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LibGfx: Keep alpha value of previous pixel for QOI_OP_RGB chunk

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I accidentally skipped this part of the spec in the QOI decoder:

> The alpha value remains unchanged from the previous pixel.

This led to incorrect rendering of some images with transparency,
visible in form of a horizontal line of non-transparent pixels (that
shouldn't exist), e.g. for the following chunk sequence:

- QOI_OP_RGBA with alpha = 0
- QOI_OP_RGB
- QOI_OP_RUN

The QOI_OP_RGB should 'inherit' the alpha value of the previous
QOI_OP_RGBA chunk, instead of always setting it to 255.
I'm unsure why the encoder added the QOI_OP_RGB chunk to the specific
image where the bug was noticed in the first place - they effectively
both had fully transparent color values.
This commit is contained in:
Linus Groh 2021-12-22 01:12:51 +01:00
parent b6d921f682
commit e4bba2fab9
1 changed files with 5 additions and 3 deletions
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8
Userland/Libraries/LibGfx/QOILoader.cpp
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@ -34,14 +34,16 @@ static ErrorOr<QOIHeader> decode_qoi_header(InputMemoryStream& stream)
return header;
}
static ErrorOr<Color> decode_qoi_op_rgb(InputMemoryStream& stream)
static ErrorOr<Color> decode_qoi_op_rgb(InputMemoryStream& stream, Color pixel)
{
u8 bytes[4];
stream >> Bytes { &bytes, array_size(bytes) };
if (stream.handle_any_error())
return Error::from_string_literal("Invalid QOI image: end of stream while reading QOI_OP_RGB chunk"sv);
VERIFY(bytes[0] == QOI_OP_RGB);
return Color { bytes[1], bytes[2], bytes[3] };
// The alpha value remains unchanged from the previous pixel.
return Color { bytes[1], bytes[2], bytes[3], pixel.alpha() };
}
static ErrorOr<Color> decode_qoi_op_rgba(InputMemoryStream& stream)
@ -163,7 +165,7 @@ static ErrorOr<NonnullRefPtr<Bitmap>> decode_qoi_image(InputMemoryStream& stream
if (stream.handle_any_error())
return Error::from_string_literal("Invalid QOI image: end of stream while reading chunk tag"sv);
if (tag == QOI_OP_RGB)
pixel = TRY(decode_qoi_op_rgb(stream));
pixel = TRY(decode_qoi_op_rgb(stream, pixel));
else if (tag == QOI_OP_RGBA)
pixel = TRY(decode_qoi_op_rgba(stream));
else if ((tag & QOI_MASK_2) == QOI_OP_INDEX)