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LibGfx: Add CanonicalCode wrapper to webp lossless decoder

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WebP lossless differs from deflate in how it handles 1-element codes.
Deflate consumes one bit from the bitstream to produce the element,
while webp lossless consumes 0 bits. Add a wrapper class to handle
this case.
This commit is contained in:
Nico Weber 2023-04-06 13:04:11 -04:00 • committed by Linus Groh
parent 60100c1389
commit b1cde0d432
1 changed files with 63 additions and 7 deletions
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70
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -298,19 +298,75 @@ static ErrorOr<VP8LHeader> decode_webp_chunk_VP8L_header(WebPLoadingContext& con
return VP8LHeader { width, height, is_alpha_used };
}
namespace {
// WebP-lossless's CanonicalCodes are almost identical to deflate's.
// One difference is that codes with a single element in webp-lossless consume 0 bits to produce that single element,
// while they consume 1 bit in Compress::CanonicalCode. This class wraps Compress::CanonicalCode to handle the case
// where the codes contain just a single element, and dispatch to Compress::CanonicalCode else.
class CanonicalCode {
public:
CanonicalCode()
: m_code(0)
{
}
static ErrorOr<CanonicalCode> from_bytes(ReadonlyBytes);
ErrorOr<u32> read_symbol(LittleEndianInputBitStream&) const;
private:
explicit CanonicalCode(u32 single_symbol)
: m_code(single_symbol)
{
}
explicit CanonicalCode(Compress::CanonicalCode code)
: m_code(move(code))
{
}
Variant<u32, Compress::CanonicalCode> m_code;
};
ErrorOr<CanonicalCode> CanonicalCode::from_bytes(ReadonlyBytes bytes)
{
auto non_zero_symbols = 0;
auto last_non_zero = -1;
for (size_t i = 0; i < bytes.size(); i++) {
if (bytes[i] != 0) {
non_zero_symbols++;
last_non_zero = i;
}
}
if (non_zero_symbols == 1)
return CanonicalCode(last_non_zero);
return CanonicalCode(TRY(Compress::CanonicalCode::from_bytes(bytes)));
}
ErrorOr<u32> CanonicalCode::read_symbol(LittleEndianInputBitStream& bit_stream) const
{
return TRY(m_code.visit(
[](u32 single_code) -> ErrorOr<u32> { return single_code; },
[&bit_stream](Compress::CanonicalCode const& code) { return code.read_symbol(bit_stream); }));
}
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#61_overview
// "From here on, we refer to this set as a prefix code group."
class PrefixCodeGroup {
public:
Compress::CanonicalCode& operator[](int i) { return m_codes[i]; }
Compress::CanonicalCode const& operator[](int i) const { return m_codes[i]; }
CanonicalCode& operator[](int i) { return m_codes[i]; }
CanonicalCode const& operator[](int i) const { return m_codes[i]; }
private:
Array<Compress::CanonicalCode, 5> m_codes;
Array<CanonicalCode, 5> m_codes;
};
}
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#621_decoding_and_building_the_prefix_codes
static ErrorOr<Compress::CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream, size_t alphabet_size)
static ErrorOr<CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream, size_t alphabet_size)
{
// prefix-code = simple-prefix-code / normal-prefix-code
bool is_simple_code_length_code = TRY(bit_stream.read_bits(1));
@ -338,7 +394,7 @@ static ErrorOr<Compress::CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPL
code_lengths[symbol1] = 1;
}
return Compress::CanonicalCode::from_bytes(code_lengths);
return CanonicalCode::from_bytes(code_lengths);
}
// This has plenty in common with deflate (cf DeflateDecompressor::decode_codes() in Deflate.cpp in LibCompress)
@ -375,7 +431,7 @@ static ErrorOr<Compress::CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPL
return context.error("WebPImageDecoderPlugin: invalid max_symbol");
}
auto const code_length_code = TRY(Compress::CanonicalCode::from_bytes({ code_length_code_lengths, sizeof(code_length_code_lengths) }));
auto const code_length_code = TRY(CanonicalCode::from_bytes({ code_length_code_lengths, sizeof(code_length_code_lengths) }));
// Next we extract the code lengths of the code that was used to encode the block.
@ -420,7 +476,7 @@ static ErrorOr<Compress::CanonicalCode> decode_webp_chunk_VP8L_prefix_code(WebPL
return Error::from_string_literal("Number of code lengths does not match the sum of codes");
dbgln_if(WEBP_DEBUG, " done reading symbols");
return Compress::CanonicalCode::from_bytes(code_lengths);
return CanonicalCode::from_bytes(code_lengths);
}
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#622_decoding_of_meta_prefix_codes