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http://www.w3.org/pub/WWW/TR/WD-png.
Whenever you use any of these unregistered chunks you should also include a tEXt chunk describing it, for example:
tEXtComment\0 This file contains a pcAL chunk written according to the format given in Version 19960914 of the PNG Proposed Chunks document.For those proposed chunks that have a "signature" field, decoders should check to ensure that the signature field is present and that its contents exactly match the specified string.
Chunk name Multiple Ordering
OK? constraints
drNG/DrNG No Before IDAT
faLT No Before IDAT
loGE/LoGE No Before IDAT
ttSC (used in multi-image format)
xxSC No Before IDAT
yySC No Before IDAT
zzSC (used in multi-image format)
pcAL No Before IDAT
This chunk can be used when the pixel values do not occupy the full range of possible values, and when bit depth scaling is not appropriate. It can also be used to enhance contrast by scaling to a larger range than the actual range of pixel values. The chunk defines the pixel values that correspond to minimum and maximum intensity, to which the pixel data is to be scaled.
The chunk can be used as a critical chunk, named DrNG, or as an ancillary chunk, named drNG. The syntax and function is exactly the same whichever name is used. The critical version should only be used if the image cannot be meaningfully displayed without performing DrNG scaling. Decoders not recognizing DrNG will not attempt to display the image at all. Encoders are strongly encouraged to scale the image data properly and to use the noncritical version, drNG, if at all possible.
This chunk's contents are
min_sample m bytes Raw sample value corresponding to
minimum intensity of the graylevel
or red channel, written as a text
floating-point value [link to
Floating-Point Values in extensions
document]
0 1 byte a zero byte to separate fields
max_sample n bytes Raw sample value corresponding to
maximum intensity for the graylevel
or red channel
(The following entries can be omitted for grayscale images, or for
color images where identical values are to be applied to all three color
channels)
0 1 byte a zero byte to separate fields
min_green m bytes raw sample value corresponding to
minimum green intensity
0 1 byte a zero byte to separate fields
max_green m bytes raw sample value corresponding to
maximum green intensity
0 1 byte a zero byte to separate fields
min_blue m bytes raw sample value corresponding to
minimum blue intensity
0 1 byte a zero byte to separate fields
max_blue m bytes raw sample value corresponding to
maximum blue intensity
If this chunk is present, graylevel or color sample values are to be
scaled for display between minimum and maximum intensity by linear
interpolation. When the sample value falls outside the range
min_value..max_value, it is to be set to min_value or max_value as
appropriate. For each graylevel or color sample, the conversion is
ratio := (2^output_bit_depth - 1)/(max_value - min_value) result := (input_sample - min_value) * ratio output_sample := LIMIT (0, result, 2^output_bit_depth-1) in which LIMIT (low, x, high) = MAX (MIN (x, high), low).In indexed-color images, output_bit_depth == 8 in this calculation, regardless of the bit depth actully used to store the indices. For other color types, output_bit_depth is the same as the bit_depth given in the IHDR chunk.
Note that min_value and max_value are permitted to be negative, positive, or zero. The only restriction is that they be different from each other.
Encoders should not use drNG/DrNG in lieu of reasonably scaling the samples. For example, if the sample values range from 0 (black) to 1000 (white), it would be an extremely bad idea to use the drNG chunk to request the decoder to scale 1000 up to 65535, because a PNG viewer that does not understand drNG/DrNG (and, since this is an extension chunk, most viewers will probably not understand drNG/DrNG) would simply display a very dark rectangle. Instead, multiply your samples by 64, and use drNG with max_sample=64000 to request the decoder to do the final adjustment.
If the tRNS chunk is present, its value is compared to the unscaled pixel value, prior to applying the drNG/DrNG scaling. The faLT, gAMA, cHRM, and alpha conversions, if present, are applied to the scaled sample values.
If present, this chunk must appear before the first IDAT chunk. Only one drNG/DrNG chunk is permitted in a PNG file.
This chunk is used with grayscale data, where the pixels are to be rendered in a false color according to the grayscale value. The grayscale sample of each pixel serves as a pointer into the false-color palette.
This chunk's contents are a zero-byte-terminated text string (purpose) that names the palette, followed (after the 20-byte signature and its null-byte separator) by a series of false-color palette entries:
purpose: n bytes (Latin-1 text)
separator: 1 byte, null
signature: 20 bytes ("PNG group 1996-09-14")
separator: 1 byte, null
index: 2 bytes, range 0 .. (2^bitdepth) -1)
red: 2 bytes, range 0 .. 65535
green: 2 bytes, range 0 .. 65535
blue: 2 bytes, range 0 .. 65535
etc.
The number of entries is determined from the remaining chunk length after
the signature field and the null separator. This length not divisible by 8 is
an error.
The "purpose" identifies the palette, which can permit applications or people to choose the appropriate one when more than one false-color palette is stored in a multiple-image file. The "purpose" string must follow the format of a tEXt keyword, i.e. 1-79 printable Latin-1 characters.
The gamma field gives the value of gamma, times 100000, that is associated with the false-color palette entries.
This chunk can appear for color type 0 or color type 4. If it appears for any other color type, it will be ignored.
The complete 2^bitdepth-entry false-color palette can be built from the chunk data. If the first entry (index value 0) is missing, it will be assumed to be {0, 0, 0} (black). If the last entry (index value 2^bitdepth - 1) is missing, it will be assumed to be {65535, 65535, 65535} (white). The red, green, blue samples for other missing entries are filled in by linearly interpolating between the samples that are present, independently for each of the three color components.
Once the complete false-color palette is established, it is used similarly to PLTE. The first entry in the completed false-color palette is referenced by the grayscale value 0, the second by grayscale value 1, etc.
If the tRNS chunk is present, its value is compared to the graylevel, not to the converted false-color of the pixels. If the bKGD chunk is present, background pixels will be displayed in the false-color corresponding to the grayscale value found in the bKGD chunk. The cHRM and alpha conversions, if present, are applied to the color samples in the converted false-color pixels. The gAMA chunk is ignored when the faLT chunk is processed, and the supplied gamma value is used instead.
Note that the gAMA and other values must be selected so that the grayscale image is meaningfully displayed when the faLT chunk is unrecognized or ignored.
If this chunk does appear, it must precede the first IDAT chunk. There can be only one faLT chunk in a PNG file.
The loGE/LoGE chunk is used to decode image data whose grayscale or color samples have been logarithmically encoded.
The chunk can be used as a critical chunk, named LoGE, or as an ancillary chunk, named loGE. The syntax and function is exactly the same whichever name is used. The critical version should only be used if the image cannot be meaningfully displayed without performing LoGE scaling. Decoders not recognizing LoGE will not attempt to display the image at all. Encoders are strongly encouraged to include a gAMA chunk that permits a meaningful if not completely accurate display, and use the noncritical version, loGE, if at all possible.
The loGE/LoGE chunk's contents are
P0 n0 bytes (First parameter, a real number
written as a text floating-point
value [link to Floating-Point
Values in extensions document]
Null separator: 1 byte
P1 n1 bytes (Second parameter)
Null separator: 1 byte
P2 n2 bytes (Third parameter)
There is no trailing zero for the final string.
The scaling algorithm is
normalized_sample_value := input_sample/(2^input_bitdepth-1) scaled_value := P0 + P1 * P2^normalized_sample_value output_sample := LIMIT (0, scaled_value, 2^output_bitdepth-1) in which LIMIT (low, x, high) = MAX (MIN (x, high), low).For color_types 0 and 4, the normalized sample value is the grayscale value of the pixel normalized to the range [0.0:1.0] by dividing it by the maximum value for the input bit depth, using floating point arithmetic.
For color_types 2, 3, and 6, the scaling algorithm is applied independently to each of the color samples similarly normalized to [0.0:1.0].
The alpha channel, if present, is not affected by the loGE/LoGE transformation. Alpha compositing is done in the normal manner, with the transformed pixels forming the foreground image [link to Decoders: Alpha Processing in core spec]
Pure logarithmic data can be expressed with
P0 := 0 P1 := min_val P2 := max_val/min_valIn which the range [min_val..max_val] includes the minimum and maximum values appearing in the source data. When the image is decoded using the loGE/LoGE data, the gamma calculation for the decoded samples should be done as though the file_gamma were 1.0, regardless of the contents of the gAMA chunk.
It is advisable to include a gAMA chunk with logarithmic data, in case a viewer does not use the loGE data to decode it. A value of gamma should be chosen that allows the image to be displayed as well as it can be with a viewer that supports gAMA but not loGE.
You can select a value of gamma (also called file_gamma) by eye as described in [link to Gamma Tutorial: General Gamma Handling in core spec], or you can calculate one as follows:
If the maximum value of the logarithmically decoded pixels can reasonably be displayed as white on the monitor, then specifying a value of gamma given by
gamma := LN( LN(0.2) / LN(max_val/min_val) + 1) / LN(0.2) (in which LN() is the natural logarithm function)causes the two values max_val and 0.2*max_val to be reproduced at correct brightness on screen - these can be thought of as white and mid-grey. Tones between white and mid-grey will be a bit too bright, ones darker than mid-grey will be increasingly too dark, but it's a reasonable approximation for viewable display. A max_val/min_val ratio of 64 gives a gamma of about 0.3, a max_val/min_val of 1000 gives a gamma of about 0.165, so a viewer assuming a default gamma of 0.45 is going to give a bright, washed-out image for any log-encoded image.
In cases where the maximum sample value is many times brighter than scene white (i.e. the image is encoded to retain specular highlight information), the formula above doesn't apply, but there's probably no way of getting such an image to display reasonably anyway with a viewer that doesn't understand loGE.
If present, the loGE/LoGE chunk must appear before the first IDAT chunk. Only one instance of the loGE/LoGE chunk is permitted in a PNG datastream.
When a PNG file is being used to store physical data other than color values, such as a two-dimensional temperature field, the pcAL chunk can be used to record the relationship between pixel values and actual physical values. The pcAL data might be used to construct a reference color bar beside the image. It is not expected to affect the way the image is displayed.
The pcAL chunk's contents are are a zero-byte-terminated text string (purpose) that names the equation, followed (after the 20-byte signature and its null-byte separator) by an equation type and a set of parameters for the equation:
purpose: n bytes (Latin-1 text)
separator: 1 byte, null
signature: 20 bytes ("PNG group 1996-10-11")
separator: 1 byte, null
Equation type 1 byte (0 for linear scaling,
1 for base-e exponential scaling,
2 for arbitrary-base exponential
scaling,
3 for hypberbolic scaling)
N 1 byte (Number of parameters)
Unit u bytes (Latin-1 symbol or description of
the unit, eg. K, Population Density,
MPa, etc). A zero-length string can
be used if the data is dimensionless.
Null separator: 1 byte
P0 p0 bytes (First parameter, a real number
written as a text floating-point
value [link to Floating-Point Values
in PNG extensions document]
Null separator: 1 byte
P1 p1 bytes (Second parameter)
Null separator: 1 byte (must appear if P2 is present)
P2 p2 bytes (Third parameter, optional)
etc.
Null separator: 1 byte (must appear if PM is present)
PM pm bytes (M+1 parameter, optional, where
M < N. If PM is present, P[M-1]
must also be present)
There is no trailing zero for the final string.
The "purpose" identifies the equation, which can permit applications or people to choose the appropriate one when more than one scaling equation is stored in a multiple-image file. The "purpose" string must follow the format of a tEXt keyword, i.e. 1-79 printable Latin-1 characters. One way the "purpose" field could be used is to identify the system of units. A "purpose" string such as "SI" or "English" could be used in the pcAL chunk as well as in the xxSC and yySC chunks, to permit a decoder to select an appropriate set of chunks based on the contents of their "purpose" fields.
The scaling algorithm is
if equation_type == 0 then scaled_value := P0 + P1 * normalized_sample_value else if equation_type == 1 then scaled_value := P0 + P1 * EXP(P2 * normalized_sample_value) else if equation_type == 2 then scaled_value := P0 + P1 * P2^normalized_sample_value else if equation_type == 3 then scaled_value := P0 + P1*SINH((normalized_sample_value - P2)/P3)in which
SINH(x) = 0.5 * ( EXP(x) - EXP(-x) )
For color_types 2, 3, and 6, the scaling algorithm is applied independently to each of the color sample values similarly normalized to [0.0:1.0].
Pure logarithmic data can be expressed either with
Equation_type =: 1 Equation_type := 2 N =: 3 or with N := 3 P0 =: 0 P0 := 0 P1 =: min P1 := min P2 =: LOGe(max/min) P2 := max/minThe pcAL data is not intended to be used by a decoder to affect the way the pixels are displayed. pcAL is simply a comment, and could be used, for example, to construct a reference color bar scale beside the image, or to extract the original physical values recorded in the file. The drNG/DrNG or loGE/LoGE chunk should be used if the encoder wants the decoder to modify the sample values for display purposes.
If present, the pcAL chunk must appear before the first IDAT chunk. Only one instance of the pcAL chunk is permitted in a PNG stream.
This is like xxSC chunk except that it gives the scale and offset in the frame-to-frame direction, which would normally be the physical time scale. It would be meaningless in a PNG stream, but in a multi-image format it could be used at the top level, to provide the physical scale (rather than the display frame rate) in the frame-to-frame direction.
This chunk relates the actual dimension of the image subject in the "x" (width) direction to the column position of a pixel. It provides some additional capability beyond that available in the sCAL [link to sCAL in PNG extensions document] chunk, which does not provide for an offset, and does not provide for different units in the width and height directions. The latter could be useful, for example, in racing photo-finish "streak" images, where one of the dimensions is time and the other is length. The chunk does not affect the image display, but could be used to construct scaled axes adjacent to the image.
The xxSC chunk's contents are
purpose: n bytes (Latin-1 text)
separator: 1 byte, null
signature: 20 bytes ("PNG group 1996-10-11")
separator: 1 byte, null
Xunit xu bytes (Latin-1 symbol or description of
the width unit, eg. milliseconds,
degrees West Longitude, etc).
A zero-length string can be used if
the data is dimensionless.
Null separator: 1 byte
Xoffset xo bytes X offset, a real number written as a
text floating-point value [link to
Floating-Point Values in PNG
extensions document] This is the
physical value of "x" corresponding
to the left edge of the image.
Null separator: 1 byte
Xscale xs bytes X scale, the x-distance corresponding
to the width of a pixel. Must be
non-zero.
There is no trailing zero for the final string.
The scaling algorithm for finding the physical x-value of the middle of a pixel is
physical_x_value := Xoffset + Xscale * (column_number + 0.5)If present, the xxSC chunk must appear before the first IDAT chunk. Only one instance of the xxSC chunk is permitted in a PNG stream.
This chunk is similar to the xxSC chunk. It relates the actual dimension of the image subject in the "y" (height) direction to the row position of a pixel, positive downward from the top.
The yySC chunk's contents are
purpose: n bytes (Latin-1 text)
separator: 1 byte, null
signature: 20 bytes ("PNG group 1996-10-11")
separator: 1 byte, null
Yunit yu bytes (Latin-1 symbol or description of
the width unit, eg. milliseconds,
degrees North Latitude, etc).
A zero-length string can be used if
the data is dimensionless.
Null separator: 1 byte
Yoffset yo bytes Y offset, a real number written as a
text floating-point value [link to
Floating-Point Values in extensions
document] This is the physical value
of "y" corresponding to the top
edge of the image.
Null separator: 1 byte
Yscale ys bytes Y scale, the y-distance corresponding
to the height of a pixel. Must be
non-zero. The positive direction is
downward.
There is no trailing zero for the final string.
The scaling algorithm for finding the physical y-value of the middle of a pixel is
physical_y_value := Yoffset + Yscale * (row_number + 0.5)If present, the yySC chunk must appear before the first IDAT chunk. Only one instance of the yySC chunk is permitted in a PNG stream.
This is like xxSC chunk except that it gives the z-scale and offset in the thickness direction, when a series of images are considered to be layers of voxels (tomographic slices). It would be meaningless in a PNG stream, but in a multi-image format it could be used to provide the physical dimension in the frame-to-frame direction, or when images are grouped as in a composite frame, to give the dimension in the plane-to-plane direction within the group.
http://www.w3.org/pub/WWW/TR/WD-png.
The same precautions taken when displaying tEXt data should be taken when displaying the text contained in the "unit" strings of the xxSC and pcAL chunks and the "purpose" string of various chunks. Viewers should not display these strings directly without first checking for the presence of nonprintable characters, and for the <ESC> character in particular.
No known additional security hazards are posed by the chunks described here.