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LibGfx/WebP: Implement uncompressed ALPH chunk reading

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ALPH chunks are only used to give lossy webp frames an alpha channel,
and lossy decompression isn't implemented yet. So this can currently
never be hit in practice -- but for debugging and testing, I put in
some code behind `#if 0` for now that fake-decompresses a lossy webp
frame by returning an empty bitmap.

But this also doesn't implement compressed ALPH chunks yet, and I
couldn't find any lossy-webp-with-alpha files that use uncompressed
alpha channels. So the code here isn't really tested.
This commit is contained in:
Nico Weber 2023-05-06 21:10:26 -04:00 • committed by Andreas Kling
parent e9f5c9ab9d
commit fab6a3915e
1 changed files with 45 additions and 3 deletions
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48
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -303,8 +303,16 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8(WebPLoadingContext&
{
VERIFY(vp8_chunk.type == FourCC("VP8 "));
// Uncomment this to test ALPH decoding for WebP-lossy-with-alpha images while lossy decoding isn't implemented yet.
#if 0
auto vp8_header = TRY(decode_webp_chunk_VP8_header(context, vp8_chunk));
// FIXME: probably want to pass in the bitmap format based on if there's an ALPH chunk.
return Bitmap::create(BitmapFormat::BGRA8888, { vp8_header.width, vp8_header.height });
#else
// FIXME: Implement webp lossy decoding.
return context.error("WebPImageDecoderPlugin: decoding lossy webps not yet implemented");
#endif
}
// https://developers.google.com/speed/webp/docs/riff_container#simple_file_format_lossless
@ -1300,6 +1308,41 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L(WebPLoadingContext&
return bitmap;
}
// https://developers.google.com/speed/webp/docs/riff_container#alpha
static ErrorOr<void> decode_webp_chunk_ALPH(WebPLoadingContext& context, Chunk const& alph_chunk, Bitmap& bitmap)
{
VERIFY(alph_chunk.type == FourCC("ALPH"));
if (alph_chunk.data.size() < 1)
return context.error("WebPImageDecoderPlugin: ALPH chunk too small");
u8 flags = alph_chunk.data.data()[0];
u8 preprocessing = (flags >> 4) & 3;
u8 filtering_method = (flags >> 2) & 3;
u8 compression_method = flags & 3;
dbgln_if(WEBP_DEBUG, "preprocessing {} filtering_method {} compression_method {}", preprocessing, filtering_method, compression_method);
ReadonlyBytes alpha_data = alph_chunk.data.slice(1);
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)
bitmap.begin()[i] |= alpha_data[i] << 24;
return {};
}
// "Lossless format compression: the byte sequence is a compressed image-stream (as described in the WebP Lossless Bitstream Format)
// 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");
}
static ErrorOr<VP8XHeader> decode_webp_chunk_VP8X(WebPLoadingContext& context, Chunk const& vp8x_chunk)
{
VERIFY(vp8x_chunk.type == FourCC("VP8X"));
@ -1594,9 +1637,8 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_image_data(WebPLoadingContext&
VERIFY(image_data.image_data_chunk->type == FourCC("VP8 "));
auto bitmap = TRY(decode_webp_chunk_VP8(context, image_data.image_data_chunk.value()));
if (image_data.alpha_chunk.has_value()) {
// FIXME: Decode alpha chunk and store decoded alpha in `bitmap`.
}
if (image_data.alpha_chunk.has_value())
TRY(decode_webp_chunk_ALPH(context, image_data.alpha_chunk.value(), *bitmap));
return bitmap;
}