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LibGfx: Implement box sampling image scaling

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Box sampling is a scaling algorithm that averages all the pixels that
form the source for the target pixel. For example, if you would resize a
9x9 image to 3x3, each target pixel would encompass a 3x3 pixel area in
the source image.

Box sampling is a near perfect scaling algorithm for downscaling. When
upscaling with this algorithm, the result is similar to nearest neighbor
or smooth pixels.
This commit is contained in:
Jelle Raaijmakers 2023-05-19 00:26:34 +02:00 committed by Andreas Kling
parent 31fa449538
commit 6242d8e023
Notes: sideshowbarker 2024-07-17 11:06:06 +09:00 
Author: https://github.com/gmta
Commit: https://github.com/SerenityOS/serenity/commit/6242d8e023
Pull-request: https://github.com/SerenityOS/serenity/pull/18921
2 changed files with 75 additions and 0 deletions
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74
Userland/Libraries/LibGfx/Painter.cpp
View file

@ -1180,6 +1180,74 @@ ALWAYS_INLINE static void do_draw_integer_scaled_bitmap(Gfx::Bitmap& target, Int
}
}
template<bool has_alpha_channel, typename GetPixel>
ALWAYS_INLINE static void do_draw_box_sampled_scaled_bitmap(Gfx::Bitmap& target, IntRect const& dst_rect, IntRect const& clipped_rect, Gfx::Bitmap const& source, FloatRect const& src_rect, GetPixel get_pixel, float opacity)
{
float source_pixel_width = src_rect.width() / dst_rect.width();
float source_pixel_height = src_rect.height() / dst_rect.height();
float source_pixel_area = source_pixel_width * source_pixel_height;
FloatRect const pixel_box = { 0.f, 0.f, 1.f, 1.f };
// FIXME: FloatRect.right() and .bottom() subtract 1 since that is what IntRect does as well.
// This is obviously wrong and causes issues with at least .intersect(). Probably the
// best course of action is to fix Rect's behavior for .right() and .bottom(), and then
// replace this with FloatRect.intersected(...).size().area().
auto float_rect_intersection_area_fixme = [](FloatRect const& a, FloatRect const& b) -> float {
float intersected_left = max(a.left(), b.left());
float intersected_right = min(a.left() + a.width(), b.left() + b.width());
float intersected_top = max(a.top(), b.top());
float intersected_bottom = min(a.top() + a.height(), b.top() + b.height());
if (intersected_left >= intersected_right || intersected_top >= intersected_bottom)
return 0.f;
return (intersected_right - intersected_left) * (intersected_bottom - intersected_top);
};
for (int y = clipped_rect.top(); y <= clipped_rect.bottom(); ++y) {
auto* scanline = reinterpret_cast<Color*>(target.scanline(y));
for (int x = clipped_rect.left(); x <= clipped_rect.right(); ++x) {
// Project the destination pixel in the source image
FloatRect const source_box = {
src_rect.left() + (x - dst_rect.x()) * source_pixel_width,
src_rect.top() + (y - dst_rect.y()) * source_pixel_height,
source_pixel_width,
source_pixel_height,
};
IntRect enclosing_source_box = enclosing_int_rect(source_box).intersected(source.rect());
// Sum the contribution of all source pixels inside the projected pixel
float red_accumulator = 0.f;
float green_accumulator = 0.f;
float blue_accumulator = 0.f;
float total_area = 0.f;
for (int sy = enclosing_source_box.y(); sy <= enclosing_source_box.bottom(); ++sy) {
for (int sx = enclosing_source_box.x(); sx <= enclosing_source_box.right(); ++sx) {
float area = float_rect_intersection_area_fixme(source_box, pixel_box.translated(sx, sy));
auto pixel = get_pixel(source, sx, sy);
area *= pixel.alpha() / 255.f;
red_accumulator += pixel.red() * area;
green_accumulator += pixel.green() * area;
blue_accumulator += pixel.blue() * area;
total_area += area;
}
}
Color src_pixel = {
round_to<u8>(min(red_accumulator / total_area, 255.f)),
round_to<u8>(min(green_accumulator / total_area, 255.f)),
round_to<u8>(min(blue_accumulator / total_area, 255.f)),
round_to<u8>(min(total_area * 255.f / source_pixel_area * opacity, 255.f)),
};
if constexpr (has_alpha_channel)
scanline[x] = scanline[x].blend(src_pixel);
else
scanline[x] = src_pixel;
}
}
}
template<bool has_alpha_channel, Painter::ScalingMode scaling_mode, typename GetPixel>
ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect const& dst_rect, IntRect const& clipped_rect, Gfx::Bitmap const& source, FloatRect const& src_rect, GetPixel get_pixel, float opacity)
{
@ -1208,6 +1276,9 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
}
}
if constexpr (scaling_mode == Painter::ScalingMode::BoxSampling)
return do_draw_box_sampled_scaled_bitmap<has_alpha_channel>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
bool has_opacity = opacity != 1.f;
i64 shift = 1ll << 32;
i64 fractional_mask = shift - 1;
@ -1307,6 +1378,9 @@ ALWAYS_INLINE static void do_draw_scaled_bitmap(Gfx::Bitmap& target, IntRect con
case Painter::ScalingMode::BilinearBlend:
do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::BilinearBlend>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
break;
case Painter::ScalingMode::BoxSampling:
do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::BoxSampling>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
break;
case Painter::ScalingMode::None:
do_draw_scaled_bitmap<has_alpha_channel, Painter::ScalingMode::None>(target, dst_rect, clipped_rect, source, src_rect, get_pixel, opacity);
break;

1
Userland/Libraries/LibGfx/Painter.h
View file

@ -45,6 +45,7 @@ public:
NearestNeighbor,
SmoothPixels,
BilinearBlend,
BoxSampling,
None,
};