It is a proposal, and the format is subject to change.
Comments on this document can be sent to the PNG specification maintainers at
png-info@uunet.uu.netor at
mpng-list@dworkin.wustl.edu.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/.
The MNG format provides a mechanism for reusing image data without having to retransmit it. Multiple images can be composed into a "frame," and an image can be used as a "sprite" that moves from one location to another in subsequent frames.
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 PND datastream.
A PND datastream defines an image in terms of a basis PNG or PND 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.
The MNG format uses the same chunk structure that is defined in the PNG specification, and shares other features of the PNG format. Any valid PNG datastream is also a valid MNG datastream.
This document includes examples that demonstrate various capabilities of MNG including simple movies, composite frames, loops, fades, tiling, scrolling, storage of voxel data, converting GIF animations to MNG format.
Note: This [proposed] specification depends on the PNG Portable Network Graphics specification. The PNG specification is available at the PNG home page,
http://quest.jpl.nasa.gov/PNG/ [about to move elsewhere...]A MNG datastream describes a sequence of single frames, each of which can be composed of one or more images defined by PNG or PND (PNG-Delta, defined herein) datastreams.
A MNG datastream consists of the MNG signature and a "MHDR" chunk, followed by one or more frame definitions, followed by the "MEND" chunk. The first frame must be a PNG datastream ("IHDR", PNG chunks, "IEND") or a group of image definitions (including at least one PNG datastream) delimited by "FRAM" chunks.
Each subsequent frame can be a PNG datastream, a PND datastream ("DHDR", PND chunks, "DEND"), a "SHOW" chunk, or a group of image definitions and "SHOW" chunks delimited by "FRAM" chunks.
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 "image/x-mng" can be used. Registration of the media type "image/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 1-4.
MNG does not yet accommodate sound or complex sequencing information,
but these capabilities might be added
at a later date, in a backwards-compatible manner. These issues are being
discussed in the mpng-list@dworkin.wustl.edu mailing list.
At some future date, support
for a lossy image format such as the proposed PNP (Portable Network Photo)
format might be added. PNP is under discussion
by pnp-list@dworkin.wustl.edu.
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.
While MNG datastreams are written so decoders can display frames properly by showing their subimages in the order they appear in the datastream, MNG also provides "level" information that allows viewers to composite the frames in a "sprite mode" that can offer faster composition and efficient use of display hardware (in this document a "sprite" is an object that has location and visibility information. We do not imply that the object is automatically redrawn whenever it changes; explicit directives to redraw a sprite are required).
Each chunk of the MNG datastream or of any image definition is an independent entity, i.e., no chunk is ever enclosed in the data segment of another chunk.
An independent PNG datastream, with a PNG signature, is also a valid MNG datastream that must be recognized and decoded by MNG-compliant decoders. This kind of MNG datastream will contain only a single image.
Because the images making up a MNG are in PNG format, MNG shares the good features of PNG:
4 bytes: max_frame_width (unsigned integer).
Maximum width of any image or frame to be
displayed.
4 bytes: max_frame_height (unsigned integer).
Maximum width of any image or frame to be
displayed.
4 bytes: ticks_per_second (unsigned integer).
This is the unit used by the FRAM chunk to
specify frame_duration and sync_timeout.
The "max_frame_width" and "max_frame_height" fields
give the intended display size (measured in pixels) and provide default
clipping boundaries (see Paragraph 2.3.3, below). These can
be set to zero if the MNG datastream contains no visible images.
The "ticks_per_second" field must be nonzero if the datastream contains more than one frame. It should be set to zero if the datastream contains exactly one frame, so that decoders and other applications like the UNIX "file" command can use this field to identify single-frame MNGs.
The "NEED" chunk contains a list of keywords that the decoder must recognize. Keywords are typically private critical chunk names.
n bytes: keyword 1 byte: null separator etc.
The "NEED" chunk should be placed early in the MNG datastream, preferably immediately 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. A null separator byte must appear after each keyword in the "NEED" chunk except for the last one.
Decoders not recognizing 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.
[During the draft phase of the development of this specification, you can specify that the datastream is written in accordance with a certain draft version of MNG, you can use "NEEDdraft nn"].
It appears after the set of chunks that define the decoder state that must be restored upon encountering a "SEEK" chunk. The "SAVE" chunk must be present when the "SEEK" chunk is present.
Only one instance of the "SAVE" chunk is permitted in a MNG datastream. It is not allowed anywhere after the first "SEEK" chunk.
When an "image_id" is defined prior to the "SAVE" chunk, it is not permitted to modify the image or any of the data associated with that "image_id" after the "SAVE" chunk. Such an image can be discarded by means of the "DISC" chunk; if so, its "image_id" must not be reused. Such an image 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 original image.
The following information must be saved. Decoders that recognize private chunks not defined in this specification might have to save additional information related to those private chunks.
n bytes: previous (number of bytes since the
previous SEEK chunk). If previous=0,
then the number of bytes is unspecified.
n bytes: next (number of bytes to the next SEEK
chunk). If next=0, then the number of
bytes is unspecified.
n is the length of the SEEK chunk, divided by two.
n must be either 4 or 8.
Applications must not use any information preceding the "SEEK"
chunk, except for
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 might need data from preceding "LOCA" or "CLIP" chunks.
When n is eight, 32-bit machines will have to interpret "previous" as a set of two integers, the first representing the number of complete 4G blocks and the second (the last four bytes of "previous") as the remainder, and will have to treat "next" similarly.
"Previous" and "next" are measured from the first length byte of one "SEEK" chunk to the first length byte of another "SEEK" chunk.
The "SEEK" chunk is not permitted within the scope of a "{LOOP, ENDL}" pair. If a decoder encounters a "SEEK" chunk while any loop is active, either as a result of an illegal "SEEK" chunk appearing inside a loop or as the result of skipping corrupted data, all display loops must be immediately terminated.
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.
1 byte: start_loop_level (unsigned integer).
1 byte: must_run_loop (unsigned integer).
0: The decoder must run the loop exactly
repeat_count times, because execution of
the loop modifies or relocates basis
images that might be needed later.
1: It is safe for a decoder to run the loop
fewer than repeat_count times, provided
that it runs the loop at least once. Every
iteration of the loop generates an
identical sequence of frames.
4 bytes: repeat_count (unsigned integer), range
[0..2^31-1].
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 basis images for any PND datastreams in the loop are the
images in existence prior to entering the "LOOP" chunk, but in
subsequent iterations these basis images might have been modified.
The "must_run_loop" field can be used to inform decoders that
it is safe to reduce the number of loop iterations or to replay the
images in the loop without recompositing them.
When the "LOOP" chunk is present, an "ENDL" chunk with the same "loop_level" must be present later in the MNG datastream. Loops can be nested. Each inner loop must have a higher value of "start_loop_level" than the loop that encloses it.
If "repeat_count" is zero, the loop is done zero times. Upon encountering a "LOOP" chunk with "repeat_count=0", decoders simply skip chunks until the matching "ENDL" chunk is found, and resume processing with the chunk immediately following it.
It is the responsibility of the encoder to make sure that the assertions made by the "must_run_loop" field are true. When "must_run_loop=1", all iterations of the loop are identical, and a viewer could choose to actually execute the loop just once, while storing copies of the composited frames for redisplay, and then replaying the stored images for the remaining "repeat_count" iterations. Also, when "must_run_loop=1", it is safe to escape from the loop upon reaching the "ENDL" chunk even though the loop has not yet been executed for "repeat_count" iterations. It is unsafe, however, to escape from the interior of a loop while it is being executed; in this event, the decoder must proceed to the next "SEEK" chunk before resuming.
The "ENDL" chunk ends a loop that begins with the "LOOP" chunk. It contains a single one-byte field:
1 byte: end_loop_level (unsigned integer),
range [0..255].
When the "ENDL" chunk is encountered,
the loop "repeat_count" is decremented. 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 "loop_level" must be present earlier in the MNG datastream.
2 bytes: image_id (unsigned integer) image identifier
to be given to the images that follow the
DEFI chunk.
1 byte: visibility (unsigned integer)
0: Make the images visible.
1: Make the images invisible.
This field can be omitted if the location fields
are also omitted. When it is omitted, the image
is made visible (visibility=0).
4 bytes: x_location (signed integer).
The x_location and y_location fields can be
omitted if the clipping boundaries are also
omitted. If so, decoders must assume default
values {x_location,y_location}={0,0}.
4 bytes: y_location (signed integer).
4 bytes: left_cb (signed integer). Left clipping
boundary. The left_cb, right_cb, top_cb, and
bottom_cb fields can be omitted as a group.
If so, decoders must assume default values
{0, max_frame_width, 0, max_frame_height}.
4 bytes: right_cb (signed integer).
4 bytes: top_cb (signed integer).
4 bytes: bottom_cb (signed integer).
If the "image_id" for an image is nonzero, subsequent "DHDR",
"SHOW", "CLON", "LOCA", "CLIP", and
"DISC" chunks can use this number to identify it. This also is
used as the level, or position in the stacking order, of the image in a
composite frame. The image having the smallest value of "image_id"
is on the bottom (it can be obscured by any other "image_id"),
while the image having the largest value is the top image (it cannot be
obscured by any other "image_id").
Negative values are permitted for the X and Y image location and clipping boundaries. The positive directions are downward and rightward from the upper left corner of the display.
The "DEFI" chunk values remain in effect until another "DEFI" chunk or a "SEEK" chunk appears. If no "DEFI" chunk is in effect, the default values for "image_id", "visibility", "x_location", and "y_location" are all zero, and the default values for the clipping boundaries are {0, max_frame_width, 0, max_frame_height}.
If "image_id" is an identifier that already exists when an "DEFI", "IHDR" or "BASI" chunk is encountered, the basis image previously associated with the identifier is discarded (or disassociated with it, if the identifier was a partial clone of another image), along with any "LOCA", "CLIP", "INHR", or "SBYK" data associated with it.
If an "IHDR-IEND" sequence is not preceded by a "DEFI" chunk, then "DEFI 0" is implied. The "BASI-IEND" sequence must be preceded by a "DEFI" chunk with a nonzero "image_id" and "visibility=1".
See the PNG specification for the format of the PNG chunks.
Any chunks between "IHDR" and "IEND" are written and decoded according to the PNG specification. The image width and height must not exceed "max_image_width" and "max_image_height" from the "MHDR" chunk.
The PNG "oFFs" chunk must be ignored by MNG viewers and simply copied by MNG editors.
If the "visibility=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.
4 bytes: width (unsigned integer). 4 bytes: height (unsigned integer). 1 byte: bit_depth (unsigned integer). 1 byte: color_type (unsigned integer). 1 byte: compression_method (unsigned integer). 1 byte: filter_type (unsigned integer). 1 byte: interlace_type (unsigned integer). 2 bytes: red sample (unsigned integer). 2 bytes: green sample (unsigned integer).. 2 bytes: blue sample (unsigned integer). 2 bytes: alpha sample (unsigned integer).The alpha sample may be 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. The decoder is responsible for converting the color and alpha samples to the appropriate format and sample depth for the specified "color_type". When "color_type=3", the decoder must generate a palette of length 2^bit_depth, whose first entry contains the given {red, green, blue} triple, and whose remaining entries are filled with zeroes.
The "BASI" introduces a datastream that contains PNG chunks, but is not necessarily a PNG datastream. It can be incomplete or it can deviate in certain ways from the PNG specification. It can serve as a basis for a PND 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:
A "BASI" chunk appearing in a MNG datastream must be preceded by a "DEFI" chunk that gives a nonzero "image_id", the location for the basis image, and "visibility=1".
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.
See Chapter 3, The PND Format, below, for the format of the PND datastream. Any chunks between "DHDR" and "DEND" are written and decoded according to the PND format, and the resulting image is displayed if its "visibility=0". The image width and height must not exceed "max_image_width" and "max_image_height" from the "MHDR" chunk.
2 bytes: image_id (nonzero unsigned integer) identifier
of the basis image to be cloned.
2 bytes: clone_id (nonzero unsigned integer) identifier
to be given to the clone (new copy) to serve as
the image_id of the new image.
1 byte: clone_type (unsigned integer)
0: full clone of LOCA and CLIP data and image
data.
1: partial clone; only the LOCA and CLIP data is
copied and a link is made to the image data.
2: renumber image (this is equivalent to
"CLON old_id new_id 1 DISC old_id"). This
can be useful for rearranging the stacking
order of images in a composite frame.
This field can be omitted if the "visibility"
field is also omitted. If it is omitted, the
clone_type defaults to zero (full clone).
1 byte: visibility (unsigned integer)
0: Make the clone visible.
1: Make the clone invisible.
This field can be omitted if the location fields
are also omitted. When it is omitted, the image
is made visible (visibility=0).
1 byte: loca_delta_type (unsigned integer)
0: LOCA data gives x_location and
y_location directly.
1: New positions are determined by adding
the LOCA data to the position of the
basis image.
This field, together with the x_location and
y_location fields, can be omitted. When they
are omitted, decoders must assume default
values {x_location,y_location}={0,0}.
4 bytes: x_location or delta_x_location
(signed integer).
4 bytes: y_location or delta_y_location
(signed integer).
Negative values are permitted for the X and Y image position. The
positive directions are downward and rightward from the upper left
corner of the display.
The clone is initially identical to the basis image and has the same "LOCA", "CLIP", "INHR", and "SBYK" data as the basis image. Subsequent "DHDR", "SHOW", "CLON", "CLIP", "LOCA", and "DISC" chunks can use the "clone_id" to identify it.
Subsequent chunks can modify, show, or discard a full clone or modify its associated "LOCA", "CLIP", "INHR", or "SBYK" data without affecting the basis image, or they can modify, show, or discard the basis image or modify its associated data without affecting the clone.
If an image has partial clones, and the image data in a basis image or any of its partial clones is modified, the basis image and all of its partial clones are changed. Decoders must take care that when the basis image or any partial clone is discarded, the image data is not discarded until the last remaining one of them is discarded. The "INHR" and "SBYK" data is also shared, so that if they are changed in any partial clone, they are changed in the others as well. Only the "LOCA" or "CLIP" data can be changed independently for each partial clone.
The chunk contains a sequence of zero or more two-byte image identifiers. The number of images to be discarded is the the chunk's data length, divided by two.
2 bytes: discard_id (nonzero unsigned integer) image
identifier that can be discarded. All
information pertaining to the corresponding
image can be discarded and the identifier can
be reused by a DEFI chunk.
etc.
If the "DISC" chunk is
empty, all images except those preceding the "SAVE" chunk
can be discarded. Note that the appearance of a "SEEK"
chunk in the datastream implies an empty "DISC" chunk.
When an image is discarded, any "LOCA", "CLIP" "INHR", or "SBYK" data associated with it is also discarded.
The appearance of an "image_id" in the "discard_id" list, when no such image has been stored, or when the image has already been discarded, should not be treated as a fatal error.
Discarding images that are defined prior to the "SAVE" chunk is permitted, but the "image_id" of such images must not be reused.
When the image is a partial clone or is the source of a partial clone that has not been discarded, only the "LOCA" or "CLIP" data can be discarded. The image data must be retained until the last remaining partial clone is discarded.
The "LOCA" chunk gives the position, measured downward and to the right of the upper left corner of the display, in pixels, where the following image is to be located.
The chunk's contents are:
2 bytes: image_id (nonzero unsigned integer).
1 byte: loca_delta_type (unsigned integer).
0: LOCA data gives x_location and
y_location directly.
1: New positions are determined by adding
the LOCA data to the position of the
basis image.
4 bytes: x_location or delta_x_location
(signed integer).
4 bytes: y_location or delta_y_location
(signed integer).
Negative values are permitted for the X and Y image position. The
positive directions are downward and rightward.
The "LOCA" 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 the display, or not displayed at
all if it falls entirely outside the display. The display
boundaries are determined as described in the specification for
the "CLIP" chunk Paragraph 2.3.3, below.
It is an error for the "LOCA" chunk to appear with an "image_id" that has not previously been defined.
1 byte: background_source (unsigned integer).
0: Viewers can use the supplied background color
as a default.
1: Viewers must use the supplied background
color.
2 bytes: red_background (unsigned integer).
2 bytes: green_background (unsigned integer).
2 bytes: blue_background (unsigned integer).
Viewers are expected to composite
every frame in the MNG datastream, whether it be a PNG or PND datastream or
a group of PNG or PND datastreams delimited by "FRAM" chunks,
against a fresh copy of the background, clipped to the frame boundaries
given in the FRAM chunk (they might actually follow some
other procedure, but the final appearance of each frame must be the same
as if they had).
Multiple instances of the "BACK" chunk are permitted in a MNG datastream. One should appear before the "SAVE" chunk, if the "SAVE" chunk is present, to give the data to be restored when a "SEEK" chunk is encountered.
The "BACK" chunk can be omitted. If a background is required and the "BACK" chunk is omitted, then the viewer must supply its own background.
The BACK components are always interpreted in linear color space (gamma=1), regardless of the presence of any "gAMA" chunk in the datastream.
2 bytes: image_id (nonzero unsigned integer).
1 byte: clip_delta_type (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 image_id.
4 bytes: left_cb or delta_left_cb (signed integer).
4 bytes: right_cb or delta_right_cb (signed integer).
4 bytes: top_cb or delta_top_cb (signed integer).
4 bytes: bottom_cb or delta_bottom_cb (signed
integer).
When no "CLIP" chunk is in effect,
the clipping boundaries are taken from the "MHDR" chunk:
left_cb := 0 right_cb := max_frame_width top_cb := 0 bottom_cb := max_frame_heightThe 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 all of the following are true:
0 <= x < max_frame_width (from the MHDR chunk) 0 <= y < max_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_cbIt is an error for the "CLIP" chunk to appear with an "image_id" that has not previously been defined. When an "image_id" is discarded, its "CLIP" data is also discarded.
An empty "FRAM" chunk is just a frame delimiter. A nonempty one is a frame delimiter, and it also changes "FRAM" parameters, either for the upcoming frame or until reset. When the "FRAM" chunk is not empty, it contains five 1-byte fields plus a variable number of optional fields.
1 byte: framing_mode.
0: Don't change framing_mode.
1: Each image is an individual frame.
2: The group of images appearing prior to
the next "FRAM" chunk form a "sprite mode"
frame (any image_id appears only once).
All images are initially invisible and
are made visible with explicit or implied
"SHOW" chunks. The level, or stacking
order, of each image is given by its
image_id.
3: The group of images appearing prior to
the next "FRAM" chunk define "sprite mode"
frames as they accumulate (any image_id
appears only once), and a frame boundary
occurs after each image is defined or
appears in a "SHOW" chunk. All images are
initially made invisible but remain
visible after being explicitly made
visible. The level, or stacking order,
of each image is given by its image_id.
4: The group of images appearing prior to
the next "FRAM" chunk is a "draw mode"
frame (an image_id can appear more than
once in the frame).
5: The group of images appearing prior to
the next "FRAM" chunk define "draw mode"
frames as they accumulate (an image_id
can appear more than once in the frame).
1 byte: change_frame_duration.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
1 byte: change_sync_timeout.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
1 byte: change_frame_boundaries.
0: no.
1: yes, for the next frame only.
2: yes, also reset default.
1 byte: change_sync_id_list.
0: no.
1: yes, for this frame only.
2: yes, also reset default list.
1 byte: fb_delta_type (unsigned integer).
(omit this and the following four fields
if change_frame_boundaries=0).
0: frame_boundary values are given directly.
1: frame_boundaries are determined by adding
the FRAM data to their previous values.
4 bytes: left_fb or delta_left_fb (signed integer).
4 bytes: right_fb or delta_right_fb (signed integer).
4 bytes: top_fb or delta_top_fb (signed integer).
4 bytes: bottom_fb or delta_bottom_fb (signed
integer).
4 bytes: frame_duration (unsigned integer) (omit
if change_frame_duration=0). The range
is [0..2^31-1] ticks.
4 bytes: sync_timeout (unsigned integer) (omit if
change_sync_timeout=0). The range is
[0..2^31-1]. The value 2^31-1 (0x7fffffff)
ticks represents an infinite timeout period.
4 bytes: sync_id (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 frame that is about to be defined, not the one that is
being terminated by the "FRAM" chunk.
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.
When a "SEEK" chunk is encountered, all "FRAM" parameters are reset to the values they had when the "SAVE" chunk was encountered, and, if a "sync_id" list existed, it is restored.
The initial values of the "FRAM" parameters are
framing_mode := 1 frame_duration := 0 left_fb := 0 right_fb := max_frame_width top_fb := 0 bottom_fb := max_frame_height sync_timeout := 0x7fffffff (infinite) sync_id list is emptyThe "LOCA" chunk can be used to specify the placement of each image within the frame. The "CLIP" chunk can be used to specify clipping boundaries for each image. If the images are transparent or do not cover the entire frame, as defined by the "max_frame_width" and "max_frame_height" fields of the "MHDR" chunk, they are composited against the background defined by the "BACK" chunk, or against an application-defined background, if the "BACK" chunk is not present.
Viewers can be designed to operate in "draw mode" or "sprite mode". A "draw mode" viewer simply displays images within a frame in the order that they appear (or corresponding "SHOW" chunks appear) in the MNG datastream. A "sprite mode" viewer accumulates images until it needs to display a complete frame, and then composites all of the images whose "visibility" byte is 0, from top to bottom, according to the "image_id" value for each image. Images will normally appear in the datastream in the "draw mode" order, which is bottom-to-top (i.e. the background image, which has the smallest value of "image_id", appears first in the datastream).
When "framing_mode=1", each image that becomes visible is a separate frame, delimited by its "IEND" chunk (in the case of PNG images) or "DEND" chunk (in the case of PND images). Each image that is made visible by a "SHOW" chunk also constitutes a separate frame. If "LOCA" chunks appear, they must appear prior to the image definition chunks or "SHOW" chunks to which they pertain.
When "framing_mode=2", viewers are expected to display all of the images at once, if possible, or as fast as can be managed, without clearing the display and restoring the background between images. The "frame_duration" value is the desired minimum time to elapse from the beginning of displaying the first image of the frame until the beginning of the next image after the next "FRAM" chunk. Each image in the frame must be given a unique "image_id", which defines its position in the stacking order. The image with the smallest value of "image_id" is the background image (which can be obscured by any other image) and the image with the largest values of "image_id" is the top image (which can not be obscured but can obscure any other image). All images are initially invisible, and are made visible by their appearance in an explicit or implied "SHOW" chunk. They must be made visibile in ascending value of "image_id". If "LOCA" or "CLIP" chunks appear, they must appear before the images to which they pertain are made visible. "Sprite mode" decoders can accumulate them and display them all at once when the next "FRAM" chunk comes along. "Draw mode" decoders can display them in the order that they appear or are mentioned in "SHOW" chunks.
When "framing_mode=3", a frame boundary occurs after each image is made visible, without clearing the display and restoring the background between frames. Each image must be assigned to a unique "image_id", in ascending order of appearance. If "LOCA" or "CLIP" chunks appear, they must appear before the images to which they pertain are made visible. "Draw mode" decoders can simply erase the display prior to displaying the first image, and then show each image as it is defined, without erasing the display. "Sprite mode" decoders can recomposite the display after each image has been made visible.
When "framing_mode=4", "draw mode" decoders can treat the frame in the same way as when "framing_mode=2". If "LOCA" or "CLIP" chunks appear, they must appear before the images to which they pertain are made visible. "Sprite mode" decoders will have to revert to "draw mode" to display this frame type, or or else they will have to make their own clones and reassign them to unique, properly ordered unused levels. Encoders are encouraged to use "framing_mode=2" instead, unless the frame contains such a large number of large images that the memory savings is significant. It would be appropriate to use "framing_mode=4" for storage of things like tomographic slices.
When "framing_mode=5", "draw mode" decoders can treat the frame in the same way as when "framing_mode=3". If "LOCA" or "CLIP" chunks appear, they must appear before the images to which they pertain are made visible. "Sprite mode" decoders will have to revert to "draw mode" to display this frame type, or or else they will have to make their own clones and reassign them to unique, properly ordered unused levels. Encoders are encouraged to use "framing_mode=3" instead, unless the frame contains such a large number of large images that the memory savings is significant.
The "frame_duration" field gives the duration of display, which is the minimum time that must elapse from the beginning of displaying one frame 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.
The value of "frame_duration" will remain in effect until another "FRAM" chunk redefines it or until a "SEEK" chunk is encountered, when the duration reverts to the default value that was stored when the "SAVE" chunk was processed.
When images in a frame overlap, viewers are expected to composite the later images against the partially completed frame that includes all earlier images.
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 "frame_duration" and the "sync_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 frame that follows the "FRAM" chunk. 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 "frame_duration=0", "sync_timeout", and "sync_id", and the second establishing the synchronization point:
FRAM 0 1 1 0 1 0000 sync_timeout sync_id FRAM
The identifier "sync_id=0" is reserved to represent synchronization with a user input from a keyboard or pointing device. If multiple channels (not defined in this specification) are not present, viewers can ignore other values appearing in the "sync_id" list.
2 bytes: first_id (nonzero unsigned integer).
2 bytes: last_id (nonzero unsigned integer).
This field can be omitted if the visibility
byte is also omitted. Decoders must assume
the default value, "last_id=first_id."
1 byte: visibility (unsigned integer)
0: Make the images visible.
1: Make the images invisible.
This field can be omitted. If so, decoders
must assume the default, "visibility=0".
When "visibility=0", the
images are shown in the order "first_id" through " last_id".
If "first_id > last_id" then the images are shown in
reverse order (this is not permitted when "framing_mode=2" or
"framing_mode=3" in the "FRAM" chunk because "sprite mode"
viewers would show them in normal order anyway).
When "framing_mode=1"
or when the "FRAM" chunk is not present, each image is
displayed as a separate frame.
When "visibility=1", the images are made invisible
(this is not permitted when "framing_mode=2" or
"framing_mode=3" in the "FRAM" chunk because "draw mode"
viewers will have already drawn them and have no way to make them invisible
without redrawing the entire frame).
An instance of each image will be displayed at the location specified by the "DEFI", "CLON", or "LOCA" chunk and clipped according to the boundaries specified by the "CLIP" chunk. When the "LOCA" 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 "LOCA" or "CLON" chunk.
It is not necessary to follow an "IHDR-IEND" or "DHDR-DEND" sequence with a "SHOW" chunk, if its visibility is the same as that given in the "DEFI" chunk that introduced the "image_id".
1 byte: fPRi_delta_type (unsigned integer).
0: Priority is given directly.
1: Priority is determined by adding
the fPRi data to the previous
values, modulo 256.
1 byte: priority or delta_priority (signed
integer). Value to be assigned to
subsequent frames until another fPRi
chunk is reached.
1 byte: max_priority (unsigned integer). Maximum
priority that will appear in any subsequent
fPRI chunk (max_priority is always given
directly without regard to fPRi_delta_type).
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 PND datastreams, the "initial"
frames could be preceded by a "fPRi" with "priority=128"
and followed by one with "priority=0", and the best representative
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 could display everything.
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 Examples 1 and 4.
When a "SEEK" chunk is encountered, the value of "priority" reverts to its value when the "SAVE" chunk was processed. Viewers that care about the priority can assume "priority=255" for any portion of the MNG datastream that is processed prior to the first "fPRi" chunk.
The "tHUM" chunk contains one or more 2-byte fields:
2 bytes: thumbnail_id (nonzero unsigned integer). etc.The number of thumbnails is determined by the length of the "tHUM" chunk, divided by two.
The images that have the given "thumbnail_id" values must have already been defined, and not discarded, prior to the appearance of the "tHUM" chunk. The thumbnail images are regular PNG or PND images, but they would normally have smaller dimensions and fewer colors than the MNG frames. They can be defined with the visibility field set to "invisible" if they are not intended to be shown as a part of the regular display.
Multiple instances of the "tHUM" chunk are permitted in a MNG datastream.
The following 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:
The following PNG chunks are also defined at the MNG top level. They provides values that take precedence over those that might be provided in subsequent PNG datastreams and provide values to be used when they are not provided in subsequent PNG datastreams:
When a decoder needs to chose 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 frame-to-frame color changes.
MNG editors that write PNG datastreams should ignore the "sPLT" and "pHYs" data from the MNG level and simply copy any "sPLT" and "pHYs" chunks appearing within the PNG datastreams.
When a "SEEK" chunk is encountered, all "sPLT" data will be restored to the state that existed when the "SAVE" chunk was encountered.
No provision is made in this specification for storing a PND datastream as a standalone file. A PND datastream will normally be found as a component of a MNG datastream. Applications that need to store a PND 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 PND datastream, and the "MEND" chunk.
The decoder must have available a basis (decoded) image from which the original chunk data is known. The basis image can be the result of decoding a PNG, another PND datastream, or it could have been generated by a PNG-like datastream introduced by a "BASI" chunk.
The new image is always of the same basic type (at present only PNG is defined) as the basis image.
The decoder must not have modified the pixel data in the basis image 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 PND decoder the unmodified pixel data along with the values for the "gAMA", "cHRM", and any other recognized chunks from the basis image datastream.
A PND datastream consists of a "DHDR" and "DEND" enclosing other optional chunks (if there are no other chunks, the decoder simply copies the basis image, and displays it if its "visibility=0").
Chunk structure (length, name, CRC) and the chunk-naming system are identical to those defined in the PNG specification. Definitions of "compression_type", "filter_type", and "interlace_type" are also the same as defined in the PNG specification.
The "DHDR" chunk can contain 4, 12, or 20 bytes:
2 bytes: image_id (nonzero unsigned integer).
Identifies the basis image from which changes
will be made. This is also the image_id
of the resulting modified image, which can be
used as the basis image for a subsequent PND
datastream.
1 byte: image_type.
0: Image type is unspecified. An IHDR or IPNG
chunk must be present.
1: Image type is PNG. IHDR and IPNG can be
omitted under certain conditions.
1 byte: delta_type.
0: Entire image replacement.
1: Block pixel addition, by samples, modulo
2^bit_depth.
2: Block alpha addition, by samples, modulo
2^bit_depth. Regardless of the color
type of the basis 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
basis image. The basis image must have
(or be promoted to via the PROM chunk)
color type 4 or color type 6.
3: Block pixel replacement.
4: Block alpha replacement.
5: No change to pixel data.
[if we want to get fancy we can add block
pixel replacement with flipping. There are
7 orientations in addition to straight
replacement:
6: left-right swap
7: up-down swap
8: left-right plus up-down swap
9: 90 degree rotation
10: 270 degree rotation
11: left-right swap plus 90 degree rotaton
12: left-right swap plus 270 degree rotation
The rotations would only be legal when
block_width = block_height]
4 bytes: block_width (unsigned integer). Omit when
delta_type=5.
4 bytes: block_height (unsigned integer). Omit when
delta_type=5.
4 bytes: block_x_location (unsigned integer),
measured in pixels from the left edge of
the basis image. Omit when delta_type=0
or when delta_type=5.
4 bytes: block_y_location (unsigned integer),
measured in pixels from the top edge of
the basis image. Omit when delta_type=0
or when delta_type=5.
The image type, whether given explicitly as 1
or implied by the presence of an "IHDR" or "IPNG"
chunk, must be the same as that of the basis image.
When "delta_type=0", the width and height of the generated image are given by the block_width and block_height fields.
For all other values of "delta_type", the width and height of the generated image are inherited from the basis image.
When "delta_type=1, 2, 3, or 4", 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 basis image. The block must fall entirely within the basis image.
The basis image must have been derived from a PNG datastream or from a sequence of PND datastreams that depend upon a PNG datastream.
The compression method, filter method, and interlace method need not be the same as those of the basis image.
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 actual pixel values are calculated using the method defined in the "delta_type" field of the "DHDR" chunk. 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.
An encoder calculates the new sample values from the samples in the basis image and those in the new image by subtracting the basis image samples from the new image samples, modulo 2^sample_bit_depth. When decoding the "IDAT" chunk, the new image bytes are obtained by adding the delta bytes to the basis image bytes, modulo 2^sample_bit_depth. This is similar in operation to the PNG SUB filter, except that it works by samples instead of by bytes.
When "color_type=3", the deltas are differences between index values, not between color samples.
The "bit_depth" of the data must match that of the basis image, and "color type" is 0 (grayscale), regardless of the "color_type" of the basis image. The basis image 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.
The "bit_depth" and "color_type" of the data must match that of the basis image. The compression method, filter method, and interlace method need not be the same.
The "DEND" chunk is empty.
1 byte: new color_type. 1 byte: new bit_depth.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.
When a grayscale or truecolor basis image is promoted to an image with an alpha channel, the alpha samples are initialized to 2^bit_depth-1 (fully opaque). When an indexed-color image is promoted, the alpha channel data is obtained from the basis image's "tRNS" chunk data, if present, or initialized to 2^bit_depth-1, if the "tRNS" chunk is not present or not recognized by the decoder.
The "PROM" chunk is not permitted to "demote" a basis image to an image with a lesser bit depth or from one with an alpha channel to one without an alpha channel.
If the "IHDR" chunk is present, its "width", "height", "bit_depth", and "color_type" fields are ignored. The values for these parameters are inherited from the basis image or from the "PROM" chunk.
The "compression_method", "interlace_type", and "filter_type" fields, if different from those of the basis image, are used in decoding any subsequent "IDAT" chunks, and the new values will be inherited by any subsequent image that uses this image as its basis.
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 basis image. Except for IDAT, any chunks appearing between "IHDR" and "IEND" are always treated as replacements or additions and not as deltas.
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_type", or "interlace_type". The purpose of this chunk is to identify the beginning of the PNG datastream, so decoders can start interpreting PNG chunks instead of PND 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 stream begins with either a "PLTE" 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 nor an "IDAT" chunk, then either the "IPNG" or "IHDR" must be present to introduce the PNG datastream.
A "gAMA", "cHRM", or similar chunk existing in the basis image would not affect the pixel data inherited by this PND 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 basis image are the raw pixel data that existed prior to any transformations that were applied while displaying the previous frame.
If the "PLTE" chunk is present, it need not have the same length as that inherited from the basis image, but it must contain the complete palette needed in the generated image. If it is shorter than the palette of the basis image, decoders can discard the remaining entries. They can also truncate any "tRNS" data inherited from an indexed-color basis image. If it is longer, and a new "tRNS" chunk is not present in an indexed-color image, the "tRNS" data should be extended with opaque entries.
[What about defining an IPLT chunk that can give an incomplete PLTE, that would only overwrite palette entries without truncating the resulting PLTE? This might be useful for palette-animation applications.]
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 basis image.
MNG viewers are expected to ignore the "oFFs" chunk. MNG editors are expected to treat the "oFFs" chunk as an unknown chunk that will be handled as described in Paragraph 3.1.5, below.
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 "INHR" 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 Paragraph 3.1.5, below. When the "INHR" chunk is present, then all chunks are considered to have appeared in the order given by the "INHR" chunk.
The "IEND" chunk can be omitted, if it would be the last chunk in the PND datastream before the "DEND", or when no PNG chunks are present.
Viewers that only process public PNG chunks and are not concerned about selecting chunks such as "tEXt" or "zTXt" by keyword also need not process the "INHR" chunks, although they must recognize them. Applications that write PND streams containing only public PNG chunks need not be concerned about writing "INHR" chunks.
The "INHR" chunk contains a series of five-byte entries, each giving a chunk name and a rule.
4 bytes: chunkname.
1 byte: rule.
0: (Replace) Inherit all instances of the chunk
in the basis image. If a new instance is found
in the PND datastream, delete all inherited
instances and replace them with the new instance
or instances.
1: (Append) Inherit all instances of the chunk in
the basis image. Append any new instances found
in the PND datastream.
2: (Ignore) Ignore any instances of the chunk in
the basis image. Insert any new instances found
in the PND datastream.
3: (Replace by Keyword) The chunk is one for which
multiple instances are permitted, and the first
field of the chunk is a null-byte terminated
Latin-1 keyword or name (tEXt, zTXt, spAL, for
example). Inherit all instances of the chunk
from the basis image. If a new instance is
found in the PND datastream, delete all
inherited instances having the same keyword or
name and replace them with the new instance or
instances.
4: (Special) Inheritance and placement of this
chunk is governed by special rules that must be
known to the PND decoder (IDAT, for example).
The "INHR" chunk provides a list of chunknames that are to be
inherited or not inherited from the basis image, regardless of the copy-safe
rules, and regardless of whether the chunk is recognized or not.
The number of entries is determined from the chunk length, divided by 5.
The "INHR" chunk also gives a template for the arrangement of chunks within the resulting PNG datastream. The decoder can place them in any order, relative to their order in the basis image, that obeys the chunk-ordering rules for copy-safe chunks given in the PNG specification, but when it encounters unknown but copy-safe chunks, it should use the "INHR" chunk data as a template to ensure that they are properly placed.
PND applications are expected to know the copying rules and order of placement of those chunks defined in the core specification for the image format corresponding to the "DHDR" "image_type", regardless of whether an "INHR" chunk is present and whether those chunks are listed in it. In particular, the PNG "IHDR" and "IEND" chunks need not be listed in the "INHR" chunk, because the position of these chunks is already well-defined. "PLTE" (with "rule=0") and "IDAT" (with "rule=4") should be listed, however, because other chunks need to be located with respect to them. It is not an error for a chunkname to appear in the list, when that chunk does not appear in the basis image or in the PND datastream.
When an unknown chunk is found in the PND datastream that appears in the basis image but is not listed in the "INHR" chunk, the decoder can copy the chunk into the same position relative to critical chunks that it occupied in the basis image. When an unknown chunk appears neither in the basis image nor in the "INHR" list, or when a chunk's "rule=4" and the PND decoder does not know the special copying rules for the chunk in question, the decoder must treat it as an unknown chunk, ignoring it if it is ancillary, and abandoning the PND datastream if it is critical.
The "INHR" chunk data itself is inherited, so an "INHR" chunk need only appear in the first of a sequence of PND datastreams that have the same "image_id", if there are no changes. When a "INHR" chunk is inherited and also appears on the PND datastream, the entire list of chunk names and rules is replaced.
Applications that reconstruct a PNG datastream from a PND datastream and a basis PNG or PND datastream must not write the "INHR" chunk itself to the resulting PNG datastream, because the "INHR" chunk would not be recognized by a PNG decoder.
There can be only one instance of the "INHR" chunk in a PND datastream. The "INHR" chunk must appear before "IHDR" if it is present.
Viewers that only process public PNG chunks and are not concerned about selecting chunks such as "tEXt/zTXt" by keyword need not process "SBYK" chunks, although they must recognize them. Applications that write PND streams containing only public PNG chunks need not be concerned about writing "SBYK" chunks.
The chunk is either empty or it contains a select/reject byte, a chunk name, and a keyword.
1 byte: sbyk_select (unsigned integer).
0: Select chunks by keyword.
1: Reject chunks by keyword.
4 bytes: chunkname (four ASCII bytes).
n bytes: keyword (Latin-1 string).
When the "SBYK" is empty, the "SBYK" list is cleared.
When the "SBYK" chunk is not empty, the specified {sbyk_select, chunk name, keyword} triple is added to the list associated with the "image_id" that is being modified.
The chunk must be one whose first field is Latin-1 string and whose second field is a null separator byte, such as the "tEXt" chunk, which begins with an Latin-1 keyword, or the proposed "fALS" chunk, which begins with an Latin-1 "purpose" string.
The keyword need not be terminated by a null byte; if it is, the null byte will be ignored. The keyword 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 name. Keywords are case-sensitive. Use caution when printing or displaying keywords (Refer to Security considerations, Chapter 6, below).
Any chunks in the basis image having the same chunk name and keyword are selected (if "sbyk_select=0") or rejected (if "sbyk_select=1") for inclusion in the resulting PND image. It is not an error to list a {chunkname, keyword} pair that does not exist in the basis image.
The "SBYK" chunk can only be used to select PNG chunks whose keyword, "purpose," or other identifying string appears first in the chunk's data segment. The practice of putting a version identifying "signature" first in the data segment of unregistered experimental chunks will defeat this mechanism, so it is better to place such a "signature" after the keyword and its zero-byte separator, if you wish to use the "SBYK" mechanism with your experimental chunk.
Applications that reconstruct a PNG datastream from a PND datastream and a basis PNG or PND datastream must not write the "SBYK" chunk itself to the resulting PNG datastream, because the "SBYK" chunk would not be recognized by a PNG decoder.
Multiple instances of the "SBYK" chunk are permitted in a PND datastream. Any instances must appear prior to the "IHDR" chunk, if the "IHDR" chunk is present.
1 byte: (unsigned integer), fade_type.
0: Fade out.
1: Fade in.
2: Fade in but don't change fully transparent
pixels.
2 bytes: (unsigned integer), alpha_delta.
When "fade_type=0", the value of "alpha_delta" is
subtracted from the alpha sample of every pixel in the basis image,
but the result
is not allowed to fall below zero.
When "fade_type=1", the value of "alpha_delta" is added to the alpha sample of every pixel in the basis image, but the result is not allowed to exceed the maximum alpha value for the image's bit depth.
When "fade_type=2", the value of "alpha_delta" is added to the alpha sample of any pixel in the basis image that has a non-zero value, and the result is not allowed to exceed the maximum alpha value for the image's bit depth.
If "color_type=3", then the value of "alpha_delta" is added to or subtracted from the alpha values that were defined by the "tRNS" chunk data in the basis image, and the resulting values become the "tRNS" data exported to any subsequent image. The maximum alpha value for this "color_type" is 255, regardless of the bit depth.
If "color_type" is 0 or 2, the "fADE" chunk is ignored.
If "IHDR" or "IPNG" is also present, the "fADE" chunk must appear before "IHDR" or "IPNG" . The fade operation is performed on the image data after decoding the chunks between "IHDR" or "IPNG" and "IEND".
The following information must be retained, for each nonzero "image_id" that is defined and not subsequently discarded:
When the encoder knows that image data will not be needed by subsequent frames, it can make life easier for decoders by using the "image_id=0" or by using the "DISC" or the "SEEK" chunk.
When an error occurs within a image datastream, such as an unknown critical PNG chunk or a missing basis image where one was required, only that image should be abandoned and the associated "image_id" should be discarded.
MNG editors, on the other hand, should be more strict and reject any datastream with errors unless the user intervenes.
If "PLTE" is present in a PND datastream, the new palette is used in displaying the image defined by the PND; 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 PND without a "PLTE" chunk, that has been declared by the "DHDR" "image_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 "spAL" chunk with a reduced global palette, to help decoders build an appropriate palette when necessary.
An infinite or just overly long loop could give the appearance of having locked up the machine, as could an unreasonably long inter-frame delay or a misplaced "sync_id" with a long "sync_timeout" value. Therefore a decoder should always provide a simple method for users to escape out of a loop or delay, either by canceling the MNG entirely or just proceeding on 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).
No known additional security concerns are raised by this format.
\212 M N G \r \n ^z \n # MNG signature
MHDR 720 468 # Width and height
30 # 30 ticks per second
tEXtTitle\0Sample Movie
fPRI 128 255 # default frame priority is "medium"
FRAM 1 2 0 0 0 3 # set frame_duration to 1/10 sec
DEFI 1 # set default image_id to 1
SAVE
SEEK 0 n1
IHDR 720 468 8 2 0 0 0 # DEFI 0 is implied
IDAT ...
IEND
DHDR 1 1 1 20 30 100 220 # A PNG-delta frame
IDAT ... # The IDAT gives the 20x30 block
DEND # 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
DEND # block at a slightly different location
SEEK n1 n2 # Ok to restart here because a
# complete PNG frame follows
fPRI 255 255 # This is the representative frame that
IHDR 720 468 ...# will be displayed by single-frame
IDAT ... # viewers.
IEND
fPRI 128 128 # Return to medium frame priority
DHDR 1 1 1 720 468 0 0 # Another PNG-delta frame
IDAT ... # The entire 720x468 rectangle changes
DEND # this time.
SEEK n2 0
MEND # End of MNG datastream
\212 M N G \r \n ^z \n # MNG signature
MHDR 1024 512 # Width, height
1 # ticklength
BACK 1 16448 16448 52800 # Must use sky blue background
DEFI 1 1 # Define invisible thumbnail image.
IHDR 64 64 4 3 0 0 0
IDAT
IEND
DEFI 2 1 # Also define a larger thumbnail.
IHDR 96 96 4 3 0 0 0
IDAT
IEND
tHUM 1 2 # Identify the thumbnail images.
DISC # Discard them.
FRAM 2 0 0 0 0 # Composite frame, 1024 x 512
DEFI 1 0 6 6 # Define first (bottom) image
IHDR 500 500 16 0 .. # A 16-bit graylevel image
gAMA 50000
IDAT ...
IEND # End of image
CLON 1 2 0 1 0 518 6 # Make a full clone
DHDR 2 1 5 # Modify it (no change to pixels).
INHR gAMA 0 tEXt 3 faLT 0 IDAT 4 # establish chunk order
gAMA 100000 # gamma value is 100000 (gamma=1.0)
tEXtComment\0The faLT chunk is described in ftp://swrinde....
faLT ... # Apply pseudocolor to basis image
DEND # End of image
DEFI 3 0 900 400 # Overlay near lower right-hand corner
IHDR 101 101 2 3 ...
gAMA 50000 # We need a new gAMA because
PLTE ... # this is not a PND datastream
tRNS ... # It's 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
\212 M N G \r \n ^z \n # MNG signature
MHDR 512 512 # Start of MNG datastream
30 # ticklength
FRAM 2 2 0 0 0 3 # First frame; set frame_duration=3 ticks
DEFI 1 # Define image 1 (LOCA 0 0 implied)
IHDR 512 512 ... # It's a full-display PNG image
etc # Chunks according to PNG spec
IEND # SHOW 0 1 1 is implied by DEFI 1
DEFI 2 # Define image 2
LOCA 2 0 300 200 # Location for image 2
IHDR 32 32 ... # It's a small PNG
gAMA 50000
IDAT ...
DEND # IEND is omitted
FRAM # Start new frame
# New location for image 1 is still 0,0
SHOW 1 # Display image 1 from previous frame
LOCA 2 1 10 5 # New (delta) location for image 2
SHOW 2 # Retrieve image 2 from previous frame,
CLON 2 3 0 # make a full clone of it as image 3
0 400 500 # Location for image 3
DHDR 3 1 3 0 0 0 0 # Modify image 3
tRNS ... # Make it semitransparent
DEND # SHOW 3 is implied by CLON visibility
FRAM # 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
LOCA 2 1 10 5 # New (delta) location for image 2
LOCA 3 1 5 -2 # New location for image 3
SHOW 1 3 # Show images 1 through 3
FRAM # Another frame
etc.
FRAM
etc. # More frames
MEND # End of MNG datastream
\212 M N G \r \n ^z \n # MNG signature
MHDR 64 64 # Width, height
30 # Tick length
BACK 0 52800 52800 52800 # "Browser gray" default background
FRAM 1 2 0 0 0 3 # First frame; set frame_duration=3 ticks
DEFI 1
IHDR ... # PNG header
PLTE ...
tRNS ... # Entries are zero for the transparent color
# and 255 for the nontransparent ones.
IDAT ...
IEND
fPRi 0 0 255 # Give the fade-in sequence a low priority
CLON 1 2 # Make a working copy of the image because
# it will be modified during the low-priority
# part of the datastream. It's a full clone.
DHDR 2 1 3 64 64 0 0
fADE 0 254 # Subtract 254 to alpha for all nontransparent
# pixels to bring them down from 255 to 1
LOOP 0 0 15
DHDR 2 1 3 64 64 0 0
fADE 2 16 # Add 16 to alpha for all nontransparent
DEND # colors.
ENDL 0 # Repeat loop. After 15 iterations, the
# opaque colors will end up with alpha=241
# and the transparent ones will still be 0.
DISC 2 # Discard the working copy
fPRi 0 255 255 # Give the final frame the highest value
FRAM 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
\212 M N G \r \n ^z \n # MNG signature
MHDR 150 150 # Width, height
1 # Tick length (unused in this example)
tEXtTitle\0Weather modeling results
tEXtComment\0The pcAL, xsCL, ysCL, zsCA, and tsCL chunks
in this file are written according to the PNG Sci-vis
chunks specification version 19960921 available at
ftp://swrinde.nde.swri.edu/pub/png-group/documents/
xsCLkilometers\0 0\0 150 # Sci-vis "xsCL" chunk
ysCLkilometers\0 0\0 150 # Sci-vis "ysCL" chunk
zsCAHeight (kilometers)\0 0\0 15
tsCLTime (hours)\0 0\0 24 # See proposed sci-vis chunks
pcAL 0 2 Degrees Celsius\0 0\0 45 # Sci-vis "pcAL" chunk
DEFI 1 # all images will have image_id = 1
SAVE
SEEK 0 3588720 # 3588720 bytes to the next SEEK chunk
FRAM 4 0 0 0 0 # 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's image 0
DHDR 1 1 0 # Source=0, PNG, pixel subtraction,
150 150 0 0 # Block is entire image
IDAT ... # IHDR is omitted; everything matches top
DEND # IEND is also omitted
etc. # Repeat DHDR through DEND 148 more times
SEEK 3588720 4621885
FRAM # End of first block
etc. # Repeat FRAM through SEEK 19 more times
SEEK 2285321 0
MEND # End of MNG
"Draw mode" version:
\212 M N G \r \n ^z \n # MNG signature MHDR 1024 768 1 # Start of MNG datastream FRAM 4 0 0 0 0 DEFI 1 1 0 -64 # Set up an offscreen copy of the tile IHDR 128 64 ... PLTE ... IDAT ... # Nothing will be displayed because it's IEND ... # outside the 1024 by 768 composite frame LOOP 0 0 12 # Y loop -- make 12 rows of tiles LOCA 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 LOCA 2 1 0 128 # Move it to the right 128 columns SHOW 2 # Use the second copy ENDL 1 ENDL 0 MENDTo make a "sprite mode" version: We must unroll the loops and make 95 partial clones:
\212 M N G \r \n ^z \n # MNG signature MHDR 1024 768 1 # Start of MNG datastream FRAM 3 0 0 0 0 DEFI 1 1 # Corner tile, image_id=1, invisible. IHDR 128 64 ... PLTE ... IDAT ... IEND ... CLON 1 2 1 1 0 128 # Make partial clones, delta located CLON 2 3 1 1 0 128 # Locate each one 128 columns to the CLON 3 4 1 1 0 128 # right of its basis image etc. CLON 1 13 1 1 0 64 # Make a partial clone below image 1 CLON 13 14 1 1 0 128 # More clones to the right etc. SHOW 1 96 # Show the image and its partial clones MEND
\212 M N G \r \n ^z \n # MNG signature
MHDR 512 256 # Width and height on screen
30 # Tick length
FRAM 2 2 0 0 0 1 # frame_duration = 1 tick
DEFI 1 1 0 256 # Define image 1 but don't display now
# Initially it's 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
FRAM
LOCA 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
SHOW 1 2 # Show the two images
ENDL 0
MEND
begin
write "MHDR" chunk
write "BACK" chunk
saved_images := 0
frame_duration := 0
first_frame := TRUE
if(loops>1) "write LOOP 0 1 loops"
for subimage in gif89a file do
if(frame_duration != gif_duration) then
frame_duration := gif_duration
write "FRAM 2 2 2 0 2 0 frame_duration 0" chunk
first_frame := FALSE
else if(first_frame == TRUE)then
write "FRAM 2 0 0 0 0" chunk
first_frame := FALSE
else
write "FRAM" chunk
endif
if(x_loc == 0 AND y_loc == 0) then
write "DEFI saved_images 1" chunk
else
write "DEFI saved_images 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" chunk
if(loops > 1)write "DISC" and "ENDL 0" chunks
write "MEND" chunk
end
Where "<image>" represents a PNG or PND datastream containing the
GIF frame converted to PNG format.
Caution: if you write such a program you might have to pay royalties in order to use it or to convey it to anyone else.
Phone: (410) 278-6554
EMail: glennrp@arl.mil or randeg@alumni.rpi.edu
End of MNG Specification. Expires 30 Jun 1997