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LibGfx/WebPWriter: Use an IsOpaque struct for tracking opacity

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No behavior change, but this makes it easy to correctly set this
flag when adding an indexing transform: Opacity then needs to be
determined based on if colors in the color table have opacity,
not if the indexes into the color table do.

With this struct, only the first time something sets opacity is
honored, giving us those semantics.
This commit is contained in:
Nico Weber 2024-05-29 18:24:41 -04:00 committed by Jelle Raaijmakers
parent c5dedbc54a
commit 804e85265e
Notes: sideshowbarker 2024-07-17 10:54:57 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/804e85265e
Pull-request: https://github.com/SerenityOS/serenity/pull/24483
Reviewed-by: https://github.com/gmta ✅
1 changed files with 24 additions and 4 deletions
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28
Userland/Libraries/LibGfx/ImageFormats/WebPWriterLossless.cpp
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@ -18,6 +18,23 @@
namespace Gfx {
namespace {
struct IsOpaque {
bool is_fully_opaque { false };
bool is_opacity_known { false };
void set_is_fully_opaque_if_not_yet_known(bool is_fully_opaque)
{
if (is_opacity_known)
return;
this->is_fully_opaque = is_fully_opaque;
is_opacity_known = true;
}
};
}
NEVER_INLINE static ErrorOr<void> write_image_data(LittleEndianOutputBitStream& bit_stream, Bitmap const& bitmap, PrefixCodeGroup const& prefix_code_group)
{
// This is currently the hot loop. Keep performance in mind when you change it.
@ -213,7 +230,7 @@ static ErrorOr<CanonicalCode> write_normal_code_lengths(LittleEndianOutputBitStr
return CanonicalCode::from_bytes(bit_lengths.span().trim(code_count));
}
static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOutputBitStream& bit_stream, Bitmap const& bitmap, bool& is_fully_opaque)
static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOutputBitStream& bit_stream, Bitmap const& bitmap, IsOpaque& is_fully_opaque)
{
// https://developers.google.com/speed/webp/docs/webp_lossless_bitstream_specification#5_image_data
// spatially-coded-image = color-cache-info meta-prefix data
@ -291,7 +308,7 @@ static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOu
prefix_code_group[i] = TRY(write_normal_code_lengths(bit_stream, code_lengths[i], alphabet_sizes[i]));
if (i == 3)
is_fully_opaque = non_zero_symbol_count == 1 && symbols[0] == 0xff;
is_fully_opaque.set_is_fully_opaque_if_not_yet_known(non_zero_symbol_count == 1 && symbols[0] == 0xff);
}
// For code #5, use a simple empty code, since we don't use this yet.
@ -303,7 +320,7 @@ static ErrorOr<void> write_VP8L_coded_image(ImageKind image_kind, LittleEndianOu
return {};
}
static ErrorOr<void> write_VP8L_image_data(Stream& stream, Bitmap const& bitmap, bool& is_fully_opaque)
static ErrorOr<void> write_VP8L_image_data(Stream& stream, NonnullRefPtr<Bitmap> bitmap, IsOpaque& is_fully_opaque)
{
LittleEndianOutputBitStream bit_stream { MaybeOwned<Stream>(stream) };
@ -323,7 +340,10 @@ static ErrorOr<void> write_VP8L_image_data(Stream& stream, Bitmap const& bitmap,
ErrorOr<ByteBuffer> compress_VP8L_image_data(Bitmap const& bitmap, bool& is_fully_opaque)
{
AllocatingMemoryStream vp8l_data_stream;
TRY(write_VP8L_image_data(vp8l_data_stream, bitmap, is_fully_opaque));
IsOpaque is_opaque_struct;
TRY(write_VP8L_image_data(vp8l_data_stream, bitmap, is_opaque_struct));
VERIFY(is_opaque_struct.is_opacity_known);
is_fully_opaque = is_opaque_struct.is_fully_opaque;
return vp8l_data_stream.read_until_eof();
}