qpa: Fix rendering issue in blitter engine (negative scaling factors)

A 180° rotation results in a TxScale QTransform with negative scaling
factors (x=-1.0 y=-1.0). This is not properly handled by blitter paint
engine yet, so use software rendering fallback in this case.

This rendering issue can be seen when using "-webkit-transform" CSS
property in WebKit with DirectFB QPA platform.

cherry-picked from qt5/qtbase 07ea3cf0b3883979e84bd91a5dc6a7a126de3123

Change-Id: I0911fd1166a3968d0a1d6bcca47ce2b26866de44
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

2 files changed, 40 insertions, 1 deletions

diff --git a/src/gui/painting/qpaintengine_blitter.cpp b/src/gui/painting/qpaintengine_blitter.cpp
index cbf42bbaac..5ee2bc9fec 100644
--- a/src/gui/painting/qpaintengine_blitter.cpp
+++ b/src/gui/painting/qpaintengine_blitter.cpp
@@ -330,7 +330,11 @@ void QBlitterPaintEnginePrivate::updateTransformState(QPainterState *s)
 {
     QTransform::TransformationType type = s->matrix.type();
 
-    caps.updateState(STATE_XFORM_COMPLEX, type > QTransform::TxScale);
+    // consider scaling operations with a negative factor as "complex" for now.
+    // as some blitters could handle axisymmetrical operations, we should improve blitter
+    // paint engine to handle them as a capability
+    caps.updateState(STATE_XFORM_COMPLEX, (type > QTransform::TxScale) ||
+        ((type == QTransform::TxScale) && ((s->matrix.m11() < 0.0) || (s->matrix.m22() < 0.0))));
     caps.updateState(STATE_XFORM_SCALE, type > QTransform::TxTranslate);
 
     hasXForm = type >= QTransform::TxTranslate;
diff --git a/tests/auto/qpainter/tst_qpainter.cpp b/tests/auto/qpainter/tst_qpainter.cpp
index fc2d0453cd..f46e50858d 100644
--- a/tests/auto/qpainter/tst_qpainter.cpp
+++ b/tests/auto/qpainter/tst_qpainter.cpp
@@ -111,6 +111,7 @@ private slots:
 
     void drawBorderPixmap();
     void drawPixmapFragments();
+    void drawPixmapNegativeScale();
 
     void drawLine_data();
     void drawLine();
@@ -1058,6 +1059,40 @@ void tst_QPainter::drawPixmapFragments()
     QVERIFY(fragment.opacity == 1);
 }
 
+void tst_QPainter::drawPixmapNegativeScale()
+{
+    // basePixmap is a 16x16 opaque white square ...
+    QPixmap basePixmap(16, 16);
+    basePixmap.fill(QColor(255, 255, 255, 255));
+    // ... with an opaque black 8x16 left strip
+    QPainter p(&basePixmap);
+    p.setCompositionMode(QPainter::CompositionMode_Source);
+    p.fillRect(QRect(0, 0, 8, 16), QColor(0, 0, 0, 255));
+    p.end();
+
+    // verify one pixel value for each strip
+    QImage baseImage = basePixmap.toImage().convertToFormat(QImage::Format_ARGB32);
+    QVERIFY(baseImage.pixel(4, 8) == qRgba(0, 0, 0, 255));
+    QVERIFY(baseImage.pixel(12, 8) == qRgba(255, 255, 255, 255));
+
+    // resultPixmap is a 16x16 square
+    QPixmap resultPixmap(16, 16);
+
+    // draw basePixmap over resultPixmap using x=-1.0 y=-1.0
+    // scaling factors (i.e. 180° rotation)
+    QPainter p2(&resultPixmap);
+    p2.setCompositionMode(QPainter::CompositionMode_Source);
+    p2.scale(qreal(-1.0), qreal(-1.0));
+    p2.translate(-resultPixmap.width(), -resultPixmap.height());
+    p2.drawPixmap(resultPixmap.rect(), basePixmap);
+    p2.end();
+
+    // check result
+    QImage resultImage = resultPixmap.toImage().convertToFormat(QImage::Format_ARGB32);
+    QVERIFY(resultImage.pixel(4, 8) == qRgba(255, 255, 255, 255)); // left strip is now white
+    QVERIFY(resultImage.pixel(12, 8) == qRgba(0, 0, 0, 255)); // and right strip is now black
+}
+
 void tst_QPainter::drawLine_data()
 {
     QTest::addColumn<QLine>("line");