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LibWeb+LibGfx: Move premultiplied alpha mixing to color.mixed_with()

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This will be needed for mixing filters in LibGfx (and may be
generally useful elsewhere).
This commit is contained in:
MacDue 2022-09-15 08:31:22 +01:00 • committed by Sam Atkins
parent ec4de1e07d
commit 7bc0c66290
3 changed files with 33 additions and 35 deletions
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42
Userland/Libraries/LibWeb/Painting/GradientPainting.cpp
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@ -134,36 +134,6 @@ LinearGradientData resolve_linear_gradient_data(Layout::Node const& node, Gfx::F
return { gradient_angle, resolved_color_stops, repeat_length };
}
static float mix(float x, float y, float a)
{
return x * (1 - a) + y * a;
}
// Note: Gfx::gamma_accurate_blend() is NOT correct for linear gradients!
static Gfx::Color color_mix(Gfx::Color x, Gfx::Color y, float a)
{
if (x.alpha() == y.alpha() || x.with_alpha(0) == y.with_alpha(0)) {
return Gfx::Color {
round_to<u8>(mix(x.red(), y.red(), a)),
round_to<u8>(mix(x.green(), y.green(), a)),
round_to<u8>(mix(x.blue(), y.blue(), a)),
round_to<u8>(mix(x.alpha(), y.alpha(), a)),
};
}
// Use slower but more visually pleasing premultiplied alpha mixing if both the color and alpha differ.
// https://drafts.csswg.org/css-images/#coloring-gradient-line
auto mixed_alpha = mix(x.alpha(), y.alpha(), a);
auto premultiplied_mix_channel = [&](float channel_x, float channel_y, float a) {
return round_to<u8>(mix(channel_x * (x.alpha() / 255.0f), channel_y * (y.alpha() / 255.0f), a) / (mixed_alpha / 255.0f));
};
return Gfx::Color {
premultiplied_mix_channel(x.red(), y.red(), a),
premultiplied_mix_channel(x.green(), y.green(), a),
premultiplied_mix_channel(x.blue(), y.blue(), a),
round_to<u8>(mixed_alpha),
};
}
void paint_linear_gradient(PaintContext& context, Gfx::IntRect const& gradient_rect, LinearGradientData const& data)
{
float angle = normalized_gradient_angle_radians(data.gradient_angle);
@ -213,17 +183,19 @@ void paint_linear_gradient(PaintContext& context, Gfx::IntRect const& gradient_r
auto& color_stops = data.color_stops;
auto start_offset = data.repeat_length.has_value() ? color_stops.first().position : 0.0f;
auto start_offset_int = round_to<int>(start_offset);
// Note: color.mixed_with() performs premultiplied alpha mixing when necessary as defined in:
// https://drafts.csswg.org/css-images/#coloring-gradient-line
for (int loc = 0; loc < gradient_color_count; loc++) {
Gfx::Color gradient_color = color_mix(
color_stops[0].color,
Gfx::Color gradient_color = color_stops[0].color.mixed_with(
color_stops[1].color,
color_stop_step(
color_stops[0],
color_stops[1],
loc + start_offset_int));
for (size_t i = 1; i < color_stops.size() - 1; i++) {
gradient_color = color_mix(
gradient_color,
gradient_color = gradient_color.mixed_with(
color_stops[i + 1].color,
color_stop_step(
color_stops[i],
@ -247,7 +219,7 @@ void paint_linear_gradient(PaintContext& context, Gfx::IntRect const& gradient_r
}
// Blend between the two neighbouring colors (this fixes some nasty aliasing issues at small angles)
auto blend = loc - static_cast<int>(loc);
auto gradient_color = color_mix(lookup_color(loc - 1), lookup_color(loc), blend);
auto gradient_color = lookup_color(loc - 1).mixed_with(lookup_color(loc), blend);
context.painter().set_pixel(gradient_rect.x() + x, gradient_rect.y() + y, gradient_color, gradient_color.alpha() < 255);
}
}