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webp-lossless-bitstream-spec: fix max_symbol definition

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If ReadBits(0) == 0, the value of max_symbol is set to the alphabet size
for each symbol type.

See vp8l_dec.c, ReadHuffmanCode(), which passes alphabet_size to
ReadHuffmanCodeLengths() as num_symbols, and ReadHuffmanCodeLengths()
then sets max_symbol to that.

Bug: webp:611
Change-Id: I662bd1d7f372e7f2e9c71cc86f87aefd02f36647
This commit is contained in:
James Zern 2023-07-07 15:35:31 -07:00
parent eac3bd5c53
commit 71916726b6
1 changed files with 10 additions and 7 deletions
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17
doc/webp-lossless-bitstream-spec.txt
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@ -925,8 +925,15 @@ for (i = 0; i < num_code_lengths; ++i) {
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Next, if `ReadBits(1) == 0`, the maximum number of different read symbols is
`num_code_lengths`. Otherwise, it is defined as:
Next, if `ReadBits(1) == 0`, the maximum number of different read symbols
(`max_symbol`) for each symbol type (A, R, G, B, and distance) is set to its
alphabet size:
* G channel: 256 + 24 + `color_cache_size`
* Other literals (A, R, and B): 256
* Distance code: 40
Otherwise, it is defined as:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
int length_nbits = 2 + 2 * ReadBits(3);
@ -948,11 +955,7 @@ to `max_symbol` code lengths.
`11 + ReadBits(7)` times.
Once code lengths are read, a prefix code for each symbol type (A, R, G, B, and
distance) is formed using their respective alphabet sizes:
* G channel: 256 + 24 + `color_cache_size`
* Other literals (A, R, and B): 256
* Distance code: 40
distance) is formed using their respective alphabet sizes.
The Normal Code Length Code must code a full decision tree, that is, the sum of
`2 ^ (-length)` for all non-zero codes must be exactly one. There is however