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LibWeb+LibGfx: Support alpha in CanvasRenderingContext2D

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This is implemented by these related changes:

  * The Skia alpha type 'Opaque' is selected for surfaces that were
    created with the intention of not having an alpha channel.
    Previously we were simply creating one with alpha.

  * Clearing now happens through Skia's `clear()` which always uses the
    source color's value for the result, instead of setting all values
    to 0.

  * CanvasRenderingContext2D selects a different clearing color based on
    the `alpha` context attribute's value.
This commit is contained in:
Jelle Raaijmakers 2025-04-28 16:12:30 +02:00
parent fac0f82031
commit 35efd4d14b
Notes: github-actions[bot] 2025-04-29 11:52:28 +00:00
7 changed files with 68 additions and 12 deletions
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23
Libraries/LibWeb/HTML/CanvasRenderingContext2D.cpp
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@ -149,7 +149,7 @@ void CanvasRenderingContext2D::clear_rect(float x, float y, float width, float h
{
if (auto* painter = this->painter()) {
auto rect = Gfx::FloatRect(x, y, width, height);
painter->clear_rect(rect, Color::Transparent);
painter->clear_rect(rect, clear_color());
did_draw(rect);
}
}
@ -270,14 +270,23 @@ void CanvasRenderingContext2D::allocate_painting_surface_if_needed()
if (m_surface || m_size.is_empty())
return;
// FIXME: implement context attribute .alpha
// FIXME: implement context attribute .color_space
// FIXME: implement context attribute .color_type
// FIXME: implement context attribute .desynchronized
// FIXME: implement context attribute .will_read_frequently
auto color_type = m_context_attributes.alpha ? Gfx::BitmapFormat::BGRA8888 : Gfx::BitmapFormat::BGRx8888;
auto skia_backend_context = canvas_element().navigable()->traversable_navigable()->skia_backend_context();
m_surface = Gfx::PaintingSurface::create_with_size(skia_backend_context, canvas_element().bitmap_size_for_canvas(), Gfx::BitmapFormat::BGRA8888, Gfx::AlphaType::Premultiplied);
m_surface = Gfx::PaintingSurface::create_with_size(skia_backend_context, canvas_element().bitmap_size_for_canvas(), color_type, Gfx::AlphaType::Premultiplied);
// https://html.spec.whatwg.org/multipage/canvas.html#the-canvas-settings:concept-canvas-alpha
// Thus, the bitmap of such a context starts off as opaque black instead of transparent black;
// AD-HOC: Skia provides us with a full transparent surface by default; only clear the surface if alpha is disabled.
if (!m_context_attributes.alpha) {
auto* painter = this->painter();
painter->clear_rect(m_surface->rect().to_type<float>(), clear_color());
}
}
Gfx::Path CanvasRenderingContext2D::text_path(StringView text, float x, float y, Optional<double> max_width)
@ -377,6 +386,12 @@ static Gfx::Path::JoinStyle to_gfx_join(Bindings::CanvasLineJoin const& join_sty
VERIFY_NOT_REACHED();
}
// https://html.spec.whatwg.org/multipage/canvas.html#the-canvas-settings:concept-canvas-alpha
Gfx::Color CanvasRenderingContext2D::clear_color() const
{
return m_context_attributes.alpha ? Gfx::Color::Transparent : Gfx::Color::Black;
}
void CanvasRenderingContext2D::stroke_internal(Gfx::Path const& path)
{
auto* painter = this->painter();
@ -561,7 +576,7 @@ void CanvasRenderingContext2D::reset_to_default_state()
// 1. Clear canvas's bitmap to transparent black.
if (surface) {
painter()->clear_rect(surface->rect().to_type<float>(), Color::Transparent);
painter()->clear_rect(surface->rect().to_type<float>(), clear_color());
}
// 2. Empty the list of subpaths in context's current default path.