Remove some uses of QImage::setAlphaChannel

Remove once where it did nothing, and once where using composition mode
masking avoids first creating a masking image.

Change-Id: Ia4c93eac6ebb11fcdab34d306d943a7f87906b7e
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 6 insertions, 2 deletions

diff --git a/src/widgets/effects/qpixmapfilter.cpp b/src/widgets/effects/qpixmapfilter.cpp
index 637c9c6aba..1f899c2660 100644
--- a/src/widgets/effects/qpixmapfilter.cpp
+++ b/src/widgets/effects/qpixmapfilter.cpp
@@ -1135,8 +1135,12 @@ void QPixmapColorizeFilter::draw(QPainter *painter, const QPointF &dest, const Q
         destImage = std::move(buffer);
     }
 
-    if (srcImage.hasAlphaChannel())
-        destImage.setAlphaChannel(srcImage.alphaChannel());
+    if (srcImage.hasAlphaChannel()) {
+        Q_ASSERT(destImage.format() == QImage::Format_ARGB32_Premultiplied);
+        QPainter maskPainter(&destImage);
+        maskPainter.setCompositionMode(QPainter::CompositionMode_DestinationIn);
+        maskPainter.drawImage(0, 0, srcImage);
+    }
 
     painter->drawImage(dest, destImage);
 }