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LibGfx: Implement most of COLOR_INDEXING_TRANSFORM for webp decoder

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Doesn't yet implement pixel packing for when the palette has fewer
than 16 colors.
This commit is contained in:
Nico Weber 2023-04-06 21:19:45 -04:00 committed by Linus Groh
parent 6f4fdd85b7
commit a915d07293
Notes: sideshowbarker 2024-07-17 07:25:39 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/a915d07293
Pull-request: https://github.com/SerenityOS/serenity/pull/18221
1 changed files with 54 additions and 1 deletions
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55
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -1062,6 +1062,58 @@ ErrorOr<void> SubtractGreenTransform::transform(Bitmap& bitmap)
return {};
}
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#44_color_indexing_transform
class ColorIndexingTransform : public Transform {
public:
static ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&);
virtual ErrorOr<void> transform(Bitmap&) override;
private:
explicit ColorIndexingTransform(NonnullRefPtr<Bitmap> palette_bitmap)
: m_palette_bitmap(palette_bitmap)
{
}
NonnullRefPtr<Bitmap> m_palette_bitmap;
};
ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebPLoadingContext& context, LittleEndianInputBitStream& bit_stream)
{
// color-indexing-image = 8BIT ; color count
// entropy-coded-image
int color_table_size = TRY(bit_stream.read_bits(8)) + 1;
IntSize palette_image_size { color_table_size, 1 };
// "When the color table is small (equal to or less than 16 colors), several pixels are bundled into a single pixel...."
// FIXME: Implement support for this pixel packing.
if (color_table_size <= 16)
return Error::from_string_literal("WebPImageDecoderPlugin: COLOR_INDEXING_TRANSFORM pixel packing not yet implemented");
auto palette_bitmap = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::EntropyCoded, BitmapFormat::BGRA8888, palette_image_size, bit_stream));
// "The color table is always subtraction-coded to reduce image entropy. [...] In decoding, every final color in the color table
// can be obtained by adding the previous color component values by each ARGB component separately,
// and storing the least significant 8 bits of the result."
for (ARGB32* pixel = palette_bitmap->begin() + 1; pixel != palette_bitmap->end(); ++pixel)
*pixel = add_argb32(*pixel, pixel[-1]);
return adopt_nonnull_own_or_enomem(new (nothrow) ColorIndexingTransform(move(palette_bitmap)));
}
ErrorOr<void> ColorIndexingTransform::transform(Bitmap& bitmap)
{
// FIXME: If this is the last transform, consider returning an Indexed8 bitmap here?
for (ARGB32& pixel : bitmap) {
// "The inverse transform for the image is simply replacing the pixel values (which are indices to the color table)
// with the actual color table values. The indexing is done based on the green component of the ARGB color. [...]
// If the index is equal or larger than color_table_size, the argb color value should be set to 0x00000000 (transparent black)."
u8 index = Color::from_argb(pixel).green();
pixel = index < m_palette_bitmap->width() ? m_palette_bitmap->scanline(0)[index] : 0;
}
return {};
}
}
// https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossless
@ -1133,7 +1185,8 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
TRY(transforms.try_append(TRY(try_make<SubtractGreenTransform>())));
break;
case COLOR_INDEXING_TRANSFORM:
return context.error("WebPImageDecoderPlugin: VP8L COLOR_INDEXING_TRANSFORM handling not yet implemented");
TRY(transforms.try_append(TRY(ColorIndexingTransform::read(context, bit_stream))));
break;
}
}