LibGfx: Fix opacity handling in Painter::draw_scaled_bitmap

If the source image had no alpha channel we'd ignore opacity < 1.0 and
blit the image as if it was fully opaque.

With this fix, adjusting the opacity of windows with mousewheel while
holding super works in hidpi mode.

2 files changed, 6 insertions, 5 deletions

diff --git a/Userland/Demos/LibGfxScaleDemo/main.cpp b/Userland/Demos/LibGfxScaleDemo/main.cpp
index aa4552fc81..a23f061070 100644
--- a/Userland/Demos/LibGfxScaleDemo/main.cpp
+++ b/Userland/Demos/LibGfxScaleDemo/main.cpp
@@ -108,10 +108,11 @@ void Canvas::draw(Gfx::Painter& painter)
 
     painter.blit({ 25, 101 }, *buggie, { 2, 30, 3 * buggie->width(), 20 });
 
-    // banner does not have an alpha channel.
-    auto banner = Gfx::Bitmap::load_from_file("/res/graphics/brand-banner.png");
-    painter.fill_rect({ 25, 122, 28, 20 }, Color::Green);
-    painter.blit({ 25, 122 }, *banner, { 314, 12, 28, 20 }, 0.8);
+    // grid does not have an alpha channel.
+    auto grid = Gfx::Bitmap::load_from_file("/res/wallpapers/grid.png");
+    ASSERT(!grid->has_alpha_channel());
+    painter.fill_rect({ 25, 122, 62, 20 }, Color::Green);
+    painter.blit({ 25, 122 }, *grid, { (grid->width() - 62) / 2, (grid->height() - 20) / 2 + 40, 62, 20 }, 0.9);
 }
 
 int main(int argc, char** argv)
diff --git a/Userland/Libraries/LibGfx/Painter.cpp b/Userland/Libraries/LibGfx/Painter.cpp
index fff871bcd4..baf2a7a7e7 100644
--- a/Userland/Libraries/LibGfx/Painter.cpp
+++ b/Userland/Libraries/LibGfx/Painter.cpp
@@ -848,7 +848,7 @@ void Painter::draw_scaled_bitmap(const IntRect& a_dst_rect, const Gfx::Bitmap& s
     if (clipped_rect.is_empty())
         return;
 
-    if (source.has_alpha_channel()) {
+    if (source.has_alpha_channel() || opacity != 1.0f) {
         switch (source.format()) {
         case BitmapFormat::RGB32:
             do_draw_scaled_bitmap<true>(*m_target, dst_rect, clipped_rect, source, src_rect, get_pixel<BitmapFormat::RGB32>, opacity);