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WebP/Lossy: Add support for images with more than one partition

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Each secondary partition has an independent BooleanDecoder.
Their bitstreams interleave per macroblock row, that is the first
macroblock row is read from the first decoder, the second from the
second, ..., until it wraps around again.

All partitions share a single prediction state though: The second
macroblock row (which reads coefficients off the second decoder) is
predicted using the result of decoding the frist macroblock row (which
reads coefficients off the first decoder).

So if I understand things right, in theory the coefficient reading could
be parallelized, but prediction can't be. (IDCT can also be
parallelized, but that's true with just a single partition too.)

I created the test image by running

    examples/cwebp -low_memory -partitions 3 -o foo.webp \
        ~/src/serenity/Tests/LibGfx/test-inputs/4.webp

using a cwebp hacked up as described in #19149. Since creating
multi-partition lossy webps requires hacking up `cwebp`, they're likely
very rare in practice. (But maybe other programs using the libwebp API
create them.)

Fixes #19149.

With this, webp lossy support is complete (*) :^)

And with that, webp support is complete: Lossless, lossy, lossy with
alpha, animated lossless, animated lossy, animated lossy with alpha all
work.

(*: Loop filtering isn't implemented yet, which has a minor visual
effect on the output. But it's only visible when carefully comparing
a webp decoded without loop filtering to the same decoded with it.
But it's technically a part of the spec that's still missing.

The upsampling of UV in the YUV->RGB code is also low-quality. This
produces somewhat visible banding in practice in some images (e.g.
in the fire breather's face in 5.webp), so we should probably improve
that at some point. Our JPG decoder has the same issue.)
This commit is contained in:
Nico Weber 2023-05-30 10:10:14 -04:00 • committed by Jelle Raaijmakers
parent 3bc0e7a7ca
commit a2d8de180c
3 changed files with 67 additions and 8 deletions
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55
Userland/Libraries/LibGfx/ImageFormats/WebPLoaderLossy.cpp
View file

@ -1142,11 +1142,16 @@ void convert_yuv_to_rgb(Bitmap& bitmap, int mb_x, int mb_y, ReadonlySpan<i16> y_
}
}
ErrorOr<void> decode_VP8_image_data(Gfx::Bitmap& bitmap, FrameHeader const& header, ReadonlyBytes data, int macroblock_width, int macroblock_height, Vector<MacroblockMetadata> const& macroblock_metadata)
ErrorOr<void> decode_VP8_image_data(Gfx::Bitmap& bitmap, FrameHeader const& header, Vector<ReadonlyBytes> data_partitions, int macroblock_width, int macroblock_height, Vector<MacroblockMetadata> const& macroblock_metadata)
{
FixedMemoryStream memory_stream { data };
BigEndianInputBitStream bit_stream { MaybeOwned<Stream>(memory_stream) };
auto decoder = TRY(BooleanDecoder::initialize(MaybeOwned { bit_stream }, data.size() * 8));
Vector<BooleanDecoder> streams;
for (auto data : data_partitions) {
auto memory_stream = make<FixedMemoryStream>(data);
auto bit_stream = make<BigEndianInputBitStream>(move(memory_stream));
auto decoder = TRY(BooleanDecoder::initialize(move(bit_stream), data.size() * 8));
TRY(streams.try_append(move(decoder)));
}
CoefficientReadingContext coefficient_reading_context;
TRY(coefficient_reading_context.initialize(macroblock_width));
@ -1167,6 +1172,8 @@ ErrorOr<void> decode_VP8_image_data(Gfx::Bitmap& bitmap, FrameHeader const& head
predicted_v_above[i] = 127;
for (int mb_y = 0, macroblock_index = 0; mb_y < macroblock_height; ++mb_y) {
BooleanDecoder& decoder = streams[mb_y % streams.size()];
coefficient_reading_context.start_new_row();
i16 predicted_y_left[16] { 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129, 129 };
@ -1223,6 +1230,40 @@ ErrorOr<void> decode_VP8_image_data(Gfx::Bitmap& bitmap, FrameHeader const& head
return {};
}
static ErrorOr<Vector<ReadonlyBytes>> split_data_partitions(ReadonlyBytes second_partition, u8 number_of_dct_partitions)
{
Vector<ReadonlyBytes> data_partitions;
// https://datatracker.ietf.org/doc/html/rfc6386#section-9.5 "Token Partition and Partition Data Offsets"
// "If the number of data partitions is
// greater than 1, the size of each partition (except the last) is
// written in 3 bytes (24 bits). The size of the last partition is the
// remainder of the data not used by any of the previous partitions.
// The partitioned data are consecutive in the bitstream, so the size
// can also be used to calculate the offset of each partition."
// In practice, virtually all lossy webp files have a single data partition.
VERIFY(number_of_dct_partitions >= 1);
VERIFY(number_of_dct_partitions <= 8);
size_t sizes_size = (number_of_dct_partitions - 1) * 3;
if (second_partition.size() < sizes_size)
return Error::from_string_literal("WebPImageDecoderPlugin: not enough data for partition sizes");
ReadonlyBytes sizes = second_partition.slice(0, sizes_size);
ReadonlyBytes data = second_partition.slice(sizes_size);
for (int i = 0; i < number_of_dct_partitions - 1; ++i) {
u32 partition_size = sizes[0] | (sizes[1] << 8) | (sizes[2] << 16);
dbgln_if(WEBP_DEBUG, "partition_size {}", partition_size);
sizes = sizes.slice(3);
if (partition_size > data.size())
return Error::from_string_literal("WebPImageDecoderPlugin: not enough data for partition data");
TRY(data_partitions.try_append(data.slice(0, partition_size)));
data = data.slice(partition_size);
}
TRY(data_partitions.try_append(data));
return data_partitions;
}
}
ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8_contents(VP8Header const& vp8_header, bool include_alpha_channel)
@ -1245,13 +1286,11 @@ ErrorOr<NonnullRefPtr<Bitmap>> decode_webp_chunk_VP8_contents(VP8Header const& v
// Done with the first partition!
if (header.number_of_dct_partitions > 1)
return Error::from_string_literal("WebPImageDecoderPlugin: decoding lossy webps with more than one dct partition not yet implemented");
auto bitmap_format = include_alpha_channel ? BitmapFormat::BGRA8888 : BitmapFormat::BGRx8888;
auto bitmap = TRY(Bitmap::create(bitmap_format, { macroblock_width * 16, macroblock_height * 16 }));
TRY(decode_VP8_image_data(*bitmap, header, vp8_header.second_partition, macroblock_width, macroblock_height, macroblock_metadata));
auto data_partitions = TRY(split_data_partitions(vp8_header.second_partition, header.number_of_dct_partitions));
TRY(decode_VP8_image_data(*bitmap, header, move(data_partitions), macroblock_width, macroblock_height, macroblock_metadata));
auto width = static_cast<int>(vp8_header.width);
auto height = static_cast<int>(vp8_header.height);