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LibGfx: Second attempt to handle max_symbol correctly in webp decoder

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The previous attempt was in commit e5e9d3b877, where I thought
max_symbol describes how many code lengths should be read.

But it looks like it instead describes how many code length input
symbols should be read. (The two aren't the same since one code length
input symbol can produce several code lengths.)

I still agree with the commit description of e5e9d3b877 that the spec
isn't very clear on this :)

This time I've found a file that sets max_symbol and with this change
here, that file decodes correctly. (It's Qpalette.webp, which I'm about
to add as a test case.)
This commit is contained in:
Nico Weber 2023-04-07 15:18:39 -04:00 • committed by Linus Groh
parent a915d07293
commit 24967b0d29
Notes: sideshowbarker 2024-07-16 21:46:50 +09:00
1 changed files with 8 additions and 4 deletions
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12
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -438,8 +438,12 @@ static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingCont
u8 last_non_zero = 8; // "If code 16 is used before a non-zero value has been emitted, a value of 8 is repeated."
// "A prefix table is then built from code_length_code_lengths and used to read up to max_symbol code lengths."
dbgln_if(WEBP_DEBUG, " reading {} symbols", max_symbol);
while (code_lengths.size() < max_symbol) {
dbgln_if(WEBP_DEBUG, " reading {} symbols from at most {} codes", alphabet_size, max_symbol);
while (code_lengths.size() < alphabet_size) {
if (max_symbol == 0)
break;
--max_symbol;
auto symbol = TRY(code_length_code.read_symbol(bit_stream));
if (symbol < 16) {
@ -472,8 +476,8 @@ static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingCont
}
}
if (code_lengths.size() != alphabet_size)
return Error::from_string_literal("Number of code lengths does not match the sum of codes");
if (code_lengths.size() > alphabet_size)
return Error::from_string_literal("Number of code lengths is larger than the alphabet size");
dbgln_if(WEBP_DEBUG, " done reading symbols");
return CanonicalCode::from_bytes(code_lengths);