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LibGfx/WebP: Implement compressed ALPH chunk reading

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A compressed ALPH chunk is a lossless webp bitstream, but without
the 5 byte "header" that stores width, height, is-alpha-channel-used
(it never is for an ALPH chunk since the ALPH chunk gets the alpha
data out of the lossless webp's green channel), and version fields.

For that reason, this cuts decode_webp_chunk_VP8L() into the
header-reading part and the remaining part, so that the remaining
part can be called by the ALPH reading routine.

Lossy webp files with a (losslessly) compressed alpha channel can
be found in the wild. Since we can't decode lossy webp data yet,
change the `#if 0` in decode_webp_chunk_VP8() to `#if 1` to test this.
This commit is contained in:
Nico Weber 2023-05-06 21:32:45 -04:00 committed by Andreas Kling
parent fab6a3915e
commit 0d2e6162db
Notes: sideshowbarker 2024-07-17 09:49:33 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/0d2e6162db
Pull-request: https://github.com/SerenityOS/serenity/pull/18690
1 changed files with 21 additions and 4 deletions
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25
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
View file

@ -1230,14 +1230,22 @@ ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<B
// https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossless
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#7_overall_structure_of_the_format
static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_contents(WebPLoadingContext& context, VP8LHeader const& vp8l_header, ReadonlyBytes lossless_data);
static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk const& vp8l_chunk)
{
VERIFY(context.first_chunk->type == FourCC("VP8L") || context.first_chunk->type == FourCC("VP8X"));
VERIFY(vp8l_chunk.type == FourCC("VP8L"));
auto vp8l_header = TRY(decode_webp_chunk_VP8L_header(context, vp8l_chunk));
ReadonlyBytes lossless_data = vp8l_chunk.data.slice(5);
return decode_webp_chunk_VP8L_contents(context, vp8l_header, lossless_data);
}
FixedMemoryStream memory_stream { vp8l_chunk.data.slice(5) };
// Decodes the lossless webp bitstream following the 5-byte header information.
static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_contents(WebPLoadingContext& context, VP8LHeader const& vp8l_header, ReadonlyBytes lossless_data)
{
FixedMemoryStream memory_stream { lossless_data };
LittleEndianInputBitStream bit_stream { MaybeOwned<Stream>(memory_stream) };
// image-stream = optional-transform spatially-coded-image
@ -1325,9 +1333,10 @@ static ErrorOr<void> decode_webp_chunk_ALPH(WebPLoadingContext& context, Chunk c
ReadonlyBytes alpha_data = alph_chunk.data.slice(1);
size_t pixel_count = bitmap.width() * bitmap.height();
if (compression_method == 0) {
// "Raw data: consists of a byte sequence of length width * height, containing all the 8-bit transparency values in scan order."
size_t pixel_count = bitmap.width() * bitmap.height();
if (alpha_data.size() < pixel_count)
return context.error("WebPImageDecoderPlugin: uncompressed ALPH data too small");
for (size_t i = 0; i < pixel_count; ++i)
@ -1339,8 +1348,16 @@ static ErrorOr<void> decode_webp_chunk_ALPH(WebPLoadingContext& context, Chunk c
// of implicit dimension width x height. That is, this image-stream does NOT contain any headers describing the image dimension.
// Once the image-stream is decoded into ARGB color values, following the process described in the lossless format specification,
// the transparency information must be extracted from the green channel of the ARGB quadruplet."
// FIXME
return context.error("WebPImageDecoderPlugin: compressed ALPH chunk processing not yet implemented");
VP8LHeader vp8l_header = { static_cast<u16>(bitmap.width()), static_cast<u16>(bitmap.height()), /*is_alpha_used=*/false };
auto lossless_bitmap = TRY(decode_webp_chunk_VP8L_contents(context, vp8l_header, alpha_data));
if (pixel_count != static_cast<size_t>(lossless_bitmap->width() * lossless_bitmap->height()))
return context.error("WebPImageDecoderPlugin: decompressed ALPH dimensions don't match VP8 dimensions");
for (size_t i = 0; i < pixel_count; ++i)
bitmap.begin()[i] |= (lossless_bitmap->begin()[i] & 0xff00) << 16;
return {};
}
static ErrorOr<VP8XHeader> decode_webp_chunk_VP8X(WebPLoadingContext& context, Chunk const& vp8x_chunk)