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LibGfx+LibWeb: Convert bitmap alpha type when creating ImmutableBitmaps

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When decoding data into bitmaps, we end up with different alpha types
(premultiplied vs. unpremultiplied color data). Unfortunately, Skia only
seems to handle premultiplied color data well when scaling bitmaps with
an alpha channel. This might be due to Skia historically only supporting
premultiplied color blending, with unpremultiplied support having been
added more recently.

When using Skia to blend bitmaps, we need the color data to be
premultiplied. ImmutableBitmap gains a new method to enforce the alpha
type to be used, which is now used by SharedResourceRequest and
CanvasRenderingContext2D to enforce the right alpha type.

Our LibWeb tests actually had a couple of screenshot tests that exposed
the graphical glitches caused by Skia; see the big smiley faces in the
CSS backgrounds tests for example. The failing tests are now updated to
accommodate the new behavior.

Chromium and Firefox both seem to apply the same behavior; e.g. they
actively decode PNGs (which are unpremultiplied in nature) to a
premultiplied bitmap.

Fixes #3691.
This commit is contained in:
Jelle Raaijmakers 2025-02-26 16:53:54 +01:00 • committed by Alexander Kalenik
parent c6644c92e7
commit bf517f9ac2
Notes: github-actions[bot] 2025-03-22 16:50:33 +00:00
14 changed files with 74 additions and 2 deletions
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2
Libraries/LibWeb/HTML/SharedResourceRequest.cpp
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@ -161,7 +161,7 @@ void SharedResourceRequest::handle_successful_fetch(URL::URL const& url_string,
Vector<AnimatedBitmapDecodedImageData::Frame> frames;
for (auto& frame : result.frames) {
frames.append(AnimatedBitmapDecodedImageData::Frame {
.bitmap = Gfx::ImmutableBitmap::create(*frame.bitmap, result.color_space),
.bitmap = Gfx::ImmutableBitmap::create(*frame.bitmap, Gfx::AlphaType::Premultiplied, result.color_space),
.duration = static_cast<int>(frame.duration),
});
}