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LibGfx/WebPWriter: Support animations with transparent pixels

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Once we see a frame with transparent pixels, we now toggle the
"has alpha" bit in the header.

To not require a SeekableStream opened for reading, we now pass the
unmodified original flag bit to WebPAnimationWriter.
This commit is contained in:
Nico Weber 2024-05-06 22:22:14 -04:00 committed by Tim Flynn
parent 3a4e0c2804
commit f506818052
Notes: sideshowbarker 2024-07-17 21:26:19 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/f506818052
Pull-request: https://github.com/SerenityOS/serenity/pull/24269
Reviewed-by: https://github.com/LucasChollet ✅
Reviewed-by: https://github.com/trflynn89
1 changed files with 45 additions and 22 deletions
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67
Userland/Libraries/LibGfx/ImageFormats/WebPWriter.cpp
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@ -199,27 +199,10 @@ struct VP8XHeader {
u32 height { 0 };
};
// https://developers.google.com/speed/webp/docs/riff_container#extended_file_format
static ErrorOr<void> write_VP8X_chunk(Stream& stream, VP8XHeader const& header)
static u8 vp8x_flags_from_header(VP8XHeader const& header)
{
if (header.width > (1 << 24) || header.height > (1 << 24))
return Error::from_string_literal("WebP dimensions too large for VP8X chunk");
if (header.width == 0 || header.height == 0)
return Error::from_string_literal("WebP lossless images must be at least one pixel wide and tall");
// "The product of Canvas Width and Canvas Height MUST be at most 2^32 - 1."
u64 product = static_cast<u64>(header.width) * static_cast<u64>(header.height);
if (product >= (1ull << 32))
return Error::from_string_literal("WebP dimensions too large for VP8X chunk");
TRY(write_chunk_header(stream, "VP8X"sv, 10));
LittleEndianOutputBitStream bit_stream { MaybeOwned<Stream>(stream) };
// Don't use bit_stream.write_bits() to write individual flags here:
// The spec describes bit flags in MSB to LSB order, but write_bits() writes LSB to MSB.
u8 flags = 0;
// "Reserved (Rsv): 2 bits
// MUST be 0. Readers MUST ignore this field."
@ -251,7 +234,30 @@ static ErrorOr<void> write_VP8X_chunk(Stream& stream, VP8XHeader const& header)
// "Reserved (R): 1 bit
// MUST be 0. Readers MUST ignore this field."
TRY(bit_stream.write_bits(flags, 8u));
return flags;
}
// https://developers.google.com/speed/webp/docs/riff_container#extended_file_format
static ErrorOr<void> write_VP8X_chunk(Stream& stream, VP8XHeader const& header)
{
if (header.width > (1 << 24) || header.height > (1 << 24))
return Error::from_string_literal("WebP dimensions too large for VP8X chunk");
if (header.width == 0 || header.height == 0)
return Error::from_string_literal("WebP lossless images must be at least one pixel wide and tall");
// "The product of Canvas Width and Canvas Height MUST be at most 2^32 - 1."
u64 product = static_cast<u64>(header.width) * static_cast<u64>(header.height);
if (product >= (1ull << 32))
return Error::from_string_literal("WebP dimensions too large for VP8X chunk");
TRY(write_chunk_header(stream, "VP8X"sv, 10));
LittleEndianOutputBitStream bit_stream { MaybeOwned<Stream>(stream) };
// Don't use bit_stream.write_bits() to write individual flags here:
// The spec describes bit flags in MSB to LSB order, but write_bits() writes LSB to MSB.
TRY(bit_stream.write_bits(vp8x_flags_from_header(header), 8u));
// "Reserved: 24 bits
// MUST be 0. Readers MUST ignore this field."
@ -329,19 +335,22 @@ ErrorOr<void> WebPWriter::encode(Stream& stream, Bitmap const& bitmap, Options c
class WebPAnimationWriter : public AnimationWriter {
public:
WebPAnimationWriter(SeekableStream& stream, IntSize dimensions)
WebPAnimationWriter(SeekableStream& stream, IntSize dimensions, u8 original_vp8x_flags)
: m_stream(stream)
, m_dimensions(dimensions)
, m_vp8x_flags(original_vp8x_flags)
{
}
virtual ErrorOr<void> add_frame(Bitmap&, int, IntPoint) override;
ErrorOr<void> update_size_in_header();
ErrorOr<void> set_alpha_bit_in_header();
private:
SeekableStream& m_stream;
IntSize m_dimensions;
u8 m_vp8x_flags { 0 };
};
static ErrorOr<void> align_to_two(SeekableStream& stream)
@ -462,6 +471,9 @@ ErrorOr<void> WebPAnimationWriter::add_frame(Bitmap& bitmap, int duration_ms, In
TRY(update_size_in_header());
if (!(m_vp8x_flags & 0x10) && !are_all_pixels_opaque(bitmap))
TRY(set_alpha_bit_in_header());
return {};
}
@ -475,6 +487,17 @@ ErrorOr<void> WebPAnimationWriter::update_size_in_header()
return {};
}
ErrorOr<void> WebPAnimationWriter::set_alpha_bit_in_header()
{
m_vp8x_flags |= 0x10;
auto current_offset = TRY(m_stream.tell());
TRY(m_stream.seek(20, SeekMode::SetPosition));
TRY(m_stream.write_value<u8>(m_vp8x_flags));
TRY(m_stream.seek(current_offset, SeekMode::SetPosition));
return {};
}
struct ANIMChunk {
u32 background_color { 0 };
u16 loop_count { 0 };
@ -510,7 +533,7 @@ ErrorOr<NonnullOwnPtr<AnimationWriter>> WebPWriter::start_encoding_animation(See
TRY(write_ANIM_chunk(stream, { .background_color = background_color.value(), .loop_count = static_cast<u16>(loop_count) }));
auto writer = make<WebPAnimationWriter>(stream, dimensions);
auto writer = make<WebPAnimationWriter>(stream, dimensions, vp8x_flags_from_header(vp8x_header));
TRY(writer->update_size_in_header());
return writer;
}