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LibGfx: Report premultiplied alpha type for opaque Skia surfaces

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We don't discern between opaque and non-opaque alpha types in LibGfx,
which at some point we might need to do. But for now, assume all opaque
Skia surfaces have premultiplied alpha.

Fixes #6892.
This commit is contained in:
Jelle Raaijmakers 2025-11-25 09:45:35 +01:00 committed by Jelle Raaijmakers
parent 6790a695da
commit 3b7eede694
4 changed files with 17 additions and 3 deletions
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6
Libraries/LibGfx/ImmutableBitmap.cpp
View file

@ -41,9 +41,11 @@ IntSize ImmutableBitmap::size() const
return { width(), height() };
}
Gfx::AlphaType ImmutableBitmap::alpha_type() const
AlphaType ImmutableBitmap::alpha_type() const
{
return m_impl->sk_image->alphaType() == kPremul_SkAlphaType ? Gfx::AlphaType::Premultiplied : Gfx::AlphaType::Unpremultiplied;
// We assume premultiplied alpha type for opaque surfaces since that is Skia's preferred alpha type and the
// effective pixel data is identical between premultiplied and unpremultiplied in that case.
return m_impl->sk_image->alphaType() == kUnpremul_SkAlphaType ? AlphaType::Unpremultiplied : AlphaType::Premultiplied;
}
SkImage const* ImmutableBitmap::sk_image() const

2
Libraries/LibGfx/ImmutableBitmap.h
View file

@ -33,7 +33,7 @@ public:
IntRect rect() const;
IntSize size() const;
Gfx::AlphaType alpha_type() const;
AlphaType alpha_type() const;
SkImage const* sk_image() const;

1
Tests/LibWeb/Text/expected/HTML/canvas-getImageData-no-alpha.txt Normal file
View file

@ -0,0 +1 @@
50, 75, 100, 255

11
Tests/LibWeb/Text/input/HTML/canvas-getImageData-no-alpha.html Normal file
View file

@ -0,0 +1,11 @@
<!DOCTYPE html>
<script src="../include.js"></script>
<script>
test(() => {
const ctx = document.createElement("canvas").getContext("2d", {alpha: false});
ctx.fillStyle = "rgba(100, 150, 200, 0.5)";
ctx.fillRect(0, 0, 300, 150);
const color = ctx.getImageData(0, 0, 300, 150);
println(`${color.data[0]}, ${color.data[1]}, ${color.data[2]}, ${color.data[3]}`);
});
</script>