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Whenever you use an unregistered chunk you should also include a tEXt chunk describing it, for example:
tEXtComment\0 This file contains a spAL chunk written according to the format given in Version 19961022 of the PNG Proposed sPLT Chunk document.
Chunk name Multiple Ordering OK? constraints spAL Yes Before IDAT
This chunk's contents are a zero-byte-terminated text string that names the palette [followed by a 20-byte signature and a zero-byte terminator] and a 1-byte spAL_sample_depth integer, followed by a series of palette entries, each a six-byte or ten-byte series, containing five unsigned integers:
n bytes: (Latin-1 text) palette_name 1 byte: (null) terminator [ 20 bytes: signature ("PNG group 1996-10-22") 1 byte: (null) separator ] 1 byte: (unsigned integer) spAL_sample_depth must be 8 or 16 1 or 2 bytes: (unsigned integer) red intensity 0: black etc. 255 or 65535: full red intensity 1 or 2 bytes: (unsigned integer) green intensity 1 or 2 bytes: (unsigned integer) blue intensity 1 or 2 bytes: (unsigned integer) alpha 0: fully transparent etc. 255 or 65535: fully opaque 2 bytes: (unsigned integer) frequency (relative frequency of occurrence) etc.There can be any number of entries; a decoder determines the number of entries from the remaining chunk length after the "palette_name" field and its zero-byte terminator [, the "signature" and its zero-byte terminator], and the spAL_sample_depth byte. This length not divisible by six (if spAL_sample_depth==8) or by ten (if spAL_sample_depth==16) is an error. Entries must appear in decreasing order of "frequency".
The "palette_name" (e.g. "256 color including Macintosh default", "256 color including Windows-3.1 default", "Optimal 512") identifies the palette, which can permit applications or people to choose the appropriate one when more than one suggested palette appears in a PNG file. The "palette_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 name. Names are case-sensitive.
The red, green, and blue values are not premultiplied by alpha, nor are they precomposited against any background. A decoder can build a palette by compositing those palette entries against any background color or set of background colors that it chooses. See [link to bKGD]
Each frequency entry is proportional to the fraction of pixels in the image that are closest to that palette entry, without regard to any compositing against a background palette. The exact scale factor is chosen by the encoder, but should be chosen so that the range of individual values reasonably fills the range 0 to 65535. It is acceptable to artificially inflate the "frequency" values for "important" colors such as those in a company logo or in the facial features of a portrait. Zero is a valid value for frequency, meaning the color is "least important" or that it is rarely if ever used. When all of the frequency values are zero, the "frequency" is undefined.
The palette uses 8 bits or 16 bits (1 or 2 bytes) per value regardless of the image bit depth specification, according to the number given in the spAL_sample_depth field. Decoders wishing to construct a palette with a different bit depth can accomplish this by scaling the RGBA entries, as described under "[link] sample depth rescaling" in the PNG specification. The palette samples have the same gamma and chromaticity values as those of the pixel samples.
Note: Earlier versions of the PNG specification recommended that the PLTE [link to PLTE] and hIST chunks be used for this purpose. While this is still allowed, to maintain backward compatibility, the spAL chunk is preferable, particularly when transparent pixels are present. When both the PLTE and spAL chunks are present, the PLTE data should only be used for decoding the indexed-color (color type 3) pixels, and the spAL data should be used for constructing the display palette, if the viewer is not capable of displaying the number of colors present in the PLTE chunk. If the hIST chunk is also present, decoders that process the spAL chunk should ignore the hIST chunk.
This chunk can appear for any color type. If this chunk does appear, it must precede the first IDAT chunk. There can be multiple spAL chunks, with different names.
The format of this chunk has not changed since version 19961022 of this document. The chunk was originally called spLT but the name was changed to spAL in the 19960927 version of the png-proposed-chunks document when the format changed from the April 1996 (960406) version of the format.
Gamma field: In the 19960927 version, a "gamma" field was added, primarily for use in multiple-image formats. This seemed to introduce more problems than it resolved, so it has been removed from the present proposal, and multiple-image formats can define a variant of sPLT (with a different chunk name such as "gPLT") that has a gamma field, if necessary.
Signature field: In the 19960927 version, a "signature" field was added to distinguish the September version from the April version. Since the 19961008 version has the identical format to the April version, no "signature" field was needed, and it was removed.
Zero "frequency": There have been changes to the interpretation of frequency==0.
Palette sample depth: Changed from 16 bits to 8 or 16 bits in the 19961022 version. Since the format changed, the chunk name was changed back to spAL again and a new "signature" field was added. Test implementations can safely distinguish among the April 1996 version (spLT, no signature), the September 27 version (spAL, 1996-09-14 signature), and this version (spAL, 1996-10-22 signature).
The same precautions taken when displaying tEXt data should be taken when displaying the text contained in the "palette_name" [and "signature"] string of the spAL chunk. Viewers should not display this string 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.