC applications are not expected to process JNG datastreams unless they have been enhanced with JNG capability.
The BASI chunk introduces a basis object that, while it might be incomplete, can serve as a parent object to which a delta image can be applied.
The first 13 bytes of the BASI chunk are identical to those of the IHDR chunk. The next 8 bytes, which can be omitted, provide sixteen-bit {red, green, blue, alpha} values that are used to fill the entire basis object when the IDAT chunk is not present, and a 1-byte "viewable" flag can also be present.
Width: 4 bytes (unsigned integer).
Height: 4 bytes (unsigned integer).
Sample_depth: 1 byte (unsigned integer).
Color_type: 1 byte (unsigned integer).
Compression_method: 1 byte (unsigned integer).
Filter_method: 1 byte (unsigned integer).
Interlace_method: 1 byte (unsigned integer).
Red_sample or
gray_sample: 2 bytes (unsigned integer).
Green_sample: 2 bytes (unsigned integer).
Blue_sample: 2 bytes (unsigned integer).
Alpha_sample: 2 bytes (unsigned integer).
Viewable: 1 byte (unsigned integer).
0: Basis object is not viewable.
1: Basis object is viewable.
The sample depth, color type, compression method, and interlace method must be valid PNG types, and the width and height must be within the valid range for PNG datastreams. The filter method must be one of the filter methods allowed in PNG datastreams or one of the additional filter methods (1, 64, or 65) allowed in PNG datastreams that are embedded in MNG datastreams.
The alpha_sample can be omitted if the viewable
field is also omitted. If so, and the color_type is one
that requires alpha, the alpha value corresponding to an opaque
pixel will be used. If the color samples are omitted, zeroes will
be used. If the viewable field is omitted, the object is not
viewable.
The decoder is responsible for converting the color and
alpha samples to the appropriate format and sample depth for the
specified color_type.
The color and alpha samples are written as four sixteen-bit samples
regardless of the color_type and sample_depth.
When the sample_depth is less than sixteen, the least
significant bits are used and the remaining bits must be zero
filled. When color_type=3, the least significant byte
of each color sample and the alpha sample is used and the upper byte must
be zero.
When color_type=0 or color_type=4, the green and blue
samples must be present but must be ignored by decoders.
When color_type=0 or color_type=2,
only the values 0 and 2sample_depth should be written.
Any other alpha value must be interpreted as fully opaque.
When color_type=3, the
decoder must generate a palette of length 2sample_depth,
whose first entry contains the given {red_sample, green_sample,
blue_sample} triple, and whose remaining entries are filled with
zeroes, together with an alpha array whose first entry is the given alpha
sample and the rest are 255 (i.e., create a one-entry tRNS chunk containing
the least significant byte of the given alpha sample, unless the given
alpha sample is fully opaque).
The BASI datastream contains PNG chunks, but is not necessarily a PNG datastream. It can be incomplete or empty and it can deviate in certain ways from the PNG specification. It can serve as a parent object for a Delta-PNG datastream, which must supply the missing data or correct the other deviations before the image is displayed. The end of the datastream is denoted by an IEND chunk.
The permitted deviations from the PNG format are:
color_type=3. If so, the subsequent Delta-PNG that uses this
as the parent object can supply a complete PLTE chunk, if the
single-entry palette that is generated is not desired.
The BASI chunk can be used to introduce such things as a library of faLT chunks from which one or another can be selected for use with any single image, or it can be used to introduce a simple blank or colored rectangle that will be immediately displayed or into which other images will be pasted by means of the PAST chunk.
A BASI chunk appearing in a MNG datastream receives its
object_id, location, and potential visibility from the
preceding DEFI chunk, if one is present, or the default
values for DEFI, if one is not present. The
concrete_flag can be either 0 (abstract) or 1 (concrete),
depending on whether the basis image is intended for subsequent use by
a Delta-PNG datastream or not. When it is abstract it must also be
viewable. When viewable=1, the
resulting image, after the pixel samples are filled in, must be a legal
PNG image. If viewable=1 and
do_not_show=0, a viewer is expected to display
it immediately, as if it were decoding a PNG datastream.
If an object already exists with the same object_id, the
contents of its object buffer are replaced with the new data.
Top-level gAMA, sRGB, cHRM, bKGD, sBIT, pHYs, iCCP, and sPLT chunks are inherited by a BASI datastream in the same manner as by a PNG datastream.
No provision is made in this specification for storing a BASI datastream as a standalone file. A BASI datastream will normally be found as a component of a MNG datastream. Applications that need to store a BASI datastream separately should use a different file signature and filename extension, or they can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, the BASI datastream, and the MEND chunk.
Create a clone (a new copy) of an image, with a new
object_id. The CLON chunk contains 4, 5, 6, 7, or
16 bytes.
Source_id: 2 bytes (nonzero unsigned integer).
Identifier of the parent object to be
cloned.
Clone_id: 2 bytes (nonzero unsigned integer).
Identifier of the child object that is
created.
Clone_type: 1 byte (unsigned integer).
0: Full clone of the object attributes
set and the object buffer.
1: Partial clone; only object attributes
set (the location, clipping boundaries,
and potential visibility) are copied
and a link is made to the object
buffer.
2: Renumber object (this is equivalent to
"CLON source_id clone_id 1
DISC source_id").
This field can be omitted if the
do_not_show field is also omitted.
If it is omitted, the clone_type defaults
to zero (full clone).
Do_not_show: 1 byte (unsigned integer).
0: Make the clone potentially visible
and display it immediately.
1: Do not make the clone potentially
visible.
This field can be omitted if the concrete
flag and location fields are also omitted.
When it is omitted, the object retains the
potential visibility of the parent object.
Concrete
flag: 1 byte (unsigned integer).
0: Concrete_flag is the same as
that of the parent object.
1: Make the clone "abstract"
(concrete_flag=0).
This field can be omitted if the location
fields are also omitted. When it is
omitted, the object retains the concrete
flag of the parent object.
Loca
delta_type: 1 byte (unsigned integer)
0: Location data gives X_location and
Y_location directly.
1: New positions are determined by
adding the location data to the
position of the parent object.
This field, together with the X_location
and Y_location fields, can be omitted.
When they are omitted, the clone has the
same location as the parent object.
X_location or delta
X_location:4 bytes (signed integer).
Y_location or delta
Y_location:4 bytes (signed integer).
The source_id must be an existing object identifier, and
the clone_id must not be an existing object identifier.
Negative values are permitted for the X and Y position. The positive directions are downward and rightward from the frame origin.
The clone is initially identical to the parent object except for the
location and potential visibility. It has the same clipping boundaries
as the parent object. Subsequent DHDR, SHOW,
CLON, CLIP, MOVE, PAST, and
DISC chunks can use the clone_id to identify it. If
the parent object is not a viewable image, neither is the clone.
Subsequent chunks can modify, show, or discard a full clone or modify its potential visibility, location and clipping boundaries without affecting the parent object, or they can modify, show, or discard the parent object or modify its object attribute set without affecting the clone.
The concrete_flag byte must be zero or omitted when the
clone_type byte is nonzero.
If an object has partial clones, and the data in the object buffer of a parent object or any of its partial clones is modified, the parent object and all of its partial clones are changed. Decoders must take care that when the parent object or any partial clone is discarded, the object buffer is not discarded until the last remaining one of them is discarded. Only the location, potential visibility, and clipping boundaries can be changed independently for each partial clone.
If viewable=1 and
do_not_show=0, the resulting image is displayed immediately.
A Delta-PNG datastream.
See The Delta-PNG Format (Chapter 6),
below, for the
format of the Delta-PNG datastream. Any chunks between DHDR
and IEND are written and decoded according to the Delta-PNG
format. The object_id of the Delta-PNG DHDR
chunk must point to an existing parent object. The resulting image
is immediately displayed if its do_not_show=0. The parent
object must be concrete (i.e., it must have concrete_flag=1).
Paste an image or images identified by source_id,
or part of it, into an existing abstract image identified by
destination_id.
The PAST chunk contains a 2-byte destination_id
and 9 bytes giving a "target location", plus one or more 30-byte source
data sequences.
Destination_id: 2 bytes (unsigned integer).
Target
delta_type: 1 byte (unsigned integer).
0: Target_x and target_y are given
directly.
1: Target_x and target_y are deltas
from their previous values in a
PAST chunk with the same
destination_id.
2: Target_x and target_y are deltas
from their previous values in the
immediately preceding PAST chunk
regardless of its destination_id.
Target_x: 4 bytes (signed integer), measured
rightward from the left edge of the
destination image.
Target_y: 4 bytes (signed integer), measured
downward from the top edge of the
destination image.
Source_id: 2 bytes (unsigned nonzero integer).
An image to be pasted in.
Composition
mode: 1 byte (unsigned integer).
0: Composite_over.
1: Replace.
2: Composite_under.
Orientation: 1 byte (unsigned integer).
The source image is flipped to
another orientation.
0: Same as source image.
2: Flipped left-right, then up-down.
4: Flipped left-right.
6: Flipped up-down.
8: Tiled with source image, to fill
the clipping boundaries. The
upper left corner of the assembly
is positioned according to the
prescribed offsets.
Offset
delta_type: 1 byte (unsigned integer).
0: Offsets are measured from the
{0,0} pixel in the destination
image.
1: Offsets are measured from the
{target_x,target_y} pixel in the
destination image.
Xoffset or
delta xoffset: 4 bytes (signed integer).
Yoffset or
delta yoffset: 4 bytes (signed integer).
Boundary
delta_type: 1 byte (unsigned integer).
0: Boundaries are measured from
the {0,0} pixel in the
destination image.
1: Boundaries are measured from the
{target_x,target_y} pixel in the
destination image.
Left_pb or delta
left_pb: 4 bytes (signed integer).
Right_pb or delta
right_pb: 4 bytes (signed integer).
Top_pb or delta
top_pb: 4 bytes (signed integer).
Bottom_pb or delta
bottom_pb: 4 bytes (signed integer).
...etc...
The destination image must have the "abstract" property
(concrete_flag=0). When destination_id=0, the
resulting image is "write-only" and therefore only "composite-over"
(composition_mode=0) operations are permitted.
The source images can be "abstract" or "concrete" and have any
color_type and sample_depth. They must
have the "viewable" property. The number of source images is
((chunk_length-11)/30).
The x_offset and y_offset distances and the
clipping boundaries are measured, in pixels, positive rightward and
downward from either the {0,0} pixel of the destination image
or the {target_x, target_y} position in the destination
image. They do not necessarily have to fall within the destination
image. Only those pixels of the source image that fall within the
destination image and also within the specified clipping boundaries
will be copied into the destination image.
Note that the coordinate
system for offsets and clipping is with respect to the upper lefthand
corner of the destination image, which is not necessarily the same
coordinate system used by the MOVE and CLIP chunks.
If the source image has been flipped or rotated, X_offset and
Y_offset give the location of its new upper left hand corner.
When it is tiled, the offsets give the location of the upper left hand
corner of the assembly of tiles.
The left and top clipping boundaries are inclusive, while the right and
bottom clipping boundaries are exclusive
(see Recommendations for encoders, below).
When composition_mode=0, any non-opaque pixels in the
source image are combined with those of the destination image. If
the destination pixel is also non-opaque, the resulting pixel will be
non-opaque.
When composition_mode=1, all pixels simply replace those
in the destination image. This mode can be used to make a transparent
hole in an opaque image.
When composition_mode=2, any non-opaque pixels in the
destination image are combined with those of the source image. If the
source pixel is also non-opaque, the resulting pixel will be non-opaque.
The order of composition is the same as the order that the
source_id's appear in the list (but a decoder can do the
composition in any order it pleases, or all at once, provided that the
resulting destination image is the same as if it had actually performed
each composition in the specified order). Decoders must be careful when
the destination image equals the source image--the pixels to be drawn
are the ones that existed before the drawing operation began.
The MOVE or CLIP information associated with the
destination_id and the source_id's is not used in
the PAST operation (but if a decoder is simultaneously updating
and displaying the destination_id, the MOVE and
CLIP for the destination_id is used in the display
operation).
This chunk provides mandatory magnification factors for existing objects and for subsequent embedded images whose object id is 0.
The chunk contains 0 to 18 bytes:
2 bytes: First magnified object_id
If omitted, any previous MAGN
chunk is nullified.
2 bytes: Last magnified object_id
If omitted, last object_id = first
object_id.
1 byte: X Method
0: No magnification
1: Pixel magnification of color and
alpha samples
2: Linear interpolation of color
and alpha samples
3: Pixel magnification of color
samples and linear interpolation
of alpha samples
4: Linear interpolation of color
samples and pixel magnification
of alpha samples
If omitted, X Method = 0.
2 bytes: MX, X magnification factor,
range 1-65535. If omitted, MX=0.
2 bytes: MY, Y magnification factor. If
omitted, MY=MX.
2 bytes: ML, left X magnification factor
If omitted, ML=MX.
2 bytes: MR, right X magnification factor.
If omitted, MR=MX.
2 bytes: MT, top Y magnification factor.
If omitted, MT=MY.
2 bytes: MB, bottom Y magnification factor.
If omitted, MB=MY.
1 byte: Y method. If omitted, Y method is
the same as X Method.
The last object_id must be greater than or equal to the
first object_id.
It is not an error to include a nonexistent object or an existing
"frozen" object in the range; decoders must do nothing
to any such objects. If an object is potentially visible and
viewable, it is displayed immediately after it is magnified. If any
object_id is nonzero the result of magnifying that object is stored in
place of the original object for later use.
If the MAGN chunk is present, all existing objects in the
specified range
must conceptually be magnified immediately in accordance with the given
magnification factors and methods (decoders may wish to save the magnification
factors and delay the magnification until display time, or until the object
is used as the parent object of a Delta-PNG, to save memory. There is
nothing preventing this, provided that the end effect is the same as if the
magnification had been accomplished immediately). If object
0 is in the specified range, then any subsequent embedded objects with
object_id=0 must be magnified immediately when they appear in
the datastream.
Magnification factors and methods for object 0 are updated by the appearance
of a subsequent MAGN chunk and magnification of object 0 is turned off by
the appearance of an empty MAGN chunk.
When X Method is 1, X magnification is done by simple pixel magnification. The leftmost pixel of each row is replicated ML-1 times. If the original width is greater than 1, the rightmost pixel is replicated MR-1 times. If the original width is greater than 2, the original interior pixels are replicated MX-1 times. The magnified width W is W = ML; if (width > 1) W = W + MR; if (width > 2) W = W + (width-2)*MX;
When X Method=2, X magnification is done by linear interpolation between pixels. If the original width of the image is greater than 1, the interval between the leftmost pixel and the second pixel of each row is subdivided into ML intervals by inserting ML-1 pixels with color and alpha values that are obtained by linear interpolation. If the original width is 1, then the pixel is simply magnified as if X Method=1. If the original width is greater than 2, the rightmost interval is subdivided into MR intervals. If the original width is greater than 3, each original interior interval is subdivided into MX intervals. The magnified width W is
/* The orginal pixels: */
W = width;
/* Add the new pixels in the left interval: */
if (width > 1) W = W + ML-1;
/* Add the new pixels in the right interval: */
if (width > 2) W = W + MR-1;
/* Add the new interior pixels: */
if (width > 3) W = W + (width-3)*(MX-1);
When X Method is 3, the color samples are magnified to the same layout as in X Method 2, but using replication as in X Method 1. The alpha samples are magnified as in X Method 2.
When X Method is 4, the color samples are magnified as in X Method 2. The alpha samples are magnified to the same layout as in X Method 2, but using replication as in X Method 1.
When Y Method=1, Y magnification is done by simple pixel magnification. The topmost pixel of each column is replicated MT-1 times. If the original height is greater than 1, the bottom pixel is replicated MR-1 times. If the original height is greater than 2, the original interior pixels of each column are replicated MY-1 times. The magnified height H is
H = MT; if (height > 1) H = H + MB; if (height > 2) H = H + (height-2)*MY;
When Y Method=2, Y magnification is done by linear interpolation between pixels. If the original height of the image is greater than 1, the interval between the topmost pixel and the second pixel of each column is subdivided into MT intervals by inserting MT-1 pixels with color and alpha values that are obtained by linear interpolation. If the original height is 1, then the pixel is simply magnified as if Y Method=1. If the original height is greater than 2, the bottom interval is subdivided into MB intervals. If the original height is greater than 3, each original interior interval is subdivided into MY intervals. The magnified height H is
H = height; if (height > 1) H = H + MT-1; if (height > 2) H = H + MB-1; if (height > 3) H = H + (height-3)*(MY-1);
When Y Method is 3, the color samples are magnified to the same layout as in Y Method 2, but using replication as in Y Method 1. The alpha samples are magnified as in Y Method 2.
When Y Method is 4, the color samples are magnified as in Y Method 2. The alpha samples are magnified to the same layout as in Y Method 2, but using replication as in Y Method 1.
When the image being magnified is a concrete object, it must not be a JNG or indexed-color PNG (the latter could be promoted to RGB or RGBA via a Delta-PNG PROM chunk first). The result of the magnification is also a concrete object. The Method 2 magnification is conceptually done first in the vertical (Y) direction, the results rounded to the sample depth, then in the horizontal (X) direction. Linear interpolation must be done on the raw pixels, prior to any color correction, using integer arithmetic, to ensure that the result is deterministic. For each channel, the m-1 interpolated samples s[i] are obtained from the two samples s0 and s1 by the following ISO C code or by any other method that obtains the identical results:
if(s1 == s0)
for (i=1; i < m; i++)
s[i] = s0;
else
for (i=1; i < m; i++)
s[i] = ((2*i*(s1-s0)+1)/(m*2) + s0;
Signed arithmetic in a precision large enough to hold the intermediate results must be used, and the final results must be modulo the sample depth.
When the image being magnified is an abstract object (which is always true of object 0), interpolation can be done by any means that achieves a visually similar but not necessarily identical result, such as rounding the results to the sample depth later, using Gouraud shading hardware, or using floating point addition in the loop instead of integer multiplication and division as in:
float delta = ((float)(s1-s0)/(float)m);
float sf= (float)s0;
for (i=1; i < m; i++) {
sf = sf+delta;
s[i]=(int)(sf+0.5);
}
If the abstract object being magnified is an indexed-color PNG, it must be converted to an RGB or RGBA abstract representation before doing the interpolation.
Because the MAGN chunk was added late in the development of MNG-1.0, and there is no simplicity flag to signal its presence, it is recommended that encoders place an empty MAGN chunk or a nEED MAGN chunk early in the datastream, to allow applications to abandon the datastream immediately if they do not recognize the MAGN chunk.
The DISC chunk can be used to inform the decoder that it can discard the object data associated with the associated object identifiers. Whether the decoder actually discards the data or not, it must not use it after encountering the DISC chunk.
The chunk contains a sequence of zero or more two-byte object identifiers. The number of objects to be discarded is the chunk's data length, divided by two.
Discard_id: 2 bytes (nonzero unsigned integer). ...etc...
If the DISC chunk is empty, all objects except those preceding the SAVE chunk (i.e., the "frozen" objects) can be discarded. If a SAVE chunk has not been encountered, all objects can be discarded. Note that each appearance of a SEEK chunk in the datastream implies an empty DISC chunk.
If the DISC chunk is not empty, the listed objects can be discarded.
When an object is discarded, any location, potential visibility, and clipping boundary data associated with it is also discarded.
It is not an error to include an object_id in the
discard_id list, when no such object has been stored, or when
the object has already been discarded.
It is an error to name explicitly any "frozen" object in the DISC list.
When the object is a partial clone or is the source of a partial clone that has not been discarded, only the object attribute set (location, potential visibility, clipping boundaries, etc.) can be discarded. The data in the object buffer must be retained until the last remaining partial clone is discarded.
The BACK chunk suggests a background color or image against which transparent, clipped, or less-than-full-frame images can be displayed.
Red_background: 2 bytes (unsigned integer).
Green_background: 2 bytes (unsigned integer).
Blue_background: 2 bytes (unsigned integer).
Mandatory
background: 1 byte (unsigned integer).
0: Background color and background
image are advisory. Applications
can use them if they choose to.
1: Background color is mandatory.
Applications must use it.
Background image is advisory.
2: Background image is mandatory.
Applications must use it.
Background color is advisory.
3: Background color and background
image are both mandatory.
Applications must use them.
This byte can be omitted if the
background_image_id is also omitted.
If so, the background color is
advisory.
Background
image_id: 2 bytes (unsigned nonzero integer).
Object_id of an image that is to be
displayed as the background of each
subframe. If the image is not
full-frame, the remainder of the area
inside the subframe boundary is filled
with the background color. This field
can be omitted; if so, no background
image is defined, and the background
image_id from a previous BACK chunk
becomes undefined. This byte must be
omitted in MNG-LC and MNG-VLC
datastreams, and when the "stored
objects" flag in the simplicity
profile is valid and zero.
Background
tiling: 1 byte (unsigned integer).
0: Do not tile the background.
1: Tile the background with the
background image. This field
can be omitted; if so, do not
tile the background.
This byte must be omitted in
MNG-LC and MNG-VLC datastreams.
Viewers are expected to composite the first layer in the MNG, MNG-LC, or MNG-VLC datastream against a background layer that fills the entire frame. The BACK chunk provides a background that the viewer can use for this purpose. If it is not "mandatory" the viewer can choose another background if it wishes. If the BACK chunk is not present, or if it has been clipped to less than full frame, the viewer must provide its own background for the first subframe. Each subframe after the first must be composited over the layers that precede it.
Viewers are expected, however, to composite every subframe in the MNG datastream against a fresh copy of the background, when the framing mode given in the FRAM chunk is 3 or 4. The images and the background are both clipped to the frame boundaries given in the FRAM chunk. Anything outside these boundaries is inherited from the previous subframe. The background image (or tiled assembly) is also clipped to its own boundaries and located like any other image. When the background image is used for tiling, the upper left tile is located according to the background image's location attributes and the entire assembly is clipped according to its clipping attributes. Viewers might actually follow some other procedure, but the final appearance of each frame must be the same as if they had filled the area within the subframe boundaries with the background color, then displayed the background image, and then displayed the foreground image (or images), without delay.
It is an error to specify a background_image_id when the
"stored objects" flag in the simplicity profile is valid and
zero.
It is not an error to specify a background_image_id
when such an image does not exist or ceases to exist for some reason.
Viewers must be prepared to fall back to using the background color in
this event. They also must be prepared for the contents, location, and
clipping boundaries of the background image to change, just like any
other object, if it has not been "frozen". The
background image is allowed to have transparency.
The three BACK components are always written as though for an RGBA PNG with 16-bit sample depth. For example, a mid-level gray background could be specified with (0x9999, 0x9999, 0x9999). The background color is interpreted in the current color space as defined by any top-level gAMA, cHRM, iCCP, sRGB chunks that have appeared prior to the BACK chunk in the MNG datastream. If no such chunks appear, the color space is unknown.
The color space of the background image, if one is used, is determined in the same manner as the color space of any other image.
When the BACK chunk appears between FRAM chunks, it applies to the upcoming frame, not to the current one. When the framing_mode is 3, it takes effect immediately prior to the next IHDR, JHDR, BASI, DHDR, PAST, or SHOW chunk in the datastream.
For the purpose of counting layers, when the background consists of both a background color and a background image, these are considered to generate a single layer and there is no delay between displaying the background color and the background image.
Multiple instances of the BACK chunk are permitted in a MNG datastream.
The BACK chunk can be omitted. If a background is needed and the BACK chunk is omitted, then the viewer must supply its own background. For the purpose of counting layers, such a viewer-supplied background layer is counted the same as a background supplied by the BACK chunk.
In practice, most applications that use MNG as part of a
larger composition should ignore the BACK data if
mandatory_background=0 and the application already has
its own background definition. This will frequently be the case in
World Wide Web pages, to achieve nonrectangular transparent animations
displayed against the background of the page.
The FRAM chunk provides information that a decoder needs for generating layers, subframes, and frames. The FRAM parameters govern how the decoder is to behave when it encounters a FRAM chunk, an embedded image, or a SHOW chunk. The FRAM chunk also delimits subframes.
Bit 1 of the MHDR simplicity profile can be used to promise that the FRAM chunk is not present.
An empty FRAM chunk is just a subframe delimiter. A nonempty one is a subframe delimiter, and it also changes FRAM parameters, either for the upcoming subframe or until reset. When the FRAM chunk is not empty, it contains a framing-mode byte, an optional name string, a zero-byte separator, plus four 1-byte fields plus a variable number of optional fields.
Framing mode: 1 byte.
0: Don't change framing mode.
1: No background layer is inserted.
The interframe delay occurs before
each image (except the very first
in the datastream) is displayed.
2: No background layer is inserted.
The interframe delay occurs before
each "FRAM" chunk. No delay occurs
before or between the images.
3: The background layer is displayed
before each image is displayed.
The interframe delay occurs before
displaying the background, and no
delay occurs between the background
and the image.
4: The background layer is inserted
after each "FRAM" chunk. The
interframe delay occurs before
the background is displayed, and no
delay occurs before or between the
images.
Subframe name: 0 or more bytes (Latin-1 Text). Can be
omitted; if so, the subframe is nameless.
Separator: 1 byte: (null). Must be omitted if all
remaining fields are also omitted.
Change interframe
delay: 1 byte.
0: No.
1: Yes, for the next subframe only.
2: Yes, also reset default.
This field and the next three must be
omitted as a group if no frame parameters
other than the framing mode are changed.
Change timeout and
termination: 1 byte
0: No.
1: Deterministic, for the next subframe
only.
2: Deterministic, also reset default.
3: Decoder-discretion, for the next
subframe only.
4: Decoder-discretion, also reset
default.
5: User-discretion, for the next subframe
only.
6: User-discretion, also reset default.
7: External-signal, for the next subframe
only.
8: External-signal, also reset default.
Change subframe clipping
boundaries: 1 byte.
0: No.
1: Yes, for the next subframe only.
2: Yes, also reset default.
Change sync id
list: 1 byte.
0: No.
1: Yes, for the next subframe only.
2: Yes, also reset default list.
Interframe
delay: 4 bytes (unsigned integer). Must
be omitted if change_interframe_delay=0.
The range is [0..2^31-1] ticks.
Timeout: 4 bytes (unsigned integer). Omit if
change_timeout=0. The range is
[0..2^31-1]. The value 2^31-1
(0x7fffffff) ticks represents an
infinite timeout period.
Subframe boundary
delta type: 1 byte (unsigned integer).
0: Subframe clipping boundary values are
given directly.
1: Subframe clipping boundaries
are determined by adding the FRAM
data to their previous values.
This and the following four
fields must be omitted if
change_frame_clipping_boundaries=0.
Left_fb or delta
left_fb: 4 bytes (signed integer).
Right_fb or delta
right_fb: 4 bytes (signed integer).
Top_fb or delta
top_fb: 4 bytes (signed integer).
Bottom_fb or delta
bottom_fb: 4 bytes (signed integer).
Sync id: 4 bytes (unsigned integer). Omit if
change_sync_id_list=0 or if the new
list is empty; repeat until all
sync_id's have been listed. The
range is [0..2^31-1].
When the FRAM parameters are changed, the new parameters affect the subframe that is about to be defined, not the one that is being terminated by the FRAM chunk.
Framing modes:
The framing mode provides information to the decoder that it uses whenever it is about to display an image, and when it is processing the next FRAM chunk.
The following events trigger the display of an image:
Any of these events generates a display update, even if the visible image is outside the clipping boundaries and no pixels are actually changed.
When a decoder is ready to perform a display update, it must check the framing mode, to decide whether it should restore the background (framing modes 3 and 4) or not (framing modes 1 and 2), and whether it needs to wait for the interframe delay to elapse before continuing (framing modes 1 and 3) or not (framing modes 2 and 4).
When the interframe delay is zero, viewers are not required actually to update the display but can continue to process the remainder of the frame and composite the next image over the existing frame before displaying anything. The final result must appear the same as if each image had been displayed in turn with no delay.
Regardless of the framing mode, encoders must insert a background layer, with a zero delay, ahead of the first image layer in the datastream, even when the BACK chunk is not present or has been clipped to less than full-frame. This layer must be included in the layer count but not in the frame count.
Also, viewers that jump or skip to a segment must insert a background layer, with a zero delay, ahead of the segment, even when the BACK chunk is not present in the prologue segment. Such layers are not included in either the layer count or the frame count.
framing_mode=1, the decoder must wait until the
interframe delay has elapsed before displaying each image.
The background layer is inserted, with no interframe delay, before the
first image in the datastream. If the interframe delay is nonzero,
the first frame in the datastream consists of two layers and the rest
consist of a single layer.
In the usual case, the interframe delay is nonzero, so each
subframe is a composite frame. When framing_mode=2, viewers are
expected to display all of the images in a frame at once, if possible, or as
fast as can be managed, without clearing the display or restoring the
background.
The next FRAM chunk delimits the subframe. A subframe
also ends when a SEEK chunk or the MEND chunk appears,
and the final frame ends when the MEND chunk appears.
framing_mode=3,
a background layer is displayed before each image layer is displayed,
without a delay between the background and the image. The decoder must
wait until the interframe delay has elapsed before displaying the background.
If the interframe delay is nonzero, each frame consists of two
layers. A subframe boundary occurs after each image appears. Otherwise,
framing_mode=3 is identical to framing_mode=1.
When the background layer is transparent or does not fill the clipping boundaries of the image layer, the application is responsible for supplying a background color or image against which the image layer is composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes (see the "background transparency" bit of the simplicity profile).
framing_mode=4, the background layer is displayed
before each frame, i.e., after each FRAM chunk, with no interframe
delay before each image. The decoder must wait until the
interframe delay has elapsed before displaying the background layer.
Otherwise, framing_mode=4 is identical to framing_mode=2.
When the background layer is transparent or does not fill the clipping boundaries of the frame, the application is responsible for supplying a background color or image against which the subframes are composited, and if the MNG is being displayed against a changing scene, the application should refresh the entire MNG frame against a new copy of the background layer whenever the application's background scene changes (see the "background transparency" bit of the simplicity profile).
The subframe name must conform to the same formatting rules as
those for a PNG tEXt keyword: It must consist only of printable
Latin-1 characters and must not have leading or trailing blanks, but
can have single embedded blanks. There must be at least one (unless
the subframe name is omitted) and no more than 79 characters in the
keyword. Keywords are case-sensitive. There is no null byte within
the keyword.
No specific use for the subframe name is specified in
this document, except that it can be included in the optional index
that can appear in the SAVE chunk.
Applications can use this
field for such purposes as constructing an external list of subframes
in the datastream. The subframe name only applies to the upcoming
subframe; subsequent subframes are unnamed unless they also have their
own frame_name field. It is recommended that the same name
not appear in any other FRAM chunk or in any SEEK
or eXPI
chunk. Subframe names should not begin with the
case-insensitive
strings "CLOCK(", "FRAME(", or "FRAMES(",
which are reserved for use in URI queries and
fragments.
The interframe delay value is the desired minimum time to elapse from the beginning of displaying one frame until the beginning of displaying the next frame. When the interframe delay is nonzero, which will probably be the usual case, subframes are frames. When it is zero, a frame consists of any number of consecutive subframes, until a nonzero delay subframe is encountered and completed. Decoders are not obligated to display such subframes individually; they can composite them offscreen and only display the complete frame.
There is no interframe delay before the first layer (the implicit background layer) in the datastream nor after the final frame, regardless of the framing mode.
The timeout field can be a number or <infinity>. Infinity can be represented by 0x7fffffff.
The termination condition given in the
change_timeout_and_termination field specifies how much
longer, after the normal interframe delay has elapsed, the frame will
endure, and whether the interframe delay can be shortened. It can take
the following values:
sync_id, but
must wait no longer than the timeout.
When the timeout is zero, the arrival of a signal can shorten the duration
of the frame to less than the interframe delay.
The sync_id list can be omitted if the termination
condition is not "external-signal".
When the sync_id list is changed, the number of
sync_id entries is determined by the remaining length of the
chunk data, divided by four. This number can be zero, which either
inactivates the existing sync_id list for one frame or
deletes it.
The initial values of the FRAM parameters are:
Framing mode = 1
Subframe name = <empty string>
Interframe delay = 1
Left subframe boundary = 0
Right subframe boundary = frame width
Top subframe boundary = 0
Bottom subframe boundary = frame height
Termination = deterministic
Timeout = 0x7fffffff (infinite)
Sync id = <empty list>
The subframe boundaries from the FRAM chunk are only used for clipping, not for placement. The DEFI or MOVE chunk can be used to specify the placement of each image within the layer. The DEFI or CLIP chunk can be used to specify clipping boundaries for each image. Even when the left and top subframe boundaries are nonzero, the image locations are measured with respect to the {0,0} position in the display area.
If the layers are transparent or do not cover the entire area defined by the subframe clipping boundaries, they are composited against whatever already occupies the area, when the framing mode is 1 or 2. When the framing mode is 3 or 4, they are composited against the background defined by the BACK chunk, or against an application-defined background, if the BACK chunk is not present or does not define a mandatory background. The images, as well as the background, are clipped to the subframe clipping boundaries. Any pixels outside the subframe clipping boundaries remain unchanged from the previous subframe.
The interframe_delay field gives the duration of
display, which is the minimum time that must elapse from the
beginning of displaying one subframe until the beginning of displaying
the next. It
is measured in "ticks" using the tick length determined from
ticks_per_second defined in the MHDR chunk.
When the interframe delay is zero, it indicates that
all subframes are to be combined into a single frame, until
a nonzero interframe delay is specified or the MEND chunk is reached.
A viewer does not actually have to follow the procedure of erasing the screen, redisplaying the background, and recompositing the images against it, but what is displayed when the frame is complete must be the same as if it had. It is sufficient to redraw the parts of the display that change from one frame to the next.
The sync_id list provides a point at which the processor
must wait for all pending processes to reach the synchronization
point having the same sync_id before resuming, perhaps
because of a need to synchronize a sound datastream (not defined
in this specification) with the display, to synchronize stereo
images, and the like. When the period defined by the sum of the
interframe_delay and the timeout fields
elapses, processing can resume even though the processor has not
received an indication that other processes have reached the
synchronization point.
Note that the synchronization point does not occur immediately, but at the end of the subframe that follows the FRAM chunk.
The identifier sync_id=0 is reserved to represent
synchronization with a user input from a keyboard or pointing device.
The sync_id values 1-255 are reserved to represent the
corresponding ASCII letter, received from the keyboard (or a simulated
keyboard), and values 256-1023 are reserved for future definition
by this specification. If multiple channels (not defined in this
specification) are not present, viewers can ignore other values
appearing in the sync_id list.
New location of an existing object or objects (replacing or incrementing the location given in the DEFI chunk).
The MOVE chunk gives the position, measured downward and to the right of the frame origin, in pixels, where the named object or group of objects is to be located.
The chunk's contents are:
First object: 2 bytes (unsigned integer).
Last object: 2 bytes (unsigned integer).
Location
delta type: 1 byte (unsigned integer).
0: MOVE data gives X_location and
Y_location directly.
1: New locations are determined
by adding the MOVE data to the
location of the parent object.
X_location or
delta X_location: 4 bytes (signed integer).
Y_location or
delta Y_location: 4 bytes (signed integer).
The new location applies to a single object, if
first_object=last_object, or to a group of consecutive
object_ids, if they are different. Last_object
must not be less than first_object. Negative values are
permitted for the X and Y location. The positive directions are
downward and rightward from the frame origin. The MOVE chunk
can specify an image placement that is partially or wholly outside the
display boundaries. In such cases, the resulting image must be clipped
to fit within its clipping boundaries, or not displayed at all if it
falls entirely outside its clipping boundaries. The clipping boundaries
are determined as described in the specification for the CLIP
chunk below (Paragraph 4.3.4).
It is not an error for the MOVE chunk to name an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to move "frozen" objects provided that the encoder includes chunks to move them back to their original positions prior to then end of the segment.
When an object is discarded, its object attribute set, which includes the MOVE data, is also discarded.
This chunk gives the new boundaries (replacing or incrementing those from the DEFI chunk) to which an existing object or group of objects must be clipped for display. It contains the following 21 bytes:
First object: 2 bytes (unsigned integer).
Last object: 2 bytes (unsigned integer).
Clip delta type: 1 byte (unsigned integer).
0: CLIP data gives boundary values
directly.
1: CLIP boundaries are determined
by adding the CLIP data to their
previous values for this object.
Left_cb or
delta_left_cb: 4 bytes (signed integer).
Right_cb or
delta_right_cb: 4 bytes (signed integer).
Top_cb or
delta_top_cb: 4 bytes (signed integer).
Bottom_cb or
delta_bottom_cb: 4 bytes (signed integer).
The new clipping boundaries apply to a single object, if
first_object=last_object, or to a group of consecutive
objects, if they are different. Last_object must not be less
than first_object.
The clipping boundaries are expressed in pixels, measured rightward and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and bottom clipping boundaries are exclusive, i.e., the pixel located at {x,y} is only displayed if the pixel falls within the physical limits of the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk) 0 <= y < frame_height Left_fb <= x < right_fb (from the FRAM chunk) Top_fb <= y < bottom_fb Left_cb <= x < right_cb (from the CLIP chunk) Top_cb <= y < bottom_cb
It is not an error for the CLIP chunk to name an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to clip "frozen" objects provided that another CLIP chunk resets them to their original values prior to the end of the segment.
When an object is discarded, its object attribute set, which includes the CLIP data, is also discarded.
The SHOW chunk is used to change the potential visibility of one or more previously-defined objects and to direct that they be displayed. It contains 2, 4, or 5 bytes, or it can be empty.
First image: 2 bytes (nonzero unsigned integer).
Last image: 2 bytes (nonzero unsigned integer).
This field can be omitted if the show_mode
byte is also omitted. If so, decoders must
assume the default values, show_mode=0
and last_image=first_image.
Show_mode: 1 byte (unsigned integer).
0: Make the images potentially visible.
and display them (set do_not_show=0).
1: Make the images invisible (set
do_not_show=1).
2: Don't change do_not_show flag;
display those that are potentially
visible.
3: Mark images "potentially visible"
(do_not_show=0), but do not display
them.
4: Toggle do_not_show flag; display
any that are potentially visible after
toggling.
5: Toggle do_not_show flag, but do
not display even if potentially visible
after toggling.
6: Step through the images in the given
range, making the next image potentially
visible (set do_not_show=0) and display
it. Set do_not_show=1 for all other
images in the range. Jump to the
beginning of the range when reaching the
end of the range. Perform one step for
each SHOW chunk (in reverse order if
last_image < first_image).
7. Make the next image in the range
(cycle) potentially visible (do_not_show=0),
but do not display it. Set
do_not_show=1 for the rest of the
images in the range.
This field can be omitted. If so, decoders
must assume the default, show_mode=0.
The decoder processes the objects (or images) named in the
SHOW chunk in the order first_image through
last_image, and resets the do_not_show flag for
each of the objects. If show_mode is even-valued, it also
displays the images if they are potentially visible and are viewable
images.
When the SHOW chunk is empty, the decoder displays
all existing potentially visible images, without changing their
do_not_show status. The empty SHOW chunk is
equivalent to
SHOW 1 65535 2
If last_image < first_image the images are processed in
reverse order.
When show_mode is odd-valued, nothing is displayed
unless a subsequent SHOW chunk with an even-valued
show_mode appears.
show_mode is even-valued, each visible image that
is displayed generates a separate layer, even
if it is offscreen and no pixels are actually displayed. In such cases,
the layer is totally transparent. When show_mode is odd, or
when show_mode=2 or 4 or is empty and no image is visible, no
layer is generated.
When show_mode is 1, 4, 5, 6, or 7, images can
be made invisible. This is not permitted when the framing mode is 2 or 4
in the FRAM chunk and the images have already appeared in the
frame, because simple viewers will have already drawn them and have no
way to make them invisible again without redrawing the entire frame.
When show_mode is 6 or 7, a single layer is generated.
The decoder must make the
next image in the "cycle" visible. To do this, it must examine
the do_not_show flag for each image in the range
first_image through last_image, and make the next
one (the one with the next higher value of image_id that
exists and is "viewable") after the first visible one it finds visible
and the rest invisible. When first_image > last_image,
the cycle is reversed, and the "next" image is the one with the next
lower value of image_id. In either case, if the first
visible one found was last_id, or none were visible, it must make
first_image visible. These modes are useful for manipulating
a group of sequential images that represent different views of an
animated icon. See Example 8, below (Chapter 17).
If no "viewable" object is in the specified range, an empty layer
must be generated.
When show_mode=0, 2, 4, or 6, separate layers will be
generated, each containing an instance of one visible image at the
location specified by the DEFI, CLON, or MOVE
chunk and clipped according to the boundaries specified by the
CLIP and FRAM chunks. When the MOVE or
CLON chunk is used in the delta form, which will frequently be
the case, each image must be displaced from its previous position by the
values given in the MOVE or CLON chunk.
Assuming a nonzero interframe delay, any of the following sequences
would cause the image identified by object_id=6 in a
composite frame to blink:
LOOP 0 0 10 FRAM 4 # Show background SHOW 1 10 # Show images 1 thru 10. FRAM # Show background SHOW 1 5 # Show images 1 thru 5. SHOW 7 10 # Show images 7 thru 10. ENDL FRAM 4 # Show background LOOP 0 0 10 SHOW 1 5 # Show images 1 thru 5. SHOW 6 6 4 # Toggle potential visibility of image 6 SHOW 7 10 # and show it; show images 7 thru 10. FRAM ENDL FRAM 4 # Show background LOOP 0 0 10 SHOW 6 6 5 # Toggle potential visibility of image 6. SHOW 1 10 2 # Show potentially visible images in 1 FRAM # through 10. ENDL
It is not necessary to follow an IHDR-IEND,
JHDR-IEND, BASI-IEND, or DHDR-IEND
sequence or PAST chunk with a SHOW chunk to
display the resulting image, if it was already caused to appear by
do_not_show=0 in the DEFI chunk that introduced the
image. Similarly, the CLON chunk need not be followed by a
SHOW chunk, if do_not_show=0 in the CLON
chunk.
It is not an error for the SHOW chunk to name a nonviewable object or an object that has not previously been defined. In such cases, nothing is done to the nonexistent object. It is permitted to change the potential visibility of "frozen" objects provided that another SHOW chunk resets them to their original values prior to the end of the segment.
The TERM chunk suggests how the end of the MNG datastream should be handled, when a MEND chunk is found. It contains either a single byte or ten bytes:
Termination
action: 1 byte (unsigned integer)
0: Show the last frame indefinitely.
1: Cease displaying anything.
2: Show the first frame after the TERM
chunk.
If processing the fPRI chunk,
use a "cost" of 255.
3: Repeat the sequence starting
immediately after the TERM chunk
and ending with the MEND chunk.
Action after
iterations: 1 byte
0: Show the last frame indefinitely after
iteration_max iterations have been
done.
1: Cease displaying anything.
2: Show the first frame after the TERM
chunk.
If processing the fPRI chunk,
use a "cost" of 255.
This and the remaining fields must be
present if termination_action is 3, and
must be omitted otherwise.
Delay: 4 bytes (unsigned integer). Delay, in
ticks, before repeating the sequence.
Iteration max: 4 bytes (unsigned integer). Maximum
number of times to execute the sequence.
Infinity is represented by 0x7fffffff.
The loop created by processing a TERM must always be treated
by the decoder as if it were a cacheable <user-discretion> loop, with
iteration_min=1.
Applications must not depend on anything that has been drawn on the output buffer or device during the previous iteration. Its contents become undefined when the TERM loop restarts.
MNG editors that extract a series of PNG or JNG files from a MNG datastream are expected to execute the TERM loop only once, rather than emitting the files repeatedly.
The TERM chunk, if present, must appear either immediately after the MHDR chunk or immediately prior to a SEEK chunk. The TERM chunk is not considered to be a part of any segment for the purpose of determining the copy-safe status of any chunk. Only one TERM chunk is permitted in a MNG datastream.
Simple viewers and single-frame viewers can ignore the TERM chunk. It has been made critical only so MNG editors will not inadvertently relocate it.
The SAVE chunk marks a point in the datastream at which objects are "frozen" and other chunk information is "saved". The SEEK chunk marks positions in the MNG datastream where a restart is possible, and where the decoder must restore the "saved" information, if they have jumped or skipped to a SEEK point. They only need to restore information that they will use, e.g., a viewer that processes gAMA and global PLTE and tRNS, but ignores iCCP and sPLT, need only restore the value of gamma and the global PLTE and tRNS data from the prologue segment but not the values of the iCCP and sPLT data.
Simple decoders that only read MNG datastreams sequentially can safely ignore the SAVE and SEEK chunks, although it is recommended that, for efficient use of memory, they at least mark existing objects as "frozen" when the SAVE chunk is processed and discard all "unfrozen" objects whenever the SEEK or empty DISC chunk is processed.
The SAVE chunk marks a point in the datastream at which objects are "frozen" and other chunk information is "saved"; a decoder skipping or jumping to a SEEK chunk must restore the "saved" chunk information if it has been redefined or discarded. In addition, the SAVE chunk can contain an optional index to the MNG datastream.
The SAVE chunk can be empty, or it can contain an index consisting of the following:
Offset size: 1 byte (unsigned integer).
4: Offsets and nominal start times are
expressed as 32-bit integers.
8: Offsets and nominal start times are
expressed as 64-bit integers.
plus zero or more of the following index entries:
Entry type: 1 byte (unsigned integer).
0: Segment with nominal start time,
nominal layer number, and nominal
frame number.
1: Segment.
2: Subframe.
3: Exported image.
Offset: 4 or 8 bytes (unsigned integer). Omit if
entry_type > 1, set equal to zero
if the offset is unknown.
Nominal start
time: 4 or 8 bytes: (unsigned integer). Start
time of the segment, measured in ticks
from the beginning of the sequence,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks. Omit if
entry_type > 0.
Nominal layer
number: 4 bytes (unsigned integer). Sequence
number of the first layer in the segment,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks; the first
layer of the first segment being layer 0.
Omit if entry_type > 0.
Nominal frame
number: 4 bytes (unsigned integer). Sequence
number of the first frame in the segment,
assuming that all prior segments were
played as intended on an ideal player,
ignoring any fPRI chunks; the first frame
of the first segment being frame 0. Omit
if entry_type > 0.
Name: 1-79 bytes (Latin-1 text). Omit for
unnamed segments.
Separator: 1 byte (null) (must be omitted after
the final entry).
The SAVE chunk must be present when the SEEK chunk is present. It appears after the set of chunks that define information that must be retained for the remainder of the datastream. These chunks, collectively referred to as the prologue segment, are no different from chunks in other segments. They can be chunks that define objects, or they can be chunks that define other information such as gAMA, cHRM, and sPLT. If any chunks appear between the SAVE chunk and the first SEEK chunk, these chunks also form a part of the prologue segment, but their contents are lost when the SEEK chunk appears.
Only one instance of the SAVE chunk is permitted in a MNG datastream. It is not allowed anywhere after the first SEEK chunk.
It is not permitted, at any point beyond the SAVE chunk, to modify or discard any object that was defined ahead of the SAVE chunk.
An object appearing ahead of the SAVE chunk can be the subject of a CLON chunk. If the clone is a partial clone, modifying it is not permitted, because this would also modify the object buffer that the original object points to.
A chunk like gAMA that overwrites a single current value is permitted after the SAVE chunk, even if the chunk has appeared ahead of the SAVE chunk. Decoders are responsible for saving a copy of the chunk data (in any convenient form) when the SAVE chunk is encountered and restoring it when skipping or jumping to a SEEK chunk. If no instance of the chunk appeared ahead of the SAVE chunk, the decoder must restore the chunk data to its original "unknown" condition when it skips or jumps to a SEEK chunk.
It is the encoder's responsibility, if it changes or discards any "saved" data, to restore it to its "saved" condition (or to nullify it, if it was unknown) prior to the end of the segment. This makes it safe for simple decoders to ignore the SAVE/SEEK mechanism.
Known chunks in this category include DEFI, FRAM, BACK, PLTE, cHRM, tRNS, fPRI, gAMA, iCCP, bKGD, sBIT, pHYg, pHYs, and sRGB. In addition, it is the responsibility of the encoder to include chunks that restore the potential visibility, location, and clipping boundaries of any "frozen" objects to their "saved" condition.
The magnification factors for object 0 must be saved when the SAVE chunk is encountered. If an encoder changes them in a segment, it must include another MAGN chunk to restore them to their "saved" condition prior to the end of the segement. This guarantees that they are safe to use by a decoder that reaches a SEEK chunk while sequentially processing a MNG datastream; but they must be restored by a decoder that jumps or skips to a SEEK chunk.
In the case of chunks like sPLT that can occur multiple times, with different "purpose" fields, additional instances of the chunk are permitted after the SAVE chunk, but not with the same keyword as any instances that occurred ahead of the SAVE chunk. The decoder is required to forget such additional instances when it skips or jumps to a SEEK chunk, but it must retain those instances that were defined prior to the SAVE chunk. Encoders are required to nullify such additional instances prior to the end of the segment. Known chunks in this category include only sPLT.
If an entry for a segment (entry type 0 or 1) appears in the optional index, there must also be an entry for every segment, whether named or not, except for the prologue segment, that precedes it. All entries must appear in the index in the same order that they appear in the MNG datastream. There must never be a segment entry (type 0 or 1) for the prologue segment, but there can be entries for named images or subframes in the prologue, placed ahead of the first segment entry. Only named images or subframes are permitted, and it is not an error to omit any or all named images or subframes. Nor is it an error to omit a contiguous set of segments at the end of the datastream from the index.
Offsets are calculated from the first byte of the MNG 8-byte signature, which has offset=0. This is true even if the MNG datastream happens to be embedded in some other file and the signature bytes are not actually present.
Applications with direct access to the datastream can use the index to find segments, subframes, and exported images quickly. After processing the prologue segment, they can jump directly to any segment and then process the remaining datastream until the desired subframe, image, or time is found. Applications that have only streaming access to the datastream can still use the index to decide whether to decode the chunks in a segment or to skip over them.
Only one instance of the SAVE chunk is permitted in a MNG datastream. If the SEEK chunk is present, the SAVE chunk must be present, prior to the first SEEK chunk. The only chunks not allowed ahead of the SAVE chunk are the SEEK chunk and the MEND chunk. The SAVE chunk must not appear inside a LOOP-ENDL pair.
The SEEK chunk marks positions ("seek points") in the MNG datastream where a restart is possible, and where the decoder must restore certain information to the condition that existed when the SAVE chunk was processed, if it has skipped or jumped to the SEEK chunk.
The SEEK chunk can be empty, or it can contain a segment name.
Segment name: 1-79 bytes (Latin-1 string).
The segment name is optional. It must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. There is no null byte terminator within the segment name, nor is there a separate null byte terminator. Segment names are case-sensitive. Use caution when printing or displaying keywords (Refer to Security considerations, below, Chapter 16). No specific use for the segment name is specified in this document, but applications can use the segment name for such purposes as constructing a menu of seek points for a slide-show viewer. It can be included in the optional index that can appear in the SAVE chunk. It is recommended that the same name not appear in any other SEEK chunk or in any FRAM or eXPI chunk. Segment names should not begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments.
Applications must not use any information preceding the SEEK chunk, except for:
When the SEEK chunk is encountered, the decoder can discard any objects appearing after the SAVE chunk, as though an empty DISC chunk were present.
In addition to providing a mechanism for skipping frames or backspacing over frames, the SEEK chunk provides a means of dealing with a corrupted datastream. The viewer would abandon processing and simply look for the next SEEK chunk before resuming. Note that looking for a PNG IHDR chunk would not be sufficient because the PNG datastream might be inside a loop or a Delta-PNG datastream, or it might need data from preceding MOVE or CLIP chunks.
When a decoder makes random access to a seek point, it must restore the information that it saved when it processed the SAVE chunk. When it encounters a SEEK chunk during normal sequential processing of a MNG datastream, it need not do anything, because the encoder will have written chunks that restore all saved information.
Multiple instances of the SEEK chunk are permitted. The SEEK chunk must not appear prior to the SAVE chunk. The SAVE chunk must also be present if the SEEK chunk is present. The SEEK must not appear between a LOOP chunk and its ENDL chunk.
This section describes ancillary MNG chunks. MNG-compliant decoders are not required to recognize and process them.
The eXPI chunk takes a snapshot of a viewable object (either concrete or abstract), associates the name with that snapshot, and makes the name available to the "outside world" (like a scripting language).
The chunk contains an object identifier (snapshot id) and a name:
Snapshot id: 2 bytes (unsigned integer).
(must be zero in MNG-LC and
MNG-VLC datastreams)
Snapshot name: 1-79 bytes (Latin-1 text).
When the snapshot_id is zero, the snapshot is the first instance of an embedded image with object_id=0 following the eXPI chunk. When the snapshot_id is nonzero, the snapshot is an already-defined object with that object_id as it already exists when the eXPI chunk is encountered.
Note that the snapshot_name is associated with the
snapshot, not with the snapshot_id nor its future contents;
changing the image identified by snapshot_id will not
affect the snapshot.
The snapshot_name means nothing inside the scope of the
MNG specification, except that it can be included in
the optional index that can appear in the SAVE chunk.
If two eXPI chunks use the same name, it is the outside world's
problem (and the outside world's prerogative to regard it as an error).
It is recommended, however, that the snapshot_name not be
the same as that appearing in any other eXPI
chunk or in any FRAM
or SEEK
chunk. A decoder that knows of no
"outside world" can simply ignore the eXPI chunk. This
chunk could be used in MNG datastreams that define libraries of related
images, rather than animations, to allow applications to extract
images by their snapshot_id.
Names beginning with the word "thumbnail" are reserved for snapshot images that are intended to make good icons for the MNG. Thumbnail images are regular PNG or Delta-PNG images, but they would normally have smaller dimensions and fewer colors than the MNG frames. They can be defined with the potential visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.
The snapshot_name string must follow the format of a
tEXt keyword: It must consist only of printable Latin-1
characters and must not have leading or trailing blanks, but can have
single embedded blanks. There must be at least one and no more than
79 characters in the keyword. Keywords are case-sensitive. There is
no null byte terminator within the snapshot_name string,
nor is there a separate null byte terminator. Snapshot names should
not begin with the case-insensitive strings
"CLOCK(", "FRAME(", or "FRAMES("
which are reserved for use in URI queries and
fragments.
Multiple instances of the eXPI chunk are permitted
in a MNG datastream, and they need not have different values of
snapshot_id.
The fPRI chunk allows authors to assign a priority to a portion of the MNG datastream. Decoders can decide whether or not to decode and process that part of the datastream based on its "priority" compared to some measure of "cost."
The fPRI chunk contains two bytes:
fPRI delta type: 1 byte (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding
the fPRI data to the previous
value, modulo 256.
Priority or
delta priority: 1 byte (signed integer). Value to be
assigned to subsequent chunks until
another fPRI chunk is reached.
While 256 distinct values of priority are possible, it
is recommended that only the values 0 (low priority), 128 (medium
priority), and 255 (high priority) be used. Viewers that can only
display a single image can look for one with priority=255 and
stop after displaying it. If the datastream contains a large number
of frames and includes periodic "initial" frames that do not contain
Delta-PNG datastreams, each "initial" frame could be preceded by a
fPRI with priority=128 and followed by one with
priority=0, and the best representative initial frame could
be preceded by a fPRI chunk with priority=255. Then
single-image viewers would just display the representative frame, slow
viewers would display just the "initial" frames, and fast viewers
would display everything.
If a viewer has established a nonzero "cost," it must skip any portion of the datastream whose priority is less than that "cost." The "cost" must be established prior to processing the proloque segment. If the decoder changes its "cost" it must process again according to the new "cost", unless it knows that there were no fPRI chunks in the prologue segment.
The SAVE, SEEK, and MEND chunks always
have priority=255; decoders must look for these chunks in
addition to the fPRI chunk while skipping a low-priority
portion of the datastream.
It is not permissible for a portion of the datastream to depend on any portion of the datastream having a lower value, because a decoder might have skipped the lower value portion. Use of the fPRI chunk is illustrated in Example 5 and Example 9.
Viewers that care about the priority must assume
priority=255 for any portion of the MNG datastream that is
processed prior to the first fPRI chunk.
Multiple instances of the fPRI chunk are permitted.
The nEED chunk can be used to specify needed resources, to provide a quick exit path for viewers that are not capable of displaying the MNG datastream.
The nEED chunk contains a list of keywords that the decoder must recognize. Keywords are typically private critical chunk names.
Keyword: 1-79 bytes. Separator: 1 byte (null). ...etc...
The nEED chunk should be placed early in the MNG datastream, preferably very shortly after the MHDR chunk.
The keywords are typically 4-character private critical chunk names, but they could be any string that a decoder is required to recognize. No critical chunks defined in this specification or in the PNG specification should be named in a nEED chunk, because MNG-compliant decoders are required to recognize all of them, whether they appear in a nEED chunk or not. The purpose of the nEED chunk is only to identify requirements that are above and beyond the requirements of this document and of the PNG specification.
Each keyword string must follow the format of a tEXt keyword: It must consist only of printable Latin-1 characters and must not have leading or trailing blanks, but can have single embedded blanks. There must be at least one and no more than 79 characters in the keyword. Keywords are case-sensitive. There is no null byte terminator within the keyword. A null separator byte must appear after each keyword in the nEED chunk except for the last one.
Decoders that do not recognize a chunk name or keyword in the list should abandon the MNG datastream or request user intervention. The normal security precautions should be taken when displaying the keywords.
The MNG pHYg chunk is identical in syntax to the PNG pHYs chunk. It applies to complete full-frame MNG layers and not to the individual images within them.
Conceptually, a MNG viewer that processes the pHYg chunk will first composite each image into a full-frame layer, then apply the pHYg scaling to the layer, and finally composite the scaled layer into the subframe. MNG datastreams can include both the pHYs chunk (either at the MNG top level or within the PNG and JNG datastreams) and the pHYg chunk (only at the MNG top level), to ensure that the images are properly displayed either when displayed by a MNG viewer or when extracted into a series of individual PNG or JNG datastreams and then displayed by a PNG or JNG application. The pHYs and pHYg chunks would normally contain the same values, but this is not necessary.
The MNG top-level pHYg chunk can be nullified by a subsequent empty pHYg chunk appearing in the MNG top level.
The namespace for MNG chunk names is separate from that of PNG. Only those PNG chunks named in this paragraph are also defined at the MNG top level. They have exactly the same syntax and semantics as when they appear in a PNG datastream:
A MNG editor that writes PNG datastreams should not include the top-level iTXt, tEXt, tIME, and zTXt chunks in the generated PNG datastreams.
These PNG chunks are also defined at the MNG top level. They provide default values to be used in case they are not provided in subsequent PNG datastreams. Any of these chunks can be nullified by the appearance of a subsequent empty chunk with the same chunk name. Such empty chunks are not legal PNG or JNG chunks and must only appear in the MNG top level.
In the MNG top level, all of these chunks are written as though for 16-bit RGBA PNG datastreams. Decoders are responsible for reformatting the chunk data to suit the actual bit depth and color type of the datastream that inherits them.
A MNG editor that writes PNG or JNG datastreams is expected to
include the top-level cHRM, gAMA, iCCP,
and sRGB chunks in the generated PNG or JNG datastreams, if
the embedded image does not contain its own chunks that define the
color space. When it writes the sRGB chunk, it should write
the gAMA chunk (and perhaps the cHRM chunk), in
accordance with the PNG specification, even though no gAMA or
cHRM chunk is present in the MNG datastream.
It is also expected to write the pHYs chunk and the reformatted
top-level bKGD chunk in the generated PNG or JNG datastreams, and
the reformatted sBIT chunk only in generated PNG datastreams, when
the datastream does not have its own bKGD, pHYs,
or sBIT chunks.
The top-level sRGB chunk nullifies the preceding
top-level gAMA and cHRM chunks, if any, and
either the top-level gAMA or the top-level cHRM chunk
nullifies the preceding top-level sRGB chunk, if any.
This PNG chunk is also defined at the MNG top
level. It
provides a value that takes precedence over those that might be provided
in subsequent PNG or JNG datastreams and provides a value to be used
when it is not provided in subsequent PNG or JNG datastreams.
It also takes precedence over the
PLTE chunk in a subsequent PNG datastream when the
PLTE and hIST chunks are being used as a suggested
palette (i.e., When a decoder needs to choose between a suggested palette
defined at the MNG level and a suggested palette defined in the PNG datastream
(either with the sPLT chunk, or with the PLTE/hIST
chunks for grayscale or truecolor images), it should give preference to
the palette from the MNG level, to avoid spurious layer-to-layer color
changes.
MNG editors that write PNG datastreams should ignore the
sPLT data from the MNG level and simply copy
any sPLT chunks appearing within the embedded PNG
datastreams.
color_type != 3). This chunk can appear for
any color type. There can be multiple sPLT chunks in a MNG
datastream. If a palette_name is repeated, the previous
palette having the same palette_name is replaced. It is
not permitted, at the MNG top level, to redefine a palette after the
SAVE chunk with the same palette_name as one that
appears ahead of the SAVE chunk. It is permitted, however, to
define and redefine other palettes with other palette_name
fields. A single empty sPLT chunk can be used to nullify all
sPLT chunks that have been previously defined in the MNG top
level, except for those that appeared ahead of the SAVE chunk,
when the SAVE chunk has been read.
JNG (JPEG Network Graphics) is the lossy sub-format for MNG objects.
MNG-LC and MNG-VLC applications can choose to support JNG or not. Those that do not can check bit 4 (JNG is present/absent) of the MHDR simplicity profile to decide whether they can process the datastream.
Note: This specification depends on the PNG Portable Network Graphics specification [PNG]. The PNG specification is available at the PNG home page,
http://www.libpng.org/pub/png/
A JNG datastream consists of a header chunk (JHDR), JDAT chunks that contain a complete JPEG datastream, optional IDAT chunks that contain a PNG-encoded grayscale image that is to be used as an alpha mask, and an IEND chunk. The alpha mask, if present, must have the same dimensions as the image itself. The JDAT and IDAT chunks can be interleaved. Some of the PNG ancillary chunks are also recognized in JNG datastreams.
While JNG is primarily intended for use as a sub-format within MNG, a single-image JNG datastream can be written in a standalone file. If so, the first eight bytes of a JNG datastream are
139 74 78 71 13 10 26 10
(decimal) which is similar to the PNG signature with "\213 J N G" instead of "\211 P N G" in bytes 0-3.
We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng can be used. It is recommended that the
file extension ".jng" (lower case preferred) be used.
JNG is pronounced "Jing."
This section specifies the critical chunks that are defined in the JNG format.
The format of the JHDR chunk introduces a JNG datastream. It contains:
Width: 4 bytes (unsigned integer, range 0..65535).
Height: 4 bytes (unsigned integer, range 0..65535).
Color type: 1 byte
8: Gray (Y).
10: Color (YCbCr).
12: Gray-alpha (Y-alpha).
14: Color-alpha (YCbCr-alpha).
Image sample
depth: 1 byte
8: 8-bit samples and quantization tables.
12: 12-bit samples and quantization
tables.
20: 8-bit image followed by a 12-bit
image.
Image compression
method: 1 byte
8: ISO-10918-1 Huffman-coded baseline JPEG.
Image interlace
method: 1 byte.
0: Sequential JPEG, single scan.
8: Progressive JPEG.
Alpha sample
depth: 1 byte.
0, 1, 2, 4, 8, or 16, if the Alpha
compression method is 0 (PNG)
8, if the Alpha compression method
is 8 (JNG).
Alpha compression
method: 1 byte.
0: PNG grayscale IDAT format.
8: JNG 8-bit grayscale JDAA format.
Alpha filter
method: 1 byte.
0: Adaptive PNG (see PNG spec) or
not applicable (JPEG).
Alpha interlace
method: 1 byte.
0: Noninterlaced PNG or sequential
single-scan JPEG.
The width, height, image_sample_depth, image_compression_method, and image_interlace_method fields are redundant because equivalent information is also embedded in the JDAT datastream. They appear in the JHDR chunk for convenience. Their values must be identical to their equivalents embedded in the JDAT chunk. We use four bytes in the width and height fields for similarity to MNG and PNG, and to leave room for future expansion, even though two bytes would have been sufficient.
When the color_type is 8 or 10 (no alpha channel), the
last four bytes, which describe the IDAT data, must be set to
zero. The alpha_sample_depth must be nonzero when the alpha
channel is present.
A JNG datastream must contain one or more JDAT chunks, whose data, when concatenated, forms a complete JNG JPEG datastream. JNG decoders are required to read all baseline JNG JPEG and eight-bit progressive JNG JPEG datastreams. Twelve-bit capability is not required.
JDAT chunks are like PNG IDAT chunks in that there may be multiple JDAT chunks, the data from which are concatenated to form a single datastream that can be sent to the decompressor. No chunks are permitted among the sequence of JDAT chunks, except for interleaved IDAT chunks. The ordering requirements of other ancillary chunks are the same with respect to JDAT as they are in PNG with respect to the IDAT chunk.
A JNG JPEG is a baseline, extended-sequential, or progressive JPEG as defined by JPEG Part 1 [ISO/IEC-10918-1]. JNG uses only JFIF-compatible [JFIF] component interpretations, and imposes a few additional restrictions that reflect limitations of many existing JPEG implementations. In particular, only Huffman entropy coding is permitted.
Actually, a JNG may contain two separate JNG JPEG datastreams (one eight-bit and one twelve-bit), each contained in a series of JDAT chunks, and separated by a JSEP chunk (see the JSEP chunk specification below, Paragraph 5.1.6). Decoders that are unable to (or do not wish to) handle twelve-bit datastreams are allowed to display the eight-bit datastream instead, if one is present.
The core of the JNG JPEG definition is baseline JNG JPEG, which is JPEG Part 1's definition of baseline JPEG further restricted by JFIF restrictions and JNG-specific restrictions. JPEG, which is also defined in JPEG Part 1 and has JNG-specific restrictions.
A baseline JPEG according to JPEG Part 1 is DCT-based (lossy) sequential JPEG, using 8-bit sample precision and Huffman entropy coding, with the following further restrictions:
The SOF marker type for baseline JPEG is SOF0.
JDAT datastreams must always follow "interchange JPEG" rules: all necessary quantization and Huffman tables must be included in the datastream; no tables can be omitted.
The image data is always stored left-to-right, top-to-bottom.
The encoded data shall have one of the two color space interpretations allowed by the JFIF specification:
Y = Luma_red*R + Luma_green*G + Luma_blue*B Cb = (B - Y) / (2 - 2*Luma_blue) + Half_scale Cr = (R - Y) / (2 - 2*Luma_red) + Half_scale
By convention, the luminance coefficients are always those defined by CCIR Recommendation 601-1:
Luma_red = 0.299 Luma_green = 0.587 Luma_blue = 0.114
The constant Half_scale is 128 when dealing with eight-bit data, 2048 for twelve-bit data. With these equations, Y, Cb, and Cr all have the same range as R, G, and B: 0 to 255 for eight-bit data, 0 to 4095 for twelve-bit data.
The JFIF convention for YCbCr differs from typical digital television practice in that no headroom/footroom is reserved: the coefficient values range over the full available 8 or 12 bits.
Intercomponent sample alignment shall be such that the first (upper leftmost) samples of each component share a common upper left corner position. This again differs from common digital TV practice, in which the first samples share a common center position. The JFIF convention is simpler to visualize: subsampled chroma samples always cover an integral number of luminance sample positions, whereas with co-centered alignment, chroma samples only partially overlap some luminance samples.
JNG imposes three additional restrictions not found in the text of either JPEG Part 1 or the JFIF specification:
In other words, the chroma components may be downsampled 2:1 or 1:2 horizontally or vertically relative to luminance, or they may be left full size. These four sampling ratios are the only ones supported by a wide spectrum of implementations (1x2 is relatively uncommon, and is usually the result of a lossless rotation of a 2x1 sampling).
For grayscale images, the sampling factors are irrelevant according to a strict reading of JPEG Part 1. Hence decoder authors should accept any sampling factors for grayscale. However, we recommend that encoders always emit sampling factors 1h1v for grayscale, since some decoders have been observed to malfunction when presented with other sampling factors.
For JNG progressive JPEG datastreams, the JPEG process is progressive Huffman coding (SOF marker type SOF2) rather than baseline (SOF0). All JNG-compliant decoders must support full progression, including both spectral-selection and successive-approximation modes, with any sequence of scan progression parameters allowed by the JPEG Part 1 standard.
Otherwise, all the restrictions listed above apply, except these:
We require full progression support since relatively little code savings can be achieved by subsetting the JPEG progression features. In particular, successive approximation offers significant gains in the visual quality of early scans. Omitting successive-approximation support from a decoder does not save nearly enough code to justify restricting JNG progressive encoders to spectral selection only.
No particular progressive scan sequence is specified or recommended by this specification. Not enough experience has been gained with progressive JPEG to warrant making such a recommendation. To allow for future experimentation with scan sequences, decoders are expected to handle any JPEG-legal sequence. Again, the code savings that might be had by making restrictive assumptions are too small to justify a limitation.
When the JSEP chunk is present, both images must be progressive if one of them is progressive.
JNG JPEGs may optionally use 12-bit sample precision as defined in JPEG Part 1.
For a sequential image, the SOF marker type must be SOF1 (extended sequential) not SOF0, and the baseline restriction of two Huffman tables is removed. Also, the encoder may use either 8-bit or 16-bit quantization tables. All other JNG baseline restrictions still apply. It is recommended that JNG encoders not use extended-sequential mode except to encode 12-bit data.
For a progressive image, the only difference between 8-bit and 12-bit modes is that the sample precision is 12 bits and the encoder may use either 8-bit or 16-bit quantization tables. All other JNG restrictions still apply.
This chunk is exactly like the IDAT chunk in a PNG grayscale
image, except that it is interpreted as an alpha mask to be applied to
the image data from the JDAT chunks,
when alpha_compression_method=0. The alpha channel, if
present, can have sample depths 1, 2, 4, 8, or 16.
The filter method can be any filter method that is defined for PNG datastreams that are embedded in MNG datastreams. Besides the filter methods defined in the PNG specification, MNG defines additional filter methods:
1: Implicit "none" filtering of every scanline;
filter bytes are omitted from the compressed
datastream in the IDAT chunk(s).
64: Adaptive filtering with five basic types,
intrapixel differencing, and leveling.
A set of level samples appears immediately
prior to the filter byte for the first
scanline in the IDAT chunk.
65: Intrapixel differencing and pixel leveling,
with implicit "none" scanline filtering.
For grayscale PNG datastreams, this means that when the filter method is
1 or 65, the filter byte is omitted from the beginning of each scanline,
and when it is 64 or 65, a single "level" sample appears as the
first byte (or first two bytes, if the sample depth is 16) of the compressed
data; this value is subtracted from each pixel (modulo the sample depth) when
encoding (prior to applying the basic filtering), and added to each pixel
(modulo the sample depth) when decoding (after undoing the basic filtering).
Because there is only one sample per pixel, there is no intrapixel differencing.
The IDAT
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
No other chunk
type can appear among the sequence of IDAT and JDAT
chunks. No other chunk type can appear between the sequences of
IDAT and JDAT chunks when they are not interleaved.
The samples in the IDAT must be presented in noninterlaced order, left
to right, top to bottom. As in PNG, zero means fully transparent and
2^alpha_sample_depth-1 means fully opaque.
The IDAT chunks must precede the JSEP chunk, if the JSEP chunk is present. Minimal viewers that ignore the twelve-bit JDAT chunks must read the IDAT chunks and apply the alpha samples to the eight-bit image that is contained in the JDAT chunks that precede the JSEP chunk. Viewers that skip the eight-bit JDAT chunks must decode the IDAT chunks that precede the JSEP chunk and apply the alpha samples to the twelve-bit image that is contained in the JDAT chunks that follow the JSEP chunk.
This chunk is exactly like the JDAT chunk in a non-progressive
JNG 8-bit grayscale image, except that it is interpreted as an alpha mask to
be applied to the image data from the JDAT chunks,
when alpha_compression_method=8. The alpha channel, if
present, can have only sample depth 8. The JDAA
chunks can be interleaved with the JDAT chunks
(see Recommendations for Encoders:
JNG interleaving below).
Like IDAT chunks, the JDAA chunks must precede the JSEP chunk, if the JSEP chunk is present, and are handled similarly.
The JNG IEND chunk is identical to its counterpart in PNG. Its data length is zero, and it serves to mark the end of the JNG datastream.
The JSEP chunk is empty.
A JSEP chunk must appear between the JDAT
chunks of an eight-bit datastream and those of a twelve-bit datastream, when
image_sample_depth=20 in the JHDR chunk. When
image_sample_depth != 20, the JSEP chunk must not
be present. The eight-bit
datastream must appear first. Both images must have the same width,
height, color type, compression method, and interlace method. Viewers can
choose to display one or the other image, but not both.
Some PNG ancillary chunks can also appear in JNG datastreams, and are used for the same purposes as described in the PNG specification [PNG] and the Extensions to the PNG Specification document [PNG-EXT].
If the bKGD chunk is present, it must be written as if it were written for a PNG datastream with sample_depth=8. It has one 2-byte entry for grayscale JNGs and three 2-byte entries for color JNGs. The first (most significant) byte of each entry must be 0.
The following chunks have exactly the same meaning and have the same syntax as given in the PNG specification: cHRM, gAMA, iCCP, sRGB, pHYs, oFFs, sCAL, iTXt, tEXt, tIME, and zTXt.
The PNG PLTE, hIST, pCAL, sBIT, sPLT, tRNS, fRAc, and gIF* chunks are not defined in JNG.
When cHRM, gAMA, iCCP, or sRGB are present, they provide information about the color space of the decoded JDAT image, and they have no effect on the decoded alpha samples from the IDAT or JDAA chunks. Any viewer that processes the gAMA chunk must also recognize and process the sRGB chunk. It can treat it as if it were a gAMA chunk containing the value .45455 and it can ignore its "intent" field.
The chunk copying and ordering rules for JNG are the same as those in PNG, except for the fact that the JDAT chunks and IDAT or JDAA chunks can be interleaved.
A Delta-PNG datastream describes a single image, by giving the changes from a previous PNG (Portable Network Graphics), a JNG (JPEG Network Graphics), or another Delta-PNG image.
No provision is made in this specification for storing a Delta-PNG datastream as a standalone file. A Delta-PNG datastream will normally be found as a component of a MNG datastream. Applications that need to store a Delta-PNG datastream separately should use a different file signature and filename extension, or they can wrap it in a MNG datastream consisting of the MNG signature, the MHDR chunk, a BASI chunk with the appropriate dimensions and an IEND chunk, the Delta-PNG datastream, and the MEND chunk.
The decoder must have available a parent (decoded) object that has an object buffer from which the original chunk data is known. The parent object can be the result of decoding a PNG, another Delta-PNG datastream, or it could have been generated by a PNG-like datastream introduced by a BASI chunk.
The child image is always of the same basic type (at present only PNG and JNG are defined) as the parent object. The child is always a viewable image even if the parent is not.
The decoder must not have modified the pixel data in the parent object by applying output transformations such as gAMA or cHRM, or by compositing the image against a background. Instead, the decoder must make available to the Delta-PNG decoder the unmodified pixel data along with the values for the gAMA, cHRM, and any other recognized chunks from the parent object datastream.
A Delta-PNG datastream consists of a DHDR and IEND
enclosing other optional chunks (if there are no other chunks, the
decoder simply copies the parent image, and displays it if its
do_not_show=0).
Chunk structure (length, name, CRC) and the chunk-naming system
are identical to those defined in the PNG specification. Definitions
of compression_method and interlace_method are
also the same as defined in the PNG specification. The definition
of filter_method is the same as for PNG datastreams that
are embedded in MNG datastreams
(see the IHDR chunk specification, above, Paragraph 4.2.3).
This section describes critical Delta-PNG chunks. MNG-compliant decoders must recognize and process them.
The DHDR chunk introduces a Delta-PNG datastream. Subsequent chunks, through the next IEND chunk, are interpreted according to the Delta-PNG format.
The DHDR chunk can contain 4, 12, or 20 bytes:
Object id: 2 bytes (nonzero unsigned integer).
Identifies the parent object from which
changes will be made. This is also the
object_id of the child image, which can
be used as the parent image for a
subsequent Delta-PNG.
Image type: 1 byte.
0: Image type is unspecified. An IHDR,
JHDR, IPNG, or IJNG chunk must be
present. If JHDR or IJNG is present,
delta_type must not be 1, 3, 4, or 6.
1: Image type is PNG. IHDR and IPNG can
be omitted under certain conditions.
2: Image type is JNG. JHDR and IJNG can
be omitted under certain conditions.
Delta_type must not be 1, 3, 4, or 6.
Delta type: 1 byte.
0: Entire image replacement.
1: Block pixel addition, by samples,
modulo 2^sample_depth.
2: Block alpha addition, by samples,
modulo 2^sample_depth. Regardless of
the color type of the parent image,
the IDAT data are written as a
grayscale image (color type 0), but
the decoded samples are used as deltas
to the alpha samples in the parent
image. The parent image must have
(or be promoted to via the PROM chunk)
a color type that has an alpha channel.
3: Block color addition. Similar to
delta type 1 except that only the
color channels are updated even when
the parent has an alpha channel.
4: Block pixel replacement.
5: Block alpha replacement.
6: Block color replacement.
7: No change to pixel data.
Block width: 4 bytes (unsigned integer). Omit when
delta_type=7.
Block height: 4 bytes (unsigned integer). Omit when
delta_type=7.
Block
X_location: 4 bytes (unsigned integer),
measured in pixels from the left edge
of the parent object. Omit when
delta_type=0 or when delta_type=7.
Block
Y_location: 4 bytes (unsigned integer),
measured in pixels from the top edge
of the parent object. Omit when
delta_type=0 or when delta_type=7.
The object_id must identify an existing object, and the
object must be a "concrete" object, i.e., it must have the property
concrete_flag=1.
The image_type, whether given explicitly as 1 or 2
or implied by the presence of an IHDR, IPNG,
JHDR, or IJNG chunk, must be the same as that of the
parent object.
When delta_type=0, the width and height of the child image
are given by the block_width and block_height
fields.
For all other values of delta_type, the width and height
of the child image are inherited from the parent object.
When delta_type=1-6, the block_width
and block_height fields give the size of the block of
pixels to be modified or replaced, and block_X_location and
block_Y_location give its location with respect to the top
left corner of the parent object. The block must fall entirely within
the parent object.
delta_type=0 in the DHDR chunk, the
pixel data in the IDAT chunks represent a completely new
image, with dimensions given by the block_width and
block_height fields of the DHDR chunk. Data from
chunks other than IDAT or JDAT can be inherited from
the parent object. If the IHDR or JHDR chunk is present,
all of its fields except width and height
(which must be ignored by decoders) provide
new values that are inherited by subsequent objects. The "pixel
sample depth" and "alpha sample depth" are also reset
equal to the
IHDR sample_depth value (in the case of a JNG object,
the new "alpha sample depth" is taken from the JHDR
alpha_sample_depth field). If the IHDR or JHDR
chunk is not present,
the IDAT chunks are decoded according to the parent object's
sample depth, and not according to the "pixel
sample depth" or "alpha sample depth" which are used for
decoding the IDAT chunks in subsequent Delta-PNG datastreams
when delta_type is nonzero.
delta_type=1 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the pixel data in
a parent object known to the decoder, including the alpha channel, if the
parent object has an alpha channel.
The IDAT chunk data contains a filtered and perhaps
interlaced set of delta pixel samples. The delta samples are presented
in the order specified by interlace_method, filtered
according to the filter_method and compressed according to
the compression_method given in the IHDR chunk.
The pixel data includes alpha
samples, if the parent object has an alpha channel.
An encoder calculates the delta sample values from the samples
in the
parent object and those in the child image by subtracting the parent
object samples from the child image samples, modulo 2^sample_depth.
When decoding the IDAT chunk, the child image bytes are
obtained by adding the delta bytes to the parent object bytes, modulo
2^sample_depth. This is similar in operation to the PNG SUB filter,
except that it works by samples instead of by bytes.
Only the pixels in the block defined by the block location and dimensions given in the DHDR chunk are changed. The size of the IDAT data must correspond exactly to this rectangle.
When the parent object has color_type=3, the
deltas are differences between index values, not between color samples.
The color type must match that of the parent,
except that
when the parent has PNG color_type=3, the delta can have
color_type=0, and vice versa, since the contents of
the IDAT chunks of either color type are indistinguishable.
If the
pixel_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth using the zero-fill method described
in the PNG specification, before performing the pixel addition.
When the IHDR chunk is present, the compression method, filter method, and interlace method need not be the same as those of the parent object. The new values are used in decoding the IDAT data, and the new values are inherited by the child object.
Whenever the sample depth differs from that of the parent object, the resulting object inherits the original value from the parent. The value from the IHDR chunk is only used for decoding the IDAT data in this and subsequent Delta-PNGs. Implicit in this is the requirement for decoders to remember in the object buffer not only the sample depth of the object but (separately) the "pixel sample depth" for use in decoding the IDAT chunks of subsequent Delta-PNG datastreams that do not contain their own IHDR chunk.
delta_type=2 in the DHDR chunk, the pixel
data in the IDAT chunks represent deltas from the alpha
data in a parent object known to the decoder. The color samples
are not changed, and the updated alpha samples are calculated in
the same manner as the updated pixel samples are calculated when
delta_type=1.
The color_type is 0 (grayscale), regardless of the
color_type of the parent object. The parent object must have
an alpha channel or must have been promoted to a type that has an alpha
channel. The compression method, filter method, and interlace method
need not be the same. If they are different, the child object inherits
the new values, and the new values will be used in decoding the data in
any subsequent IDAT chunks. Neither the parent object nor the
delta object can have alpha samples that were carried in JPEG-encoded
JDAA chunks.
The sample_depth value from the IHDR chunk is
interpreted as a new value of alpha_sample_depth and is
only used for decoding the IDAT data in this and subsequent
Delta-PNGs. Implicit in this is the requirement for decoders to remember
in the object buffer not only the sample depth of the object
but (separately) the alpha_sample_depth
for use in decoding the IDAT chunks in any subsequent Delta-PNG
datastreams.
If the
alpha_sample_depth
does not match the
object_sample_depth, the delta must be scaled to the
object_sample_depth,
using the zero-fill method described in the PNG specification, before
performing the pixel subtraction.
delta_type=3 is similar to delta_type=1
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=4 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values for the
pixel samples in the rectangle given by the block location and dimension
fields in the DHDR chunk, including the alpha channel, if the
parent object has an alpha channel.
If the pixel_sample_depth does not match
the object_sample_depth,
the pixel data must be linearly scaled to the object_sample_depth
before making the replacements. This can be accomplished by
using the left bit replication method described in the PNG specification,
or by the right shift method in the unlikely event that the
pixel_sample_depth is larger than
the object_sample_depth.
The color type must match that of the parent, except for the cases mentioned for delta type 1, above.
delta_type=5 in the DHDR chunk, the pixel
data in the IDAT chunks represent replacement values of the
alpha samples in the rectangle given by the block location and dimension
fields in the DHDR chunk. The sample depth of the
data (i.e. the "alpha sample depth") need not match the sample depth
of the parent object, and color_type is
0 (grayscale), regardless of the color_type of the parent
object.
If the sample depths differ, the samples must be scaled
to the object_sample_depth by linear scaling, which can be achieved
with the left bit replication method or right shift method described
in the PNG specification, depending on whether
the alpha_sample_depth is larger or smaller than
the object_sample_depth.
The parent object must have an alpha channel or must have been
promoted to a type that has an alpha channel. The compression method,
filter method, and interlace method need not be the same. If they
differ, the child object inherits the new values.
It is permitted to use JPEG-encoded JDAA chunks to convey the new alpha data. If this is done, then the alpha channel of the object can no longer be used as the parent for block-alpha-addition.
delta_type=6 is similar to delta_type=4
except that the alpha channel is not included in the IDAT
pixels; the alpha channel is inherited from the parent object.
The color type of the parent must be one that has an alpha channel
(4 or 6) and the color type of the delta must be the corresponding color type
(0 or 2) that does not have an alpha channel.
delta_type=7 in the DHDR chunk, there is
no change to the pixel data, and it is an error for
IDAT, or JDAT to appear. If the IHDR
or JHDR chunk appears, the width, height, and color_type fields are
ignored, the PNG sample depth
(or JNG alpha_sample_depth) is used to update the pixel_sample_depth
and alpha_sample_depth, and the data in the remaining fields
are inherited by the child object.
delta_type != 0. The decoder needs to remember the
pixel_sample_depth and alpha_sample_depth to use with each
object. They are initialized to the sample_depth value
from the IHDR chunk that appears when the object is first
created but can be changed by the appearance of the IHDR chunk in
a Delta-PNG datastream that has delta_type != 0.
If the object is a JNG image, they are initialized from the value
of alpha_sample_depth from
the original JHDR chunk, and can be changed by the appearance
of the JHDR chunk in a Delta-PNG datastream that has
delta_type != 0.
End of Delta-PNG datastream. An IEND chunk must be present for each DHDR chunk in a MNG datastream.
The IEND chunk is empty.
This chunk is used to "promote" a parent object to a higher bit depth or to add an alpha channel, before making changes to it.
New color type: 1 byte.
New sample depth: 1 byte.
Fill method: 1 byte.
0: Left-bit-replication
1: Zero fill
When a decoder encounters the PROM chunk, it must promote the pixel data. The cases are:
color_type 0 -> 4)
color_type 2 -> 6)
color_type 0 -> 2)
color_type 4 -> 6)
color_type 0 -> 6)
color_type 3 -> 2)
color_type 3 -> 6)
JNG color_type 8 -> 10)
JNG color_type 8 -> 12)
JNG color_type 8 -> 14)
JNG color_type 10 -> 14)
JNG color_type 12 -> 14)
color_type
If the sample depth has been changed, the sample values must be
widened. The decoder must use left-bit-replication or zero-fill
according to the specified fill_method to fill the additional
bits of each sample. If cheap transparency information is present in a
grayscale or truecolor object, its sample values must also be widened in
the same manner. If the image type is JNG, then the new sample depth
refers to the alpha_sample_depth and only the alpha channel
is affected, if one is present. If the color_type has been
promoted from indexed-color, the original bit depth is always considered to be
8. See the PNG specification [PNG]
for
further information on these filling methods. Any alpha channel added
in this manner is eligible to be updated by block-alpha-addition in this
or a subsequent Delta-PNG.
If the basis object contains data from the PNG bKGD chunk,
this data must be promoted as well. If a grayscale object is being
promoted to a truecolor object, the background RGB samples are set equal
to the grayscale background sample. If the bit depth has been changed,
the background samples are widened in accordance with the specified
fill_method. If the basis object is a JNG, the bKGD
chunk is not affected.
If the basis object contains data from the PNG sBIT chunk, this data must also be promoted. If a grayscale object is being promoted to a truecolor object, the new RGB bytes are set equal to the grayscale byte. When an alpha channel is added, the alpha byte is set equal to the sample depth of the basis image. If the sample depth has been changed, the sBIT bytes do not change.
The PROM chunk is not permitted to "demote" a parent object to an object with a lesser bit depth or from one with an alpha channel to one without an alpha channel.
The PROM chunk must appear ahead of the IHDR chunk, if IHDR is present, and ahead of any chunks that would have followed IHDR, if IHDR is omitted.
Inside a Delta-PNG datastream, the IHDR chunk introduces an incomplete PNG (Portable Network Graphics) datastream. The parent object must be a PNG or PNG-based Delta-PNG. The datastream can be introduced by a complete PNG IHDR chunk or by an IPNG chunk, which is empty.
If the IHDR chunk is present, its width,
and height
fields are ignored. The values for these parameters are inherited from
the parent object or from the PROM chunk.
different from those of the parent, they are only used for decoding
the data in the IDAT chunks, and the child object inherits
the original values from the parent.
The sample_depth, color_type,
compression_method, interlace_method, and
filter_method fields, if different from those of the parent
object, are used in decoding any subsequent IDAT chunks, and
the new values will be inherited by any subsequent image that uses this
object as its parent. These do not change the sample_depth
and color_type of the object itself; those can only be
changed by using the PROM chunk or by using delta_type=0.
See the PNG specification for the format of the PNG chunks. The PNG datastream must contain at least IHDR and IEND (whether actually present in the datastream or omitted and included by implication, as described below), but can inherit other chunk data from the parent object. Except for IDAT and PPLT, any chunks appearing between IHDR and IEND are always treated as replacements or additions and not as deltas.
The IPNG chunk is empty.
The IPNG chunk can be used instead of the IHDR
chunk if the IHDR chunk is not needed for resetting the
value of compression_method, filter_method, or
interlace_method. The purpose of this chunk is to identify
the beginning of the PNG datastream, so decoders can start interpreting
PNG chunks instead of Delta-PNG chunks. The decoder must treat this
datastream as though the IHDR chunk were present in the
location occupied by the IPNG chunk.
The IHDR chunk can also be omitted when
image_type=1 and the PNG datastream begins with a
PLTE chunk, a PPLT chunk, or an IDAT chunk.
In this case, no IPNG chunk is required, either. The decoder
must treat this datastream as though the IHDR chunk were
present, immediately preceding the first PNG chunk. If the first PNG
chunk is neither a PLTE chunk, a PPLT chunk, nor an
IDAT chunk, then either the IPNG or IHDR must
be present to introduce the PNG datastream.
If the PLTE chunk is present, it need not have the same
length as that inherited from the parent object, but it must contain
the complete palette needed in the child image. If it is shorter than
the palette of the parent object, decoders can discard the remaining
entries and the child image must not refer to them. Decoders can also
truncate any tRNS data inherited from an indexed-color parent
object. If the new palette is longer than the parent palette, and
a new tRNS chunk is not present in an indexed-color image,
the tRNS data must be extended with opaque entries. The new
palette must not be longer than the object's sample_depth
would allow, and must not have more than 256 entries.
When processing the tRNS chunk, if color_type=3
and PLTE is not supplied, then the number of allowable entries
is determined from the number of PLTE entries in the parent
object. A tRNS chunk appearing in a Delta-PNG datastream is
always treated as a complete replacement for the tRNS chunk
data in the parent object. All entries beyond those actually supplied
are overwritten with the "opaque" value (255).
If it is desired only to overwrite or add palette entries, the PPLT chunk can be used. This might be useful for palette-animation applications. This chunk can also be used to overwrite or add entries to the transparency (alpha) data from the parent's tRNS chunk.
The PPLT chunk contains a delta_type byte and one
or more groups of palette entries:
PPLT delta type: 1 byte.
0: Values are replacement RGB samples.
1: Values are delta RGB samples.
2: Values are replacement alpha
samples.
3: Values are delta alpha samples.
4: Values are replacement RGBA
samples.
5: Values are delta RGBA samples.
First index,
first group: 1 byte.
Last index,
first group: 1 byte.
First set of
samples: 1, 3, or 4 bytes.
...etc...
Last set of
samples: 1, 3, or 4 bytes.
First index,
second group: 1 byte.
...etc...
The last_index must be equal to or greater than
first_index. The groups are not required to appear in
ascending order. If any index of any group is beyond the end of the
parent object's palette, the palette and tRNS data must be
extended just as if a longer complete PLTE chunk had appeared.
If there are gaps in the resulting extended palette, the colors must be
filled with {0,0,0} and the alphas filled with 255. If alpha samples
are supplied (PPLT_delta_type > 1) and no tRNS
data is present in the parent object, a tRNS chunk must be
created in the child object as though a complete tRNS chunk
had appeared. The new palette must not be longer than the object's
sample_depth would allow.
When PPLT_delta_type=0, the values are replacements for
the existing samples in the palette.
When PPLT_delta_type=1, the values are added to the
existing samples (modulo 256) to obtain the new samples.
If the new entry is beyond the range of the original palette,
the values are simply appended, regardless of the contents of
PPLT_delta_type.
Inside a Delta-PNG datastream, the JHDR chunk introduces an incomplete JNG (JPEG Network Graphics) datastream. The parent object must be a JNG or JNG-based Delta-PNG. The datastream is introduced by a complete JNG JHDR chunk.
If the JHDR chunk is present, its width,
height, image_sample_depth,
image_color_type, image_filter_method, and
image_interlace_method fields are ignored. The values for these
parameters are inherited from the parent object.
The alpha_compression_method,
alpha_interlace_method, and alpha_filter_method fields,
if different from those of the parent object, are used in decoding any
subsequent IDAT chunks, and the new values will be inherited by
any subsequent image that uses this object as its parent. If the
alpha_sample_depth differs, it will be used in decoding the
IDAT chunk data of the Delta-PNG and subsequent Delta-PNG
datastreams; but the child object itself will retain the original sample depth,
and must also retain the "alpha sample depth" for use in decoding
subsequent Delta-PNG datastreams. The decoded alpha samples must be
scaled to the object's sample depth before the replacements or delta
calculations are done.
See the JNG specification above for the format of the JNG chunks. The PNG datastream must contain at least JHDR and IEND, but can inherit other chunk data from the parent object. Except for IDAT, any chunks appearing between IHDR and IEND are always treated as replacements or additions and not as deltas.
The IJNG chunk is empty.
The IJNG chunk can be used instead of the JHDR chunk if the JHDR chunk is not needed for resetting the value of any of the JHDR fields. The purpose of this chunk is to identify the beginning of the JNG datastream, so decoders can start interpreting JNG chunks instead of Delta-PNG chunks. The decoder must treat this datastream as though the JHDR chunk were present in the location occupied by the IJNG chunk.
The JHDR chunk can also be omitted when
image_type=2 and the JNG datastream begins with a
JDAT or JDAA chunk. Note: Be sure that the first
JDAT or JDAA chunk
precedes the first IDAT chunk. In this case, no IJNG
chunk is required, either. The decoder must treat this datastream as
though the JHDR chunk were present, immediately preceding
the first JDAT chunk. If the first JNG chunk is not a
JDAT or JDAA chunk, then either the IJNG
or JHDR must be present to introduce the JNG datastream.
All chunks in the parent object with the specified name are inhibited from being copied into the child image.
Chunk name: 4 bytes (ASCII text). etc.
If multiple names appear in the DROP chunk, it is shorthand for multiple DROP chunks.
Chunk name: 4 bytes (ASCII text).
Polarity: 1 byte (unsigned integer).
0: Only.
1: All-but.
Keywords (null-separated Latin-1 text strings).
The chunk name must be the name of a chunk whose data begins with a null-terminated text string. Some parent object chunks with the specified chunk name are inhibited from being copied into the child image. If polarity is <only>, then any parent chunk whose keyword appears in the keywords list is inhibited. If polarity is <all-but>, then any parent object chunk whose keyword does not appear in the keywords list is inhibited.
The format of the keyword is the same as that specified for the parent chunk. Comparisons of keywords in the parent chunk and the DBYK chunk are case sensitive.
Use caution when printing or displaying keywords (Refer to Security considerations, below, Chapter 16).
The ORDR chunk informs the applier of the Delta-PNG of the ordering restrictions for ancillary chunks. It contains one or more 5-byte sequences:
Chunk name: 4 bytes (ASCII text).
Order type: 1 byte.
0: Anywhere.
1: After IDAT and/or JDAT or JDAA.
2: Before IDAT and/or JDAT or JDAA.
3: Before IDAT but not before PLTE.
4: Before IDAT but not after PLTE.
etc.
Critical chunk names must not appear in the ORDR chunk. The applier needs to know everything about them anyway.
If a chunk name appears in the ORDR chunk, it is a promise that any chunk of that name appearing in the parent object which is not inhibited by DROP/DBYK will not be broken by this Delta-PNG, and therefore the applier must copy it into the child image at a location compatible with its ordering restrictions.
If any ancillary chunk appears in the parent object, and it is not inhibited, and its name does not appear in the ORDR chunk, then the applier should copy it into the child only if it knows the chunk well enough to be sure that it is consistent with the changes made by the Delta-PNG, and knows where it can be placed in the child. Those conditions are always true of safe-to-copy chunks.
If any critical chunk defined in neither this specification nor the PNG specification appears in the parent object or in the Delta-PNG, it is a fatal error unless the applier knows how to handle it. The specification of the critical chunk can include provisions for this scenario.
This section describes ancillary Delta-PNG chunks. MNG-compliant decoders should recognize and process them, but are not required to.
A gAMA, cHRM, iCCP, sRGB or similar chunk existing in the parent object would not affect the pixel data in a concrete object inherited by this Delta-PNG datastream because they are not used in decoding the pixel data. Applications are responsible for ensuring that the pixel values that are inherited from the parent object are the raw pixel data that existed prior to any transformations that were applied while displaying the parent image. These color transformations are applied to the resulting pixel data for display purposes.
MNG viewers must ignore oFFs and pHYs chunks that appear inside a PNG or JNG datastream or are inherited from the MNG top level. MNG editors are expected to treat them as if they were unknown copy-safe chunks.
Any other ancillary PNG chunks that the decoder recognizes when processing a PNG datastream should also be recognized and handled when processing a delta-PNG datastream. Any chunks that it does not recognize should be processed as instructed by the ORDR, DROP, and DBYK chunks. MNG viewers are free to ignore any ancillary chunks, while MNG editors should handle them in accordance with the copying rules.
The PNG specification places ordering requirements on many chunks with respect to the PLTE and IDAT chunks. If neither of these two chunks is present, and the ORDR chunk is not present, known chunks (always including all standard chunks described in the PNG specification) are considered to have appeared in their proper order with respect to the critical chunks. Unknown chunks are ordered as described in above (Paragraph 6.1.12).
The chunk copying rules for MNG are the same as those in PNG, except that a MNG editor is not permitted to move unknown chunks across any of the following chunks:
The copy-safe status of an unknown chunk is determined from the chunk name, just as in PNG. If bit 5 of the first byte of the name is 0 (Normally corresponding to an uppercase ASCII letter), the unknown chunk is critical and cannot be processed or copied. If it is 1 (usually corresponding to a lowercase ASCII letter), the unknown chunk is ancillary and its copy-safe status is determined by bit 5 of the fourth byte of the name, 0 meaning copy-unsafe and 1 meaning copy-safe.
If an editor makes changes to the MNG datastream that render unknown chunks unsafe-to-copy, this does not affect the copy-safe status of any chunks beyond the next SEEK chunk or prior to the previous one. However, if it makes such changes prior the SAVE chunk, this affects the copy-safe status of all top-level unknown chunks in the entire MNG datastream.
Changes to the MHDR chunk do not affect the copy-safe status of any other chunk.
The SAVE, SEEK, and TERM chunks are not considered to be a part of any segment. Changes to the data in the SAVE or SEEK chunks do not affect the copy-safe status of any other chunks. Adding or removing a SEEK chunk affects the copy-safe status of unknown chunks in the newly-merged or newly-separated segments. Adding, removing, or changing the TERM chunk has no effect on the copy-safe status of any chunk.
As in PNG, unsafe-to-copy ancillary chunks in the top-level MNG datastream can have ordering rules only with respect to critical chunks. Safe-to-copy ancillary chunks in the top-level MNG datastream can have ordering rules only with respect to the SAVE, SEEK, SHOW, and PAST chunks, DHDR-IEND, BASI-IEND, IHDR-IEND, JHDR-IEND sequences, or with respect to any other critical "header-end" sequence that might be defined in the future that could contain IDAT or similar chunks.
The copying rules for unknown chunks inside IHDR-IEND, BASI-IEND, DHDR-IEND, and JHDR-IEND sequences are governed by the PNG and JNG specifications, and any changes inside such sequences have no effect on the copy-safe status of any top-level MNG chunks.
The copy-safe status of chunks inside a DHDR-IEND sequence depends on the copy-safe status of the chunks in its parent object.
This section specifies the minimum level of support that is expected of MNG, MNG-LC, or MNG-VLC-compliant decoders, and provides recomendations for viewers that will support slightly more than the minimum requirements. All critical chunks must be recognized, but some of them can be ignored after they have been read and recognized. Ancillary chunks can be ignored, and do not even have to be recognized.
Anything less than this level of support requires subsetting. Applications that provide less than minimal MNG support should check the MHDR "simplicity profile" for the presence of features that they are unable to support or do not wish to support. A specific subset, in which "complex MNG features" and JNG are absent, is called "MNG-LC". In MNG-LC datastreams, bit 0 of the simplicity profile must be 1 and bits 2 and 4 must be 0. Another subset is called "MNG-VLC". In MNG-VLC datastreams, "simple MNG features" are also absent, and bit 1 must therefore also be 0.
Subsets are useable when the set of MNG datastreams to be processed is known to be (or is very likely to be) limited to the feature set in MNG-LC or MNG-VLC. Limiting the feature set in a widely-deployed WWW browser to anything less than MNG with 8-bit JNG support would be highly inappropriate.
Some subsets of MNG support are listed in the following table, more or less in increasing order of complexity.
Level of support MHDR Profile bits Profile
31 30-6 5 4 3 2 1 0 value
MNG-VLC without transparency 0 0 0 0 0 0 0 1 1
MNG-VLC 0 0 0 0 1 0 0 1 9
MNG-VLC with JNG 0 0 0 1 1 0 0 1 25
MNG-LC 0 0 0 0 1 0 1 1 11
MNG-LC with JNG 0 0 0 1 1 0 1 1 19
MNG without JNG and Delta-PNG 0 0 0 0 1 1 1 1 15
MNG without Delta-PNG 0 0 0 1 1 1 1 1 31
MNG without JNG 0 0 1 0 1 1 1 1 47
MNG 0 0 1 1 1 1 1 1 63 (or 0)
MNG with 12-bit JNG support 0 0 1 1 1 1 1 1 63 (or 0)
One reasonable path for an application developer to follow might be to develop and test the application at each of the following levels of support in turn:
An equally reasonable development path might be
On the other hand, a developer working on an application for storing multi-page fax documents might have no need for more than "MNG-VLC without transparency".
We are allowing conformant decoders to skip twelve-bit JNGs because those are likely to be rarely encountered and used only for special purposes. There is no profile flag to indicate the presence or absence of 12-bit JNGs.
ticks_per_second must be supported by animation viewers.
The simplicity profile, frame count, layer count, and nominal play time
can be ignored. Decoders that provide less than minimal support can use
the simplicity profile to identify datastreams that they are incapable
of processing.
iteration_count must be supported. The
nest_level should be used as a sanity check but is not
required. When iteration_min=1 either explicitly or
when it is omitted and termination_condition != 0, the
LOOP chunk
and its ENDL chunk can be ignored (bit 2 of the simplicity profile
can be used to promise that this is true for all loops).
framing_mode and clipping parameters must be
supported. The interframe_delay must be supported
except by single-frame viewers. The sync_id and
timeout data can be ignored.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported. Alpha can be ignored if bit 3 of the simplicity profile is
0.
Bit 4 of the simplicity profile can be used to promise that JNG chunks are not present. Viewers that choose not to support JNG can check this bit before deciding to proceed. MNG-compliant decoders must support JNG, but MNG-LC and MNG-VLC decoders are not required to support JNG.
color_type, bit_depth,
compression_method, filter_method and
interlace_method must be supported (interlacing, as in PNG,
need not necessarily be displayed on-the-fly; the image can be displayed
after it is fully decoded). The alpha-channel must be supported, at
least to the degree that fully opaque pixels are opaque and fully
transparent ones are transparent. It is recommended that alpha be fully
supported.
image_sample_depth=8 must be supported. The JSEP
chunk must be recognized and must be used by minimal decoders to select
the eight-bit version of the image, when both eight-bit and twelve-bit versions
are present, as indicated by image_sample_depth=20 in the
JHDR chunk. When image_sample_depth=12, minimal
decoders are not obligated to display anything. Such decoders can
choose to display nothing or an empty rectangle of the width and height
specified in the JHDR chunk.
This can be done by processing the JNG as though a
viewable transparent BASI object had appeared:
BASI width height 1 4 0 0 0 0 00 00 00 00 1 IEND
MNG-compliant decoders are required to support Delta-PNG, but MNG-LC and MNG-VLC decoders are not.
The following recommendations do not form a part of the specification.
It is a good idea to use a single color space for all of the layers in an animation, where speed and fluidity are more important than exact color rendition. This is best accomplished by defining a single color space at the top level of MNG, using and either an sRGB chunk or the gAMA and cHRM chunks and perhaps the iCCP chunk, and removing any color space chunks from the individual images after converting them to the common color space.
When the encoder converts all images to a single color space before putting them in the MNG datastream, this will allow decoders to improve the speed and consistency of the display.
For single-frame MNG datastreams, however, where decoding speed is less important and exact color rendition might be more important, it is best to leave the images in their original color space, as recommended in the PNG specification, to avoid any loss of data due to conversion, and to retain the individual color space chunks if the images have different color spaces.
Always use framing mode 1 or 2 when all of the images are opaque. This avoids unnecessary screen clearing, which can cause flickering.
If it
is necessary to establish a synchronization point immediately, this can
be done by using two consecutive FRAM chunks, the first setting
a temporary interframe_delay=0, timeout, and
sync_id, and the second establishing the synchronization
point:
FRAM 2 0 1 1 0 1 0000 timeout sync_id FRAM 0 name
Embedded images should not be enclosed in loops unless absolutely necessary. It is better to store them ahead of time and then use SHOW chunks inside the loops. Otherwise, decoders will be forced to repeatedly decode them. See Examples 2, 8, 11, and 12, below (Chapter 17).
Authors of MNG files that are intended for transmission over a network should consider whether it is more economical for the client to rebuild the index from scratch than it is to transmit it. Web pages that are likely to be downloaded over slow lines, and whose clients are unlikely to use the index anyway, generally should have empty SAVE chunks. No information is lost by deleting the index, because the MNG datastream contains all of the information needed to build the index. If an application does build an index, and the file is going to be kept as a local file, the application should replace the empty SAVE chunk with one containing the index. See above (Paragraph 4.4.1).
When a JNG datastream contains an alpha channel, and the file is intended for transmission over a network, it is useful to interleave the IDAT or JDAA and the JDAT chunks. In the case of sequential JPEG, the interleaving should be arranged so that the alpha data arrives more or less in sync with the color data for the scanlines. In the case of progressive JPEG, the alpha data should be interleaved with the first JPEG pass, so that all of the alpha data has arrived before beginning the second JPEG pass.
If a decoder reads an ENDL chunk for which the matching
LOOP chunk is missing, or has been skipped for some reason, any
active loops with a higher nest_level should be terminated,
and processing can resume after the next SEEK chunk. Simple
viewers that do not process the SAVE chunk should abandon the
MNG datastream. See above.
The PNG specification gives a good explanation of how to composite a partially transparent image over an opaque image, but things get more complicated when both images are partially transparent.
Pixels in PNG and JNG images are represented using gamma-encoded RGB (or gray) samples along with a linear alpha value. Alpha processing can only be performed on linear samples. This chapter assumes that R, G, B, and A values have all been converted to real numbers in the range [0..1], and that any gamma encoding has been undone.
For a top pixel {Rt,Gt,Bt,At} and a bottom pixel {Rb,Gb,Bb,Ab}, the composite pixel {Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab)
if (Ac != 0) then
s = At / Ac
t = (1 - At) Ab / Ac
else
s = 0.0
t = 1.0
endif
Rc = s Rt + t Rb
Gc = s Gt + t Gb
Bc = s Bt + t Bb
When the bottom pixel is fully opaque (Ab = 1.0), the function reduces to:
Ac = 1 Rc = At Rt + (1 - At) Rb Gc = At Gt + (1 - At) Gb Bc = At Bt + (1 - At) Bb
When the bottom pixel is not fully opaque, the function is much simpler if premultiplied alpha is used. A pixel that uses non-premultiplied alpha can be converted to premultiplied alpha by multiplying R, G, and B by A.
For a premultiplied top pixel {Rt,Gt,Bt,At} and a premultiplied bottom pixel {Rb,Gb,Bb,Ab}, the premultiplied composite pixel {Rc,Gc,Bc,Ac} is given by:
Ac = 1 - (1 - At)(1 - Ab) Rc = Rt + (1 - At) Rb Gc = Gt + (1 - At) Gb Bc = Bt + (1 - At) Bb
As mentioned in the PNG specification, the equations become much simpler when no pixel has an alpha value other than 0.0 or 1.0, and the RGB samples need not be linear in that case.
The decoder must retain information about each object (except for
objects with object_id=0) for possible redisplay with the
SHOW chunk or for possible use as the parent object for a
subsequent Delta-PNG datastream.
The following information must be retained, for each nonzero object that is defined and not subsequently discarded:
target_x and
target_y, if the object was the destination of a
PAST chunk.
When the encoder knows that data in the object buffer will not be
needed by subsequent frames, it can make life easier for decoders by
using object_id=0 or by using the DISC or the
SEEK chunk. Abstract images rather than concrete objects
should be used if the encoder knows that the data will not later be used
as the parent object for a Delta-PNG.
When a fatal error is encountered, such as a bad CRC or an unknown critical MNG chunk, minimal viewers that do not implement the SAVE/SEEK mechanism should simply abandon the MNG datastream. More capable MNG viewers should attempt to recover gracefully by abandoning processing of the segment and searching for a SEEK chunk. If such errors occur before the SAVE chunk is reached, the viewer should abandon the MNG datastream.
When an error occurs within a image datastream, such as an unknown critical PNG chunk or a missing parent object where one was required, only that image should be abandoned and the associated object should be discarded. If a bad CRC is found, indicating a corrupted datastream, the entire segment should be abandoned, as above.
MNG editors, on the other hand, should be more strict and reject any datastream with errors unless the user intervenes.
Decoders are required to be able to interpret datastreams that contain interlaced PNG images, but are only required to display the completed frames; they are not required to display the images as they evolve. Viewers that are decoding datastreams coming in over a slow communication link might want to do that, but MNG authors should not assume that the frames will be displayed in other than their final form.
When a PLTE chunk is received, it only affects the display of the PNG datastream that includes or inherits it. Decoders must take care that it does not retroactively affect anything that has already been decoded.
If PLTE or PPLT is present in a Delta-PNG datastream, the new palette is used in displaying the image defined by the Delta-PNG; if no IDAT chunk is present and the image type is PNG indexed-color, then the resulting image is displayed using the old pixel samples as indices into the new palette, which provides a "palette animation" capability.
If a frame contains two or more images, the PLTE chunk in
one image does not affect the display of the
other, unless one image
is a subsequent Delta-PNG without a PLTE chunk, that has been
declared by the DHDR object_id field to depend on the
other.
A composite frame consisting only of indexed-color images should not be assumed to contain 256 or fewer colors, since the individual palettes do not necessarily contain the same set of colors. Encoders can supply a top-level sPLT chunk with a suggested reduced global palette, to help decoders build an appropriate palette when necessary.
Viewers that can only display a single frame must display the first frame that they encounter. It is strongly recommended that such viewers understand the fPRI chunk above (Paragraph 4.5.2), which will enable them to find and display the best representative frame, if the encoder has identified one.
MNG provides four types of clipping, in addition to any clipping that might be required due to the physical limitations of the display device.
frame_width and frame_height are defined in
the MHDR chunk and cannot be changed by any other MNG chunk.
This is the only type of clipping available in MNG-VLC datastreams.
Decoders can use these parameters to establish the size of
a window in which to display the MNG frames. When the frame_width
or frame_height exceeds the physical dimensions of the
display hardware, the contents of the area outside those dimensions is
undefined. If a viewer chooses, it can create "scroll bars" or the
like, to enable persons to pan and scroll to the offscreen portion
of the frame. If this is done, then the viewer is responsible for
maintaining and updating the offscreen portion of the frame.
In the case of a MNG datastream that consists of a PNG or JNG
datastream, with the PNG or JNG signature, the frame_width
and frame_height are defined by the width and
height fields of the IHDR (or JHDR) chunk.
frame_width and frame_height)
will remain on the display from frame to frame without being explicitly
redisplayed.
See Example 8,
which displays a
large background image once, and then, in each frame, only redisplays
the portion of the background surrounding the moving sprite.
The clipping boundaries are expressed in pixels, measured rightward and downward from the frame origin.
The left and top clipping boundaries are inclusive and the right and bottom clipping boundaries are exclusive, i.e., the pixel located at {x,y} is only displayed if the pixel falls within the physical limits of the display hardware and all of the following are true:
0 <= x < frame_width (from the MHDR chunk) 0 <= y < frame_height Left_fb <= x < right_fb (from the FRAM chunk) Top_fb <= y < bottom_fb Left_cb <= x < right_cb (from the DEFI or CLIP chunk) Top_cb <= y < bottom_cb
Editors must recreate or delete the optional SAVE chunk index whenever they make any change that affects the offsets of chunks following the portion of the datastream that is changed. If the changes do not involve the addition, deletion, or relocation of segments, frames, and images, then it is sufficient to zero out the offsets.
The SAVE chunk is not considered to be in any MNG segment, so changing it has no effect on the copy-safe status of unknown chunks in any other part of the MNG datastream.
When the SAVE chunk is expanded to include an index, all chunks that follow will have their offsets changed by an amount equal to the change in the length of the data segment of the SAVE chunk, so the offset table will have to be adjusted accordingly. If a SAVE chunk is already present with zero offsets, the correct offsets can be written without adjustment.
Editors that create a series of PNG or JNG datastreams from a MNG datastream
should check the termination condition of any LOOP chunks and execute
loops only iteration_min times. The loop created by the TERM
chunk should be executed only once.
On systems where file names customarily include an extension
signifying file type, the extension .mng is recommended for
MNG (including MNG-LC and MNG-VLC)
files. Lowercase .mng is
preferred if file names are case-sensitive. The extension .jng is
recommended for JNG files.
When and if the MNG format becomes finalized, the
MNG authors intend to register video/mng
as the Internet Media Type for
MNG [RFC-2045],
[RFC-2048].
At the date of this document, the media
type registration process had not been started. It is recommended
that implementations also recognize the interim media type
video/x-mng.
Although we define a standalone JNG format, we recommend that such
files be used only temporarily while compiling or disassembling MNG
datastreams. We may at some future time register an Internet Media
Type for JNG files. Until then, the interim media type
image/x-jng can be used.
Segments, subframes, and objects are externally accessible via named SEEK, eXPI, and FRAM chunk names. They can be referred to by URI, as in
SRC=file.mng#segment_name SRC=file.mng#subframe_name SRC=file.mng#snapshot_name SRC=file.mng?segment_name#segment_name SRC=file.mng?snapshot_name#snapshot_name
When the URI specializer ("#" or "?") is "#", and the fragment identifier (the string following the specializer) is the name of a segment, i.e., a named SEEK chunk, the viewer should display the sequence from the beginning of the named segment up to the next segment. When it refers to a subframe or an image, i.e., a named FRAM or eXPI chunk, it should display the single frame (as it exists when the next FRAM chunk is encountered) or image that is identified by the fragment identifier. If the SAVE chunk is present and contains the optional index, this can help the client find the needed segment quickly.
When the URI specializer is "?" (server side query), the "query component" is the string following the "?" specializer and up to but not including the "#" if the "#" specializer is also present. The server should find the segment that is named in the query component or the segment that contains the subframe or image named in the query component, and it should return a datastream consisting of:
If no SAVE chunk is present, the server must simply return the entire MNG datastream. Servers that are unwilling to parse the MNG datastream and are unconcerned about bandwidth can return the entire MNG datastream even when the SAVE chunk is present. Authors should defend against this behavior by including both a query and a fragment in the URI even when a segment is being requested.
The client can process this as a complete MNG datastream, either displaying the entire segment, if no fragment identifier is present, or extracting the segment, frame or image that is named in a fragment identifier and displaying it, if a fragment identifier is present (a fragment identifier must be present if a frame or image is being requested). To "extract a frame" means to decode the returned datastream through the end of the frame that contains the named subframe and to display the result as a single still image. If the named subframe does not cover the entire frame, pixels from the background and from earlier subframes must be included in the resulting composition.
A part of the MNG datastream can also be requested by timecode, as in
SRC=file.mng#clock(10s-20s) SRC=file.mng#clock(0:00-0:15) SRC=file.mng?clock(0:00-0:15)#clock(0:00-0:15)
or by frame number, as in
SRC=file.mng#frame(10) SRC=file.mng#frames(30-60) SRC=file.mng?frames(30-60)#frames(30-60)
The timecode must consist of starting and ending clock values, as defined in the W3C SMIL recommendation, separated by a hyphen (ASCII code 45).
When the URI specializer is "#", the viewer should play that part of the sequence beginning and ending at the requested times, measuring from zero time at the beginning of the MNG datastream, or beginning and ending with the specified frame numbers. To do this it must start with the segment containing the requested time and decode any part of the segment up to that time, composing but not displaying the frames; this will provide the background against which the desired frames are displayed.
When the URI specializer is "?", the server can send the entire MNG datastream, or, preferably, it should construct a complete MNG file containing:
If the server does not send the entire MNG datastream, and the first segment after the SAVE chunk is not sent but a later segment is sent, the optional index must be written even if it does not exist in the source file. The index must contain at least one "type 0" entry that gives the nominal start time and frame number for the first segment that is sent after the SAVE chunk. The offset field can be set to zero and the segment name can be omitted.
The query component should always be repeated as a fragment identifier, so clients can find the requested item in case the server sends more than what was requested.
MNG datastreams should not contain segment, subframe, or image names that begin with the case-insensitive strings "CLOCK(", "FRAME(", or "FRAMES(", which are reserved for use in URI queries and fragments.
See [RFC-2396] and the W3C SMIL recommendation at http://www.w3.org/TR/.
This (incomplete as of version 0.97) section does not form a part of the specification. It provides the rationale behind some of the design decisions in MNG.
Filter method 64 could have been implemented as a new critical chunk in PNG or in MNG, e.g.
FILT type L0 L1 L2 L3in which L0-L3 are 16-bit values that convey the levels for the various color channels, type is one of the additional filter methods, and an empty FILT chunk turns off this type of filtering.
The choice of using a new filter method instead of a new critical chunk was made based on simplicity of implementation and possible eventual inclusion of this method in PNG.
We did not have any experimental evidence suggesting that it would
be useful to have a separate level set for each scanline. It
is possible that this would be effective for very wide images, however,
and more filter methods could be defined to achieve this.
We considered additional filter methods in which the level samples were implicitly zero, but this saves an insignificant amount of space (up to 8 bytes per PNG datastream) and the additional complexity is not warranted.
Explain why the interframe delay has to be provided before the subframe images are defined, instead of having a simpler DELA chunk that occurs in the stream where the delay is wanted.
Some delta types are not allowed when the parent object is a JNG image. Explain why types 4 and 6 (pixel replacement and color channel replacement) are not allowed under these circumstances.
Q. Why not just use a BASI chunk to encode solid-color rectangles?
A. The MAGN chunk also allows encoding of gradient-filled rectangles.
Q. Why not just use PNG to encode gradient-filled rectangles?
A. While PNG can encode vertical and horizontal gradients fairly
efficiently, it cannot do diagonal ones efficiently, and none
are as efficient as a 30-byte MAGN chunk plus a 4-pixel PNG.
Q. Why not use full-scale low-quality JPEG/JNG?
A. Low-quality JPEG with reduced dimensions can be much smaller than even
the lowest-quality full-sized JPEG. For example, color lena converted
to a 512x512 JPEG using ImageMagick software, quality 1, is 2944 bytes,
while converted to a 64x64 JPEG, quality 50, it's only 1171 bytes[*] and
looks less posterized when magnified 8 times using the linear interpolation
method. This is a compression ratio of 671:1. Similar results were
obtained with the greyscale lena (2217, 941 bytes[*], and 279:1,
respectively). [Show PSNR numbers] [*]including 21 bytes for a
"MAGN 00 00 2 08 07" chunk.
Such images can then be magnified to full scale with the MAGN
chunk, for
use as preview ("LOWSRC") images. For natural images, this has been
demonstrated to be about 40 to 50 times as efficient as using Adam7
interlacing. For example, the size increase of the color lena image due
to interlacing is 45460 bytes, compared to the 1171 bytes needed for a
reduced JNG in front of a noninterlaced color lena.
It appears that in general, usable preview images of truecolor
photographic images can be made at compression ratios from M*800:1 to
M*2500:1, where M is the number of megapixels in the original image, by
reducing the original image spatially to width and height in the range
64 to 200 pixels and then compressing the result to a medium-quality JNG.
Q. Why not use the pHYg chunk?
A. It is not mandatory for decoders to process the pHYg chunk
and it does not apply to individual images; it
is used to scale the entire MNG frame. The pHYs chunk cannot be
used either because MNG decoders are required to ignore it.
Q. Why not 4-byte magnification factors instead of 2-byte ones?
A. Encoders can start with a larger object or, except for
object 0, magnify it twice.
Q. Why not 1-byte magnification factors, then?
A. With typical screen widths currently 1280 or 1620 pixels and film and
printer pages currently about 3000 pixels wide, magnifying a 1x1 image
to a width of more than 255 pixels would not be uncommon.
Q. I want to magnify a "frozen" object.
A. You can make a full clone and magnify that.
Q. Why define Methods 3 and 4?
A. Method 4 is useful for magnifying an alpha-encoded image while maintaining
binary transparency. Method 3 is useful for making an alpha-gradient
while preserving sharp edges in the main image.
It has been suggested that the JNG alpha data should optionally be
transmitted as a grayscale JPEG. Why not?
We will be running some tests with a proposed JDAA chunk to
evaluate this suggestion. But we are reluctant to add new critical chunks
to the JNG specification now.
It has been suggested that a new global MNG chunk, JTAB,
be defined to hold global JPEG quantization and huffman tables that
could be inherited by JNG datastreams from which these have been
omitted. This has not been tested, and we are reluctant to add new
critical chunks to the MNG specification now.
Proposed 8 September 2000
Added filter method 65 for use in embedded PNG datastreams.
Eliminated filter methods 32 and 48. If leveling is used, the
level samples must always be present.
Allow the new filter methods (1, 64, 65) in JNG
alpha_filter_method.
Editorial changes in the LOOP chunk specification.
Changed "ASCII" to "character codes" in the
specification of the the sync field of the LOOP chunk.
Proposed 5 September 2000
Eliminated color types 18 and 22.
Added new filter methods for use in embedded PNG datastreams.
Proposed 2 September 2000
Added MAGN chunk.
Clarified wording about sRGB nullifying gAMA and
cHRM chunks, to make it clear that it is preceding
chunks that are nullified, not later ones.
Fixed typo in JNG signature bytes, to be the decimal equivalent
of "JNG" instead of "PNJ".
Added color types 18 and 22, to carry color samples that
have been transformed to improve compressibility.
Disallow the JSEP chunk when Proposed 29 August 2000
Relocated misplaced TERM chunk in Example 14.
Added a note that top-level color-space chunks do not have any effect
on already-decoded objects.
Changed "immediately after the MHDR chunk" to "very shortly
after the MHDR chunk" in the nEED chunk specification.
Clarified some wording in the SEEK chunk specification, and
added a cross reference to the existing requirement to insert a background
layer when making random access to a segment.
Proposed 27 August 2000
Global sRGB nullifies global gAMA and cHRM,
and vice versa.
This will be handled by informal consensus, i.e., by a Call for Objection.
Added a terminology entry for "nullify".
It is permitted to change the potential visibility,
location, and clipping boundaries of "frozen" objects, provided
that the encoder writes chunks to restore them to their "frozen"
values prior to the end of the segment.
Clarified treatment of the alpha sample in the BASI chunk
when the color type is 0, 2, or 3, and clarified that the BASI
chunk inherits default DEFI values if no DEFI chunk
is present.
Proposed 22 August 2000
Added JPEG-encoded alpha channel in JNG and Delta-PNG datastreams,
stored in a new JDAA chunk. This requires a formal vote.
The meaning of the transparency flag (bit 2 of the simplicity profile)
was restored to the meaning that it had in version 0.97.
A "semitransparency" flag was added to the simplicity profile.
This will be included in the vote on the "background transparency"
flag.
Revised the MNG-LC and MNG-VLC definitions to include the possibility of
the two new transparency flags and the new validity flag being set.
The discussion of handling a changing background scene was revised.
URLs were changed to "libpng.org".
Instances of "mpng-list" were updated to "mng-list".
Proposed 21 August 2000
Changed "Repeat count" to "Iteration count" in
the LOOP chunk specification, and "times to
repeat" to "times to execute" in the description of
the "Iteration max" field in the TERM chunk, and
added a statement about representing infinity. This is a minor
proposal that will be handled by informal consensus.
Separates the "transparency" profile bit
into "transparency"
and "background transparency", and adds discussion
of "background
transparency" to the BACK and FRAM chunk specifications. This
proposal requires a formal vote.
Adds a "stored objects" flag to promise that even
of "complex MNG features" are present, it is not necessary
to create object buffers. This proposal requires a formal vote.
Adds a "validity" flag to maintain backward compatibility
of the simplicity profile. If it is zero, then the "background
transparency" and "stored objects" flags do not
make any promises.
Released 28 February 2000.
Minor editorial changes only.
A new example was added.
Released 18 July 1999.
The changes that are not simple editorial
changes were approved by votes of the
PNG Development group that closed 16 July 1999 (pHYg
and change to treatment of the pHYs chunk), 14 July 1999 (global
bKGD and sBIT) and 25 June 1999 (change to LOOP
chunk and treatment of the DEFI chunk and nonviewable objects).
Initial public release, approved by the PNG Development Group
on 11 May 1999.
See also
Pennebaker, William B., and Joan L. Mitchell,
"JPEG : Still Image Data Compression Standard"
Van Nostrand Reinhold, ISBN:0442012721, September 1992
See also the PNG-1.2 draft:
Security considerations are addressed in the PNG specification.
An infinite or just overly long loop could give the appearance
of having locked up the machine, as could an unreasonably long
interframe delay or a misplaced Some people may experience epileptic seizures when they are exposed
to certain kinds of flashing lights or patterns that are common in
everyday life. This can happen even if the person has never had any
epileptic seizures. All graphics software and file formats that
support animation and/or color cycling make it possible to encode
effects that may induce an epileptic seizure in these individuals.
It is the responsibility of authors and software publishers to issue
appropriate warnings to the public in general and to animation creators
in particular.
No known additional security concerns are raised by this format.
We use the "#" character to denote commentary in these
examples; such comments are not present in actual MNG datastreams.
The simplest MNG datastream is a single-image PNG datastream. The
simplest way to create a MNG from a PNG is:
The resulting MNG file looks like:
If file.png contains an sRGB chunk and
also gAMA and cHRM chunks that are recommended in the
PNG specification for "fallback" purposes, you can remove
those gAMA and cHRM chunks from file.mng
because any MNG viewer that processes the gAMA chunk is also
required to recognize and process the sRGB chunk, so those
chunks will always be ignored. Any MNG editor that converts the MNG file
back to a PNG file is supposed to insert the recommended gAMA
and cHRM chunks.
This example demonstrates a very simple movie, such as might result
from directly converting an animated GIF that contains a simple series
of full-frame images:
This slideshow gives exactly the same output as Example 3, but the
storage in the datastream is more efficient (the IDAT chunks will be
smaller) while the memory requirements in the decoder are larger. Image
ID 1 is used to store the ornate logos and frame design that appear on
every slide. The DHDR-IEND datastreams only contain deltas due to the
text and other information that is unique to each slide.
This movie is still fairly simple, but it capitalizes on
frame-to-frame similarities by use of Delta-PNG datastreams, and also
demonstrates the use of the fPRI chunk.
Here is an example single-composite-frame MNG, with thumbnails, which
takes a grayscale image and draws it side-by-side with a false-color
version of the same image:
Here is another movie, illustrating the use of Delta-PNG datastreams
as sprites:
This movie illustrates the use of several abstract images with
Show_mode=6 to describe an animated sprite, and the PAST chunk to turn
it around. The sprite runs back and forth across the background ten
times. The FRAM clipping boundaries restrict the screen updates to the
small region that changes, with a little "wiggle room" to make sure the
disturbed part of the background gets updated.
The opaque parts of this image will "fade in" gradually. This
example also illustrates the use of the PPLT and fPRI
chunks.
In this example, we store a series of twenty-four 150 x 150 x 150
blocks of eight-bit voxels. Each block is stored as a composite frame with
the first image being a PNG whose pixels represent the top layer of
voxels, which is followed by 149 Delta-PNG images representing the rest
of the layers of voxels. Only one image is defined, through which the
parent image is passed along from PNG to Delta-PNG to Delta-PNG. This
example also illustrates the use of unregistered ancillary chunks that
describe the x, y, and z scales and pixel calibration.
Here is another composite frame, illustrating the use of the LOOP
syntax to tile a large (1024 by 768) image area with a small (128 by 64)
image.
Here is a better approach, which creates a reusable tiled image by
means of the PAST chunk.
Here is an example of scrolling a 3000-line-high image (perhaps an
image of some text, but could be anything) through a 256-line-high
window with an alpha-blended border.
Alternatively, we can declare the scrolling object to be the
background and use framing-mode 3:
This demonstrates the use of the SHOW chunk with
This will cycle through the ten objects in the forward direction,
100 times, unless terminated sooner by the user or the decoder.
This will cycle through the ten objects, back and forth,
50 times, unless terminated sooner by the user or the decoder.
Outline of a program to convert GIF animations to MNG format:
Where "<image>" represents a PNG or Delta-PNG containing a GIF
frame converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
Outline of a program to convert simple GIF animations that do not
use the "restore-to-previous" disposal method to "simple" MNG
(or "MNG-LC") format:
Where "<image>" represents a PNG datastream containing a GIF
frame that has been converted to PNG format.
Caution: if you write such a program, you might have to pay royalties
in order to convey it to anyone else.
This demonstrates the determination of the layer-count and frame-count
that should be written in the MHDR chunk.
For framing_modes 1 and 2, the FRAM chunks themselves
don't generate layers. For framing_modes 3 and 4, they do
generate layers ("B" for background), and also generate frames if
there is no embedded image with which to combine the background layer.
Note that every framing_mode creates a "B" layer at the beginning.
Given the following chunk stream:
in which
This table shows the layer count and frame count for each of
the four possible values of framing-mode:
Here is an example of storing a library of icons in a
MNG-LC datastream.
All of the icons use the same palette, transparency, and colorspace,
so these are put in global chunks at the beginning. Empty PLTE
chunks in the embedded images are used to import the global palette and
transparency data.
This is similar, but it uses Delta PNG datastreams to create modified
versions by replacing the palette. This can be more storage-efficient,
but requires a full MNG decoder because of the presence of Delta PNG
datastreams.
This example demonstrates the use of MNG to display Region of Interest (ROI)
at a higher quality than the rest of the frame, and the MAGN chunk to convey
a highly-compressed but very lossy image, a drop shadow, and a diagonal
gradient background.
For the particular image used in this example (the 512x512 color Lena
from Bragzone [URL]), the resulting 600x600 frame occupies about 2.6
times the file size when written as a simple JNG and about 26 times the
file size when written as a simple PNG.
Contributors' names are presented in alphabetical order:
This document was built from the file mng-master-20000908
on 08 September 2000.
Copyright © 1998, 1999, 2000 by: Glenn Randers-Pehrson
This specification is being provided by the copyright holder
under the following license. By obtaining, using and/or copying this
specification, you agree that you have read, understood, and will comply
with the following terms and conditions:
Permission to use, copy, and distribute this specification for any
purpose and without fee or royalty is hereby granted, provided that the
full text of this NOTICE appears on ALL copies
of the specification or portions thereof, including modifications, that
you make.
THIS SPECIFICATION IS PROVIDED "AS IS," AND
COPYRIGHT HOLDER
MAKES NO REPRESENTATIONS OR WARRANTIES, EXPRESS OR IMPLIED. BY WAY OF
EXAMPLE, BUT NOT LIMITATION, COPYRIGHT HOLDERS MAKE NO REPRESENTATIONS
OR WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PARTICULAR PURPOSE
OR THAT THE USE OF THE SPECIFICATION WILL NOT INFRINGE ANY THIRD PARTY
PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER RIGHTS. COPYRIGHT HOLDER WILL
BEAR NO LIABILITY FOR ANY USE OF THIS SPECIFICATION.
The name and trademarks of copyright holder may NOT be
used in advertising or publicity pertaining to the specification
without specific, written prior permission. Title to copyright in this
specification and any associated documentation will at all times remain
with copyright holder.
JNG alpha channel
Global JPEG tables
14. Revision History
14.1. Version 0.97g
14.2. Version 0.97f
14.3. Version 0.97e
image_sample_depth != 20
14.4. Version 0.97d
14.5. Version 0.97c
14.6. Version 0.97b
14.7. Version 0.97a
14.8. Version 0.97
14.9. Version 0.96
JDAT_sample_depth" to
"image_sample_depth" and
"IDAT_sample_depth" to
"alpha_sample_depth", etc.
14.10. Version 0.95
15. References
ftp://ftp.isi.edu/in-notes/rfc2083.txt
also available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
This
specification has also been published as a W3C Recommendation, which is
available at
http://www.w3.org/TR/REC-png.html.
Randers-Pehrson, G., et. al., "PNG (Portable Network Graphics
Format) Version 1.2", which is available at
ftp://swrinde.nde.swri.edu/pub/png/documents/.
ftp://swrinde.nde.swri.edu/pub/png/documents/pngext-*.
ftp://ftp.isi.edu/in-notes/rfc2045.txt
ftp://ftp.isi.edu/in-notes/rfc2048.txt
ftp://ftp.isi.edu/in-notes/rfc2396.txt
16. Security Considerations
sync_id with a long
timeout value. Therefore a decoder should always
provide a simple method for users to escape out of a loop or delay,
either by abandoning the MNG entirely or just proceeding to the next
SEEK chunk. (The SEEK chunk makes it safe for a
viewer to resume processing after it encounters a corrupted portion of a
MNG datastream or jumps out of the interior of a segment for any reason.)
17. Appendix: Examples
17.1. Example 1: A single image
copy file.png file.mng
\211 P N G \r \n ^z \n # PNG signature.
sRGB 2
IHDR 720 468 8 0 0 0 0 # Width and Height, etc.
IDAT ...
IEND
17.2. Example 2: A very simple movie
\212 M N G \r \n ^z \n # MNG signature.
MHDR 256 300 # Width and height.
1 # 1 tick per second.
5 4 4 # Layers, frames, play time
7 # Simplicity profile
DEFI 1 0 0 IHDR ... IDAT ... IEND # Four PNG datastreams
DEFI 2 0 0 IHDR ... IDAT ... IEND # are read and stored
DEFI 3 0 0 IHDR ... IDAT ... IEND # and are displayed as
DEFI 4 0 0 IHDR ... IDAT ... IEND # they are read.
SAVE # This is needed so we can place TERM before SEEK.
TERM 3 0 120 10 # When done, repeat from TERM 10 times.
SEEK
SHOW
MEND
17.3. Example 3: A simple slideshow
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 1 # Width and height, 1 tick per second.
6 5 5 # Layers, frames, play time.
1 # Simplicity profile (MNG-VLC)
FRAM 1 0 2 2 0 2 1 600 0 # Set interframe_delay to 1,
# timeout to 600 sec, and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
IHDR ... IDAT ... IEND # DEFI 0, visible, abstract
SEEK "Outline" # is implied.
IHDR ... IDAT ... IEND
SEEK "Our Vision" IHDR ... IDAT ... IEND
SEEK "Our Mission" IHDR ... IDAT ... IEND
SEEK "Downsizing Plans" IHDR ... IDAT ... IEND
MEND
17.4. Example 4: A more storage-efficient slideshow
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
1 6 5 5 15 # 1 tick per second, complex, no JNG.
DEFI 1 1 1 # Define image 1, invisible, concrete.
IHDR ... IDAT ... IEND
FRAM 1 0 2 2 0 2 1 600 0 # set interframe_delay to 1,
# timeout to 600 sec and sync_id list to {0}.
SAVE
SEEK "Briefing to the Workforce"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Outline"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Vision"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Our Mission"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
SEEK "Downsizing Plans"
CLON 1 2 DHDR 2 ... IDAT ... IEND SHOW 2
MEND
17.5. Example 5: A simple movie
\212 M N G \r \n ^z \n # MNG signature.
MHDR 720 468 # Width and height.
30 6 5 15 # 30 ticks per second.
47 # Delta-PNG, transparent, complex
tEXtTitle\0Sample Movie
fPRI 0 128 # Default frame priority is "medium".
FRAM 1 0 2 0 0 0 3 # Set interframe_delay to 1/10 sec.
DEFI 1 0 1 # Set default image to 1 (concrete).
SAVE
SEEK "start"
IHDR 720 468 8 2 0 0 0 # DEFI 1 is implied.
IDAT ...
IEND
DHDR 1 1 1 20 30 100 220 # A PNG-delta frame.
IDAT ... # The IDAT gives the 20x30 block
IEND # of deltas.
DHDR 1 1 1 20 30 102 222 # Another PNG-delta frame.
IDAT ... # This time the deltas are in a 20 x 30
IEND # block at a slightly different location.
SEEK "frame 3" # OK to restart here because a
# complete PNG frame follows.
fPRI 0 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 0 128 # Return to medium frame priority.
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame.
IDAT ... # The entire 720x468 rectangle changes
IEND # this time.
SEEK "end"
MEND # End of MNG datastream.
17.6. Example 6: A single composite frame
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 512 0 # Width, height, ticks per second
4 1 0 47 # Layers, frames, time, simplicity
BACK 16448 16448 52800 1 # Must use sky blue background.
PLTE ... # Define global PLTE
gAMA 50000 # Define global gAMA
DEFI 1 1 # Define invisible abstract thumbnail image.
IHDR 64 64 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 1"
DEFI 1 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0 PLTE IDAT ... IEND # use global PLTE
eXPI 1 "thumbnail 2"
DISC # Discard the thumbnail image.
FRAM 4 "Two views of the data"
DEFI 1 0 1 6 6 # Define first (bottom) image.
IHDR 500 500 16 0 .. # A 16-bit graylevel image.
IDAT ...
IEND # End of image.
CLON 1 2 0 1 0 0 518 6 # Make full invisible concrete clone.
SHOW 2 2 3 # Mark it for immediate display during
# the upcoming delta-PNG operation.
DHDR 2 1 7 # Modify it (no change to pixels).
ORDR faLT 2 # Establish chunk placement.
gAMA 100000 # Local gamma value is 100000 (gamma=1.0).
tEXtComment\0The faLT chunk is described in ftp://swrinde...
faLT ... # Apply pseudocolor to parent image.
IEND # End of image.
DEFI 3 0 0 900 400 # Overlay near lower right-hand corner.
IHDR 101 101 2 3 ...
PLTE ... # Use a local PLTE and global gAMA.
tRNS ... # It is transparent (maybe a logo).
IDAT ... # Note that the color type can differ
IDAT ... # from that of the other images.
IEND # End of image.
MEND # End of MNG datastream.
17.7. Example 7: A movie with sprites
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 47 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame
# sets interframe_delay=3 ticks.
DEFI 1 # Define image 1 (abstract, LOCA 0 0).
IHDR 512 512 ... # It is a full-display PNG image.
etc # Chunks according to PNG spec.
IEND # SHOW 1 is implied by DEFI 1.
DEFI 2 0 1 300 200 # Define image 2, concrete.
IHDR 32 32 ... # It is a small PNG.
gAMA 50000
IDAT ...
IEND
FRAM 0 "frame 2" # Start new frame.
# New location for image 1 is still 0,0.
SHOW 1 # Display image 1 from previous frame.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
SHOW 2 # Retrieve image 2 from previous frame,
CLON 2 3 0 1 0 # make a full clone of it as image 3.
0 400 500 # Location for image 3.
DHDR 3 1 7 0 0 0 0 # Modify image 3 (no change to pixels).
tRNS ... # Make it semitransparent.
IEND # SHOW 3 is implied by CLON visibility.
FRAM 0 "frame 3" # Next frame (repeat this FRAM-SHOW 1 3
# sequence with different locations to
# move the images around).
# New location for image 1 is still 0,0.
MOVE 2 2 1 10 5 # New (delta) location for image 2.
MOVE 3 3 1 5 -2 # New location for image 3.
SHOW 1 3 # Show images 1 through 3.
FRAM 0 "frame 4" # Another frame.
etc.
FRAM 0 "frame 99"
etc. # More frames.
MEND # End of MNG datastream.
17.8. Example 8: A movie with an animated sprite
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 512 30 0 0 0 15 # Start of MNG datastream.
FRAM 2 "frame 1" 0 2 0 0 0 3 # First frame.
DEFI 1 IHDR 512 512 ... # Background PNG image.
etc ... IEND # Chunks according to PNG spec.
DEFI 10 1 0 x0 y0 # Static part of sprite.
IHDR 64 64 ... IDAT ... IEND
DEFI 11 1 0 x0 y1 # View 1 of animated part.
IHDR 64 32 ... IDAT ... IEND # (y1=y0+64)
DEFI 12 1 0 x0 y1 # View 2 of animated part.
IHDR 64 32 ... IDAT ... IEND
DEFI 13 1 0 x0 y1 # View 3 of animated part.
IHDR 64 32 ... IDAT ... IEND
FRAM 0 0 0 0 2 0 0 x0-dx x0+64+dx y0-dy y1+32+dy
LOOP 0 0 10
LOOP 1 0 150
FRAM 0 "left-to-right" 0 0 2 0 1 dx dx dy dy
MOVE 10 13 1 dx dy # Move animated icon {dx, dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
FRAM SHOW 1
PAST 10 0 0 0 10 1 4 0 0 0 0 0 64 64
PAST 11 0 0 0 11 1 4 0 0 0 0 0 64 32
PAST 12 0 0 0 12 1 4 0 0 0 0 0 64 32
PAST 13 0 0 0 13 1 4 0 0 0 0 0 64 32
LOOP 1 0 150
FRAM 0 "right-to-left" 0 0 2 0 1 -dx -dx -dy -dy
MOVE 10 13 1 -dx -dy # Move animated icon {-dx, -dy}.
SHOW 1 SHOW 10 # Show background and static part.
SHOW 11 13 6 # Select the next view of the
ENDL 1 # animated part and show it.
ENDL 0 FRAM
MEND
17.9. Example 9: "Fading in" a transparent image
\212 M N G \r \n ^z \n # MNG signature.
MHDR 64 64 30 0 0 0 47 # Width, height, ticklength, ....
BACK 52800 52800 52800 # "Browser gray" default background.
FRAM 3 0 2 0 0 0 3 # Set interframe_delay=3 ticks. Use
# framing mode 3 so background gets restored.
DEFI 1 1 1 # Invisible and "concrete".
IHDR ... # PNG header.
PLTE ...
tRNS 0 # Entries are zero for the transparent (0)
# color and 255 for the nontransparent ones.
IDAT ...
IEND
fPRI 0 0 # Give the fade-in sequence a low priority.
CLON 1 2 # Make a working concrete copy of the image
# that will be modified during the low-priority
# part of the datastream. It is a full clone.
DHDR 2 1 7 # No change to pixel data.
tRNS 0 0 0 0 0 0 ... # Make all pixels fully transparent.
IEND
SHOW 2 2 3 # Make it visible but don't show it now.
LOOP 0 0 15
DHDR 2 1 7 # A Delta-PNG.
# Delta-type 7 means no change to pixels.
PPLT 1 10 3 16 16 16 16 ... # Increment all alphas except
IEND # for entry 0 by 16.
SHOW 2
ENDL 0 # Nontransparent pixel alpha=15, 31, ... 240.
DISC 2 # Discard the working copy.
fPRI 0 255 # Give the final frame the highest value
FRAM 0 0 1 0 0 0 60 # Hold the last frame for at least
# 60 ticks (2 sec). Applications might show it longer.
SHOW 1 # This copy still has alpha=255 for the
# opaque pixels and alpha=0 for the others.
MEND # End of MNG.
17.10. Example 10: Storing three-dimensional images
\212 M N G \r \n ^z \n # MNG signature.
MHDR 150 150 1 # Width, height, ticklength.
0 0 0 47 # Layers, frames, time, simplicity.
tEXtTitle\0Weather modeling results
tEXtComment\0The xxSC, yySC, zzSC, and ttSC chunks
in this file are written according to the Proposed
chunk specifications version 19970203 and Sci-Vis
chunks specification version 19970203 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xxSC kch\0 [sig\0] kilometers\0 0\0 150
yySC kch\0 [sig\0] kilometers\0 0\0 150
zzSC kch\0 [sig\0] Height (kilometers)\0 0\0 15
ttSC kch\0 [sig\0] Time (hours)\0 0\0 24
pCAL kch\0 0 255 0 2 Degrees Celsius\0 0\0 45
DEFI 1 0 1 # All images will have image = 1
SAVE # and be visible and "concrete".
SEEK
FRAM 2 # Initial composite image.
IHDR 150 150 16 # Width, height, bit depth for top layer.
0 0 0 0 # Color, comp, filter, interlace.
IDAT ...
IEND # No DEFI chunk, so it is image 0.
DHDR 1 1 0 # Source=0, PNG, pixel addition,
150 150 0 0 # Block is entire image.
IDAT ... # IHDR is omitted; everything matches top.
IEND # IEND is also omitted.
etc. # Repeat DHDR through IEND 148 more times.
SEEK
FRAM # End of first block.
etc. # Repeat FRAM through SEEK 19 more times.
SEEK
MEND # End of MNG.
17.11. Example 11: Tiling
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
98 1 0 15 # Layers, frames, time, simplicity.
FRAM 2
DEFI 1 1 0 0 -64 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
LOOP 0 0 12 # Y loop -- make 12 rows of tiles.
MOVE 1 1 1 0 64 # Move the first copy down 64 rows.
SHOW 1 # Display it.
CLON 1 2 1 # Create a partial clone of the tile.
LOOP 1 0 7 # X loop - 7 additional columns.
MOVE 2 2 1 0 128 # Move it to the right 128 columns.
SHOW 2 # Use the second copy.
ENDL 1
ENDL 0
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 1024 768 0 # Start of MNG datastream.
3 1 0 15 # Layers, frames, time, simplicity.
DEFI 1 1 # Set up an offscreen "abstract" copy
IHDR 128 64 ... PLTE ... IDAT ... IEND # of the tile.
DEFI 2 # The abstract, visible, viewable image to
BASI 1024 768 8 2 0 0 0 0 0 0 0 1 # be tiled. Initially
IEND # all pixels are zero.
PAST 2 0 0 0 # Destination and target location.
# src mod orient offset clipping
1 0 8 0 0 512 0 0 1024 0 768
# End of PAST chunk data.
MEND
17.12. Example 12: Scrolling
\212 M N G \r \n ^z \n # MNG signature.
MHDR 512 256 30 # Width, height, ticks per second
6513 3257 3257 15 # Layers, frames, time, simplicity.
BACK 50000 50000 50000 0 # advisory gray background
DEFI 1 1 0 0 256 # Define image 1 but don't display now.
# Initially it is offscreen, just
# below the 512 by 256 window.
IHDR 512 3000 1 0 ... # A PNG datastream containing the
PLTE ... # text (or whatever) to be scrolled.
IDAT ...
IEND
DEFI 2
IHDR 512 256 8 6 ... # A PNG datastream containing some kind
PLTE ... # of alpha-blended border that is
tRNS ... # transparent in the center.
IDAT ...
IEND
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack image 1 up one scanline, 3256 times.
# It ends up just above the 512 by 256 window.
# The border does not move.
FRAM 1 0 2 0 0 0 0 # Interframe_delay = 0 ticks.
# We use Framing_mode=1 to avoid unnecessary
# screen clearing between frames.
SHOW 1 # Show first image and continue without delay.
FRAM 1 0 2 0 0 0 1 # Interframe_delay = 1 tick.
SHOW 2 # Composite second image over first, wait 1 tick.
ENDL 0
MEND
(Same as above down to the LOOP chunk.)
BACK 50000 50000 50000 2 1 # Advisory gray background.
# Mandatory image background.
FRAM 3 0 2 0 0 0 1 # Interframe_delay = 1 tick.
LOOP 0 0 3256
MOVE 1 1 1 0 -1 # Jack background up one scanline, 3256 times.
SHOW 2 # Composite the second image over it, wait 1 tick.
ENDL 0
MEND
17.13. Example 13: Cycling animations
show_mode=6 to create animations that cycle through
a series of ten objects.
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 30 # Width, height, ticks per second
11 1001 1001 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
LOOP 0 100 6 # 100 iterations, user-discretion, cacheable
SHOW 1 10 6
ENDL 0
MEND
\212 M N G \r \n ^z \n # MNG signature.
MHDR 400 88 6 # Width, height, ticks per second
11 901 901 7 # Layers, frames, time, simplicity.
DEFI 1 ...
etc. # Define 10 objects.
DEFI 10 ...
CLON 11 9 1 # Make partial clones of objects 2-9
etc. # in reverse order, as objects 11-18.
CLON 18 2 1
LOOP 0 50 6 # 50 iterations, user-discretion, cacheable
SHOW 1 18 6
ENDL 0
MEND
17.14. Example 14: Converting a GIF animation
begin
write "MHDR" chunk
saved_images := 0; Interframe_delay := 0
First_frame := TRUE
if(loops>1) "write TERM 3 0 0 loops" chunk
write "BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 4 0 2 2 0 2 0 interframe_delay 0" chunk
First_frame := FALSE
else if(First_frame == TRUE)then
write "FRAM 4" chunk
First_frame := FALSE
else
write "FRAM" chunk
endif
if(X_loc == 0 AND Y_loc == 0) then
write "DEFI saved_images 1 1" chunk
else
write "DEFI saved_images 1 1 X_loc Y_loc" chunk
write "<image>"
write "SHOW 0 saved_images" chunk
if (gif_disposal_method == 0
OR gif_disposal_method == 2) then
/* (undefined or restore background) */
write "DISC" chunk
saved_images := 0
else if (gif_disposal_method == 1) then
/* (keep) */
saved_images := saved_images + 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
write "DISC saved_images" chunk
endif
endfor
write "FRAM" and "MEND" chunks
end
17.15. Example 15: Converting a simple GIF animation
begin
write "MHDR" chunk
Interframe_delay := 0; Previous_mode := 1
Framing_mode := 1
if(loops>1) "write TERM 3 0 0 loops"
write "mandatory BACK" chunk
for subimage in gif89a file do
if(interframe_delay != gif_duration) then
interframe_delay := gif_duration
write "FRAM 0 0 2 2 0 2 0 interframe_delay 0"
endif
if(X_loc != 0 OR Y_loc != 0) then
write "DEFI 0 0 0 X_loc Y_loc" chunk
endif
write "<image>"
if (gif_disposal_method < 1) then
/* (none or keep) */
Framing_mode := 1
else if (gif_disposal_method == 2) then
/* (restore background) */
write "FRAM 4 0 1 0 1 0 0 L R T B"
Previous_mode := 4; Framing_mode := 1
else if (gif_disposal_method == 3) then
/* (restore previous) */
error ("can't do gif_disposal method = previous.")
endif
if(Framing_mode != Previous_mode) then
write "FRAM Framing_mode" chunk
Previous_mode := Framing_mode
endif
end
write "MEND" chunk
end
17.16. Example 16: Counting layers and frames
MHDR sRGB Fn F I I I F F I I I F F I I I MEND
Fn represents a FRAM chunk with framing_mode n
F represents an empty FRAM chunk;
I represents an embedded image
Framing Layer count Frame count
mode
1 B,I,I,I, I,I,I, I,I,I = 10 BI,I,I, I,I,I, I,I,I = 9
2 B,I,I,I, I,I,I, I,I,I = 10 BIII,III,III = 3
3 3*(B, B,I, B,I, B,I) = 21 3*(B,BI,BI,BI) = 12
4 3*(B,B,I,I,I) = 15 B,BIII,B,BIII,B,BIII = 6
17.17. Example 17: Storing an icon library
MHDR 96 96 1 6 5 5 11 # Profile 11 is MNG-LC
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
eXPI 0 "left arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "right arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "up arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 0 "down arrow"
IHDR 96 96 ... PLTE IDAT ... IEND
MEND
MHDR 96 96 1 6 5 5 47 # Profile 47 is MNG without JNG
sRGB 2 # Global sRGB
PLTE ... # Global PLTE
tRNS 0 # Global tRNS
eXPI 0 "thumbnail"
IHDR 32 32 ... PLTE IDAT ... IEND
SAVE
<index>
SEEK "left arrows"
DEFI 1
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red left arrow"
DHDR 1 1 7 PPLT ... IEND # Change some palette entries.
eXPI 1 "blue left arrow"
SEEK "right arrows"
IHDR 96 96 ... PLTE IDAT ... IEND
eXPI 1 "red right arrow"
DHDR 1 1 7 PPLT ... IEND
eXPI 1 "blue right arrow"
MEND
17.18. Example 18: MAGN chunks and ROI
MHDR 600 600 0 5 1 0 19
# Gradient background
MAGN 00 00 2 599
sRGB 1
IHDR IDAT IEND
18. Credits
Editor
Contributors
Trademarks
Document source
Copyright Notice
End of MNG Specification.