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LibGfx: Use premultiplied alpha when scaling images

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This commit replaces usages of `Color::interpolate()` with
`Color::mixed_with()` in image scaling functions. The latter
uses premultiplied alpha, which results in more visually
pleasing edges when images are scaled against a transparent
background.

These changes affect the SmoothPixels and BilinearBlend scaling modes
of `Painter::draw_scaled_bitmap()` as well as the `Bitmap::scaled()`
function.

Fixes #17153.
This commit is contained in:
Tim Ledbetter 2023-02-16 22:22:21 +00:00 • committed by Jelle Raaijmakers
parent 2480b94ae7
commit 5a6b995444
2 changed files with 11 additions and 11 deletions
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12
Userland/Libraries/LibGfx/Painter.cpp
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@ -1230,10 +1230,10 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
auto bottom_left = get_pixel(source, scaled_x0, scaled_y1);
auto bottom_right = get_pixel(source, scaled_x1, scaled_y1);
auto top = top_left.interpolate(top_right, x_ratio);
auto bottom = bottom_left.interpolate(bottom_right, x_ratio);
auto top = top_left.mixed_with(top_right, x_ratio);
auto bottom = bottom_left.mixed_with(bottom_right, x_ratio);
src_pixel = top.interpolate(bottom, y_ratio);
src_pixel = top.mixed_with(bottom, y_ratio);
} else if constexpr (scaling_mode == Painter::ScalingMode::SmoothPixels) {
auto scaled_x1 = clamp(desired_x >> 32, clipped_src_rect.left(), clipped_src_rect.right());
auto scaled_x0 = clamp(scaled_x1 - 1, clipped_src_rect.left(), clipped_src_rect.right());
@ -1251,10 +1251,10 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
auto bottom_left = get_pixel(source, scaled_x0, scaled_y1);
auto bottom_right = get_pixel(source, scaled_x1, scaled_y1);
auto top = top_left.interpolate(top_right, scaled_x_ratio);
auto bottom = bottom_left.interpolate(bottom_right, scaled_x_ratio);
auto top = top_left.mixed_with(top_right, scaled_x_ratio);
auto bottom = bottom_left.mixed_with(bottom_right, scaled_x_ratio);
src_pixel = top.interpolate(bottom, scaled_y_ratio);
src_pixel = top.mixed_with(bottom, scaled_y_ratio);
} else {
auto scaled_x = clamp(desired_x >> 32, clipped_src_rect.left(), clipped_src_rect.right());
auto scaled_y = clamp(desired_y >> 32, clipped_src_rect.top(), clipped_src_rect.bottom());