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LibGfx: Fix off-by-some in Painter::draw_scaled_bitmap_with_transform()

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Before this, drawing a 1x1 bitmap scaled up to MxN would only fill
M/2 x N/2 pixel, due to source_point going outside (0, 0).
This commit is contained in:
Nico Weber 2024-01-09 21:29:16 -05:00 committed by Andreas Kling
parent 822b7720e1
commit 7fb32b6682
Notes: sideshowbarker 2024-07-17 05:01:20 +09:00 
Author: https://github.com/nico
Commit: https://github.com/SerenityOS/serenity/commit/7fb32b6682
Pull-request: https://github.com/SerenityOS/serenity/pull/22678
3 changed files with 53 additions and 1 deletions
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1
Tests/LibGfx/CMakeLists.txt
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@ -6,6 +6,7 @@ set(TEST_SOURCES
TestGfxBitmap.cpp
TestICCProfile.cpp
TestImageDecoder.cpp
TestPainter.cpp
TestParseISOBMFF.cpp
TestRect.cpp
TestScalingFunctions.cpp

48
Tests/LibGfx/TestPainter.cpp Normal file
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@ -0,0 +1,48 @@
/*
* Copyright (c) 2024, Nico Weber <thakis@chromium.org>
*
* SPDX-License-Identifier: BSD-2-Clause
*/
#include <LibTest/TestCase.h>
#include <LibGfx/Painter.h>
TEST_CASE(draw_scaled_bitmap_with_transform)
{
auto bitmap = MUST(Gfx::Bitmap::create(Gfx::BitmapFormat::BGRx8888, { 40, 30 }));
bitmap->fill(Gfx::Color::White);
Gfx::Painter painter(bitmap);
auto source_bitmap = MUST(Gfx::Bitmap::create(Gfx::BitmapFormat::BGRx8888, { 1, 1 }));
source_bitmap->fill(Gfx::Color::Black);
auto dest_rect = source_bitmap->rect();
auto source_rect = source_bitmap->rect().to_rounded<float>();
// Identity transform: Lower left pixel is black, rest stays white.
Gfx::AffineTransform transform;
painter.draw_scaled_bitmap_with_transform(dest_rect, source_bitmap, source_rect, transform);
for (int y = 0; y < bitmap->height(); ++y) {
for (int x = 0; x < bitmap->width(); ++x) {
if (x == 0 && y == 0)
EXPECT_EQ(bitmap->get_pixel(x, y), Color::Black);
else
EXPECT_EQ(bitmap->get_pixel(x, y), Color::White);
}
}
// Scale up 1x1 source bitmap 10x in x and 5x in y and paint at 10, 20. Should fill that rect:
bitmap->fill(Gfx::Color::White);
transform = transform.translate(10, 20).scale(10, 5);
painter.draw_scaled_bitmap_with_transform(dest_rect, source_bitmap, source_rect, transform);
for (int y = 0; y < bitmap->height(); ++y) {
for (int x = 0; x < bitmap->width(); ++x) {
dbgln("{} {}: {}", x, y, bitmap->get_pixel(x, y));
if (x >= 10 && x < 10 + 10 && y >= 20 && y < 20 + 5)
EXPECT_EQ(bitmap->get_pixel(x, y), Color::Black);
else
EXPECT_EQ(bitmap->get_pixel(x, y), Color::White);
}
}
}

5
Userland/Libraries/LibGfx/Painter.cpp
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@ -2489,7 +2489,10 @@ void Painter::draw_scaled_bitmap_with_transform(IntRect const& dst_rect, Bitmap
for (int x = 0; x < clipped_bounding_rect.width(); ++x) {
auto point = Gfx::IntPoint { x, y };
auto sample_point = point + start_offset;
auto source_point = sample_transform.map(sample_point).to_rounded<int>();
// AffineTransform::map(IntPoint) rounds internally, which is wrong here. So explicitly call the FloatPoint version, and then truncate the result.
auto source_point = Gfx::IntPoint { sample_transform.map(Gfx::FloatPoint { sample_point }) };
if (!source_rect.contains(source_point))
continue;
auto source_color = bitmap.get_pixel(source_point);