Fix incorrect QImage transformation when its devicePixelRatio != 1

QImage::transformed compensates for unwanted translation. This compensation
is performed in "pixel space". However, a possible code path to perform the
transformation uses QPainter which is devicePixelRatio-aware and expects the
transformation matrix to be in logical coordinates.

For example, image.transformed(QTransform().rotate(45)) will result in
cropped out image if devicePixelRatio == 2.

Change-Id: I830ff3ffa25531d842dc9c77f1d0e8d4bd502c9d
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@theqtcompany.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/gui/image/qimage.cpp b/src/gui/image/qimage.cpp
index 4e10b4cb4b..ea23954a49 100644
--- a/src/gui/image/qimage.cpp
+++ b/src/gui/image/qimage.cpp
@@ -4480,7 +4480,6 @@ QImage QImage::transformed(const QTransform &matrix, Qt::TransformationMode mode
 
     dImage.d->dpmx = dotsPerMeterX();
     dImage.d->dpmy = dotsPerMeterY();
-    dImage.d->devicePixelRatio = devicePixelRatio();
 
     switch (bpp) {
         // initizialize the data
@@ -4502,13 +4501,19 @@ QImage QImage::transformed(const QTransform &matrix, Qt::TransformationMode mode
     }
 
     if (target_format >= QImage::Format_RGB32) {
+        // Prevent QPainter from applying devicePixelRatio corrections
+        const QImage sImage = (devicePixelRatio() != 1) ? QImage(constBits(), width(), height(), format()) : *this;
+
+        Q_ASSERT(sImage.devicePixelRatio() == 1);
+        Q_ASSERT(sImage.devicePixelRatio() == dImage.devicePixelRatio());
+
         QPainter p(&dImage);
         if (mode == Qt::SmoothTransformation) {
             p.setRenderHint(QPainter::Antialiasing);
             p.setRenderHint(QPainter::SmoothPixmapTransform);
         }
         p.setTransform(mat);
-        p.drawImage(QPoint(0, 0), *this);
+        p.drawImage(QPoint(0, 0), sImage);
     } else {
         bool invertible;
         mat = mat.inverted(&invertible);                // invert matrix
@@ -4520,6 +4525,8 @@ QImage QImage::transformed(const QTransform &matrix, Qt::TransformationMode mode
         int dbpl = dImage.bytesPerLine();
         qt_xForm_helper(mat, 0, type, bpp, dImage.bits(), dbpl, 0, hd, sptr, sbpl, ws, hs);
     }
+
+    dImage.d->devicePixelRatio = devicePixelRatio();
     return dImage;
 }