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LibWeb: Use bitmap's alpha type instead of assuming unpremultiplied

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When converting a `Gfx::Bitmap` to a Skia bitmap, we cannot assume the
color data is unpremultiplied. For example, everything canvas-related
uses premultiplied color data:

  https://html.spec.whatwg.org/multipage/canvas.html#premultiplied-alpha-and-the-2d-rendering-context

We were probably assuming unpremultiplied since that is what the PNG
decoder gives us. Since we now make `Gfx::Bitmap` identify what alpha
type is being used, we can instruct Skia a bit better :^)

Update our `EdgeFlagPathRasterizer` to use premultiplied alpha instead
of unpremultiplied so we can apply alpha correctly for path masks.

This fixes the dark borders sometimes visible when SVGs are blended
with a colored background.

This also exposed an issue with our `CanvasRenderingContext2D`, which is
supposed to hold a bitmap with premultiplied alpha internally but expose
a bitmap with unpremultiplied alpha in `CanvasImageData`. Expand our C2D
test to include the alpha channel as well.

Finally, this also exposed an off-by-one issue in
`EdgeFlagPathRasterizer` which caused the last scanlines for edges to
render incorrectly. We had some reference images which included these
corruptions (they were almost unnoticeable), so update them as well.
This commit is contained in:
Jelle Raaijmakers 2024-08-02 13:15:45 +02:00 • committed by Alexander Kalenik
parent a430ae6dcf
commit 5865cf5864
Notes: github-actions[bot] 2024-08-07 18:16:57 +00:00
12 changed files with 29 additions and 13 deletions
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2
Userland/Libraries/LibWeb/HTML/ImageData.cpp
View file

@ -30,7 +30,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<ImageData>> ImageData::create(JS::Realm& re
// 2. Initialize this given sw, sh, and settings set to settings.
// 3. Initialize the image data of this to transparent black.
auto data = TRY(JS::Uint8ClampedArray::create(realm, sw * sh * 4));
auto bitmap = TRY_OR_THROW_OOM(vm, Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::RGBA8888, Gfx::AlphaType::Premultiplied, Gfx::IntSize(sw, sh), sw * sizeof(u32), data->data().data()));
auto bitmap = TRY_OR_THROW_OOM(vm, Gfx::Bitmap::create_wrapper(Gfx::BitmapFormat::RGBA8888, Gfx::AlphaType::Unpremultiplied, Gfx::IntSize(sw, sh), sw * sizeof(u32), data->data().data()));
return realm.heap().allocate<ImageData>(realm, realm, bitmap, data);
}