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LibGfx: Implement meta prefix code support in webp decoder

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Nico Weber 2023-04-04 14:40:22 -04:00 • committed by Linus Groh
parent 6d38824985
commit d354c1b007
1 changed files with 52 additions and 8 deletions
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60
Userland/Libraries/LibGfx/ImageFormats/WebPLoader.cpp
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@ -533,14 +533,41 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_image(WebPLoadingCo
TRY(color_cache.try_resize(color_cache_size));
}
int num_prefix_groups = 1;
RefPtr<Gfx::Bitmap> entropy_image;
int prefix_bits = 0;
if (image_kind == ImageKind::SpatiallyCoded) {
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#622_decoding_of_meta_prefix_codes
// In particular, the "Entropy image" subsection.
// "Meta prefix codes may be used only when the image is being used in the role of an ARGB image."
// meta-prefix = %b0 / (%b1 entropy-image)
bool has_meta_prefix = TRY(bit_stream.read_bits(1));
dbgln_if(WEBP_DEBUG, "has_meta_prefix {}", has_meta_prefix);
if (has_meta_prefix)
return context.error("WebPImageDecoderPlugin: VP8L meta_prefix not yet implemented");
if (has_meta_prefix) {
prefix_bits = TRY(bit_stream.read_bits(3)) + 2;
dbgln_if(WEBP_DEBUG, "prefix_bits {}", prefix_bits);
int block_size = 1 << prefix_bits;
IntSize prefix_size { ceil_div(size.width(), block_size), ceil_div(size.height(), block_size) };
entropy_image = TRY(decode_webp_chunk_VP8L_image(context, ImageKind::EntropyCoded, BitmapFormat::BGRx8888, prefix_size, bit_stream));
// A "meta prefix image" or "entropy image" can tell the decoder to use different PrefixCodeGroup for
// tiles of the main, spatially coded, image. It's a bit hidden in the spec:
// "The red and green components of a pixel define the meta prefix code used in a particular block of the ARGB image."
// ...
// "The number of prefix code groups in the ARGB image can be obtained by finding the largest meta prefix code from the entropy image"
// That is, if a meta prefix image is present, the main image has more than one PrefixCodeGroup,
// and the highest value in the meta prefix image determines how many exactly.
u16 largest_meta_prefix_code = 0;
for (ARGB32& pixel : *entropy_image) {
u16 meta_prefix_code = (pixel >> 8) & 0xffff;
if (meta_prefix_code > largest_meta_prefix_code)
largest_meta_prefix_code = meta_prefix_code;
}
dbgln_if(WEBP_DEBUG, "largest meta prefix code {}", largest_meta_prefix_code);
num_prefix_groups = largest_meta_prefix_code + 1;
}
}
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#52_encoding_of_image_data
@ -551,7 +578,9 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_image(WebPLoadingCo
// data = prefix-codes lz77-coded-image
// prefix-codes = prefix-code-group *prefix-codes
PrefixCodeGroup group = TRY(decode_webp_chunk_VP8L_prefix_code_group(context, color_cache_size, bit_stream));
Vector<PrefixCodeGroup, 1> groups;
for (int i = 0; i < num_prefix_groups; ++i)
TRY(groups.try_append(TRY(decode_webp_chunk_VP8L_prefix_code_group(context, color_cache_size, bit_stream))));
auto bitmap = TRY(Bitmap::create(format, size));
@ -582,8 +611,21 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_image(WebPLoadingCo
// lz77-coded-image =
// *((argb-pixel / lz77-copy / color-cache-code) lz77-coded-image)
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#623_decoding_entropy-coded_image_data
ARGB32* pixel = bitmap->begin();
ARGB32* begin = bitmap->begin();
ARGB32* end = bitmap->end();
ARGB32* pixel = begin;
auto prefix_group = [prefix_bits, begin, &groups, size, &entropy_image](ARGB32* pixel) {
if (!prefix_bits)
return groups[0];
size_t offset = pixel - begin;
int x = offset % size.width();
int y = offset / size.width();
int meta_prefix_code = (entropy_image->scanline(y >> prefix_bits)[x >> prefix_bits] >> 8) & 0xffff;
return groups[meta_prefix_code];
};
auto emit_pixel = [&pixel, &color_cache, color_cache_size, color_cache_code_bits](ARGB32 color) {
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#523_color_cache_coding
@ -594,6 +636,8 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_image(WebPLoadingCo
};
while (pixel < end) {
auto const& group = prefix_group(pixel);
auto symbol = TRY(group[0].read_symbol(bit_stream));
if (symbol >= 256u + 24u + color_cache_size)
return context.error("WebPImageDecoderPlugin: Symbol out of bounds");
@ -653,13 +697,13 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8L_image(WebPLoadingCo
distance = distance - 120;
}
if (pixel - bitmap->begin() < distance) {
dbgln_if(WEBP_DEBUG, "invalid backref, {} < {}", pixel - bitmap->begin(), distance);
if (pixel - begin < distance) {
dbgln_if(WEBP_DEBUG, "invalid backref, {} < {}", pixel - begin, distance);
return context.error("WebPImageDecoderPlugin: Backward reference distance out of bounds");
}
if (bitmap->end() - pixel < length) {
dbgln_if(WEBP_DEBUG, "invalid length, {} < {}", bitmap->end() - pixel, length);
if (end - pixel < length) {
dbgln_if(WEBP_DEBUG, "invalid length, {} < {}", end - pixel, length);
return context.error("WebPImageDecoderPlugin: Backward reference length out of bounds");
}