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LibGfx: Implement color index pixel bundling in webp decoder

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See the lengthy comment added in this commit for details.

With this, the webp lossless decoder is feature complete :^)

(...except for bug fixes and performance improvements, as always.)
This commit is contained in:
Nico Weber 2023-04-07 18:14:19 -04:00 • committed by Linus Groh
parent 7309441b31
commit 1dab480015
1 changed files with 71 additions and 20 deletions
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91
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -1086,12 +1086,29 @@ public:
static ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> read(WebPLoadingContext&, LittleEndianInputBitStream&);
virtual ErrorOr<NonnullRefPtr<Bitmap>> transform(NonnullRefPtr<Bitmap>) override;
// For a color indexing transform, the green channel of the source image is used as the index into a palette to produce an output color.
// If the palette is small enough, several output pixels are bundled into a single input pixel.
// If the palette has just 2 colors, every index needs just a single bit, and the 8 bits of the green channel of one input pixel can encode 8 output pixels.
// If the palette has 3 or 4 colors, every index needs 2 bits and every pixel can encode 4 output pixels.
// If the palette has 5 to 16 colors, every index needs 4 bits and every pixel can encode 2 output pixels.
// This returns how many output pixels one input pixel can encode after the color indexing transform.
//
// The spec isn't very explicit about this, but this affects all images after the color indexing transform:
// If a webp file contains a 32x32 image and it contains a color indexing transform with a 4-color palette, then the in-memory size of all images
// after the color indexing transform assume a bitmap size of (32/4)x32 = 8x32.
// That is, the sizes of transforms after the color indexing transform are computed relative to the size 8x32,
// the main image's meta prefix image's size (if present) is comptued relative to the size 8x32,
// the main image is 8x32, and only applying the color indexing transform resizes the image back to 32x32.
int pixels_per_pixel() const { return m_pixels_per_pixel; }
private:
explicit ColorIndexingTransform(NonnullRefPtr<Bitmap> palette_bitmap)
: m_palette_bitmap(palette_bitmap)
ColorIndexingTransform(unsigned pixels_per_pixel, NonnullRefPtr<Bitmap> palette_bitmap)
: m_pixels_per_pixel(pixels_per_pixel)
, m_palette_bitmap(palette_bitmap)
{
}
int m_pixels_per_pixel;
NonnullRefPtr<Bitmap> m_palette_bitmap;
};
@ -1102,35 +1119,64 @@ ErrorOr<NonnullOwnPtr<ColorIndexingTransform>> ColorIndexingTransform::read(WebP
int color_table_size = TRY(bit_stream.read_bits(8)) + 1;
dbgln_if(WEBP_DEBUG, "colorindexing color_table_size {}", color_table_size);
// "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");
IntSize palette_image_size { color_table_size, 1 };
auto palette_bitmap = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::EntropyCoded, BitmapFormat::BGRA8888, palette_image_size, bit_stream));
// "When the color table is small (equal to or less than 16 colors), several pixels are bundled into a single pixel...."
int pixels_per_pixel = 1;
if (color_table_size <= 2)
pixels_per_pixel = 8;
else if (color_table_size <= 4)
pixels_per_pixel = 4;
else if (color_table_size <= 16)
pixels_per_pixel = 2;
// "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)));
return adopt_nonnull_own_or_enomem(new (nothrow) ColorIndexingTransform(pixels_per_pixel, move(palette_bitmap)));
}
ErrorOr<NonnullRefPtr<Bitmap>> ColorIndexingTransform::transform(NonnullRefPtr<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;
if (pixels_per_pixel() == 1) {
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 bitmap;
}
return bitmap;
// Pixel bundling case.
IntSize unbundled_size = { bitmap->size().width() * pixels_per_pixel(), bitmap->size().height() };
auto new_bitmap = TRY(Bitmap::create(BitmapFormat::BGRA8888, unbundled_size));
unsigned bits_per_pixel = 8 / pixels_per_pixel();
unsigned pixel_mask = (1 << bits_per_pixel) - 1;
for (int y = 0; y < bitmap->height(); ++y) {
ARGB32* bitmap_scanline = bitmap->scanline(y);
ARGB32* new_bitmap_scanline = new_bitmap->scanline(y);
for (int x = 0, new_x = 0; x < bitmap->width(); ++x, new_x += pixels_per_pixel()) {
u8 indexes = Color::from_argb(bitmap_scanline[x]).green();
for (int i = 0; i < pixels_per_pixel(); ++i) {
u8 index = indexes & pixel_mask;
new_bitmap_scanline[new_x + i] = index < m_palette_bitmap->width() ? m_palette_bitmap->scanline(0)[index] : 0;
indexes >>= bits_per_pixel;
}
}
}
return new_bitmap;
}
}
@ -1162,6 +1208,8 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#4_transformations
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#72_structure_of_transforms
auto stored_size = context.size.value();
// optional-transform = (%b1 transform optional-transform) / %b0
// "Each transform is allowed to be used only once."
u8 seen_transforms = 0;
@ -1195,22 +1243,25 @@ static ErrorOr<void> decode_webp_chunk_VP8L(WebPLoadingContext& context, Chunk c
switch (transform_type) {
case PREDICTOR_TRANSFORM:
TRY(transforms.try_append(TRY(PredictorTransform::read(context, bit_stream, context.size.value()))));
TRY(transforms.try_append(TRY(PredictorTransform::read(context, bit_stream, stored_size))));
break;
case COLOR_TRANSFORM:
TRY(transforms.try_append(TRY(ColorTransform::read(context, bit_stream, context.size.value()))));
TRY(transforms.try_append(TRY(ColorTransform::read(context, bit_stream, stored_size))));
break;
case SUBTRACT_GREEN_TRANSFORM:
TRY(transforms.try_append(TRY(try_make<SubtractGreenTransform>())));
break;
case COLOR_INDEXING_TRANSFORM:
TRY(transforms.try_append(TRY(ColorIndexingTransform::read(context, bit_stream))));
case COLOR_INDEXING_TRANSFORM: {
auto color_indexing_transform = TRY(ColorIndexingTransform::read(context, bit_stream));
stored_size.set_width(stored_size.width() / color_indexing_transform->pixels_per_pixel());
TRY(transforms.try_append(move(color_indexing_transform)));
break;
}
}
}
auto format = vp8l_header.is_alpha_used ? BitmapFormat::BGRA8888 : BitmapFormat::BGRx8888;
context.bitmap = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::SpatiallyCoded, format, context.size.value(), bit_stream));
context.bitmap = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::SpatiallyCoded, format, stored_size, bit_stream));
// Transforms have to be applied in the reverse order they appear in in the file.
// (As far as I can tell, this isn't mentioned in the spec.)