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Merge changes Id56ca4fd,I662bd1d7 into main

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* changes:
  webp-lossless-bitstream-spec: add validity note for max_symbol
  webp-lossless-bitstream-spec: fix max_symbol definition
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James Zern 2023-07-13 20:43:51 +00:00 committed by Gerrit Code Review
commit 29b9eb1517
1 changed files with 13 additions and 7 deletions
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doc/webp-lossless-bitstream-spec.txt
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@ -925,14 +925,24 @@ for (i = 0; i < num_code_lengths; ++i) {
} }
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Next, if `ReadBits(1) == 0`, the maximum number of different read symbols is Next, if `ReadBits(1) == 0`, the maximum number of different read symbols
`num_code_lengths`. Otherwise, it is defined as: (`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); int length_nbits = 2 + 2 * ReadBits(3);
int max_symbol = 2 + ReadBits(length_nbits); int max_symbol = 2 + ReadBits(length_nbits);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
If `max_symbol` is larger than the size of the alphabet for the symbol type, the
bitstream is invalid.
A prefix table is then built from `code_length_code_lengths` and used to read up A prefix table is then built from `code_length_code_lengths` and used to read up
to `max_symbol` code lengths. to `max_symbol` code lengths.
@ -948,11 +958,7 @@ to `max_symbol` code lengths.
`11 + ReadBits(7)` times. `11 + ReadBits(7)` times.
Once code lengths are read, a prefix code for each symbol type (A, R, G, B, and 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: 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
The Normal Code Length Code must code a full decision tree, that is, the sum of 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 `2 ^ (-length)` for all non-zero codes must be exactly one. There is however