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Fix reading GIF images without CLEAR code

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The encoder may chose not to emit a CLEAR code when the alphabet is full
according to the specification, but wxGIFDecoder assumed it always did,
preventing it from reading some valid GIF files (in practice, most often
animated ones).

This commit is best viewed ignoring whitespace-only changes.

Closes #22799.

Closes #24234.
This commit is contained in:
Mark Wagner 2024-01-20 16:29:43 -08:00 • committed by Vadim Zeitlin
parent c72f4efc9c
commit 8eb3729fec
1 changed files with 19 additions and 21 deletions
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40
src/common/gifdecod.cpp
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@ -443,28 +443,26 @@ wxGIFDecoder::dgif(wxInputStream& stream, GIFImage *img, int interl, int bits)
// make new entry in alphabet (only if NOT just cleared)
if (lastcode != -1)
{
// Normally, after the alphabet is full and can't grow any
// further (ab_free == 4096), encoder should (must?) emit CLEAR
// to reset it. This checks whether we really got it, otherwise
// the GIF is damaged.
if (ab_free > ab_max)
return wxGIF_INVFORMAT;
// This assert seems unnecessary since the condition above
// eliminates the only case in which it went false. But I really
// don't like being forced to ask "Who in .text could have
// written there?!" And I wouldn't have been forced to ask if
// this line had already been here.
wxASSERT(ab_free < allocSize);
ab_prefix[ab_free] = lastcode;
ab_tail[ab_free] = code;
ab_free++;
if ((ab_free > ab_max) && (ab_bits < 12))
// The GIF specification does not require sending a CLEAR code
// once the alphabet is full. Instead, the encoder can continue
// with the alphabet as-is, making no further updates until a
// CLEAR code is emitted.
if (ab_free <= ab_max)
{
ab_bits++;
ab_max = (1 << ab_bits) - 1;
// It should be impossible to trigger this assert, since the
// above check should prevent the alphabet from growing
// beyond 4096 entries.
wxASSERT(ab_free < allocSize);
ab_prefix[ab_free] = lastcode;
ab_tail[ab_free] = code;
ab_free++;
if ((ab_free > ab_max) && (ab_bits < 12))
{
ab_bits++;
ab_max = (1 << ab_bits) - 1;
}
}
}