Reverts using composition mode when using DeviceCoordinateMode cache.

Going back to always blitting the newly painted areas to the cache (and
not blending it like before). To avoid painting artifacts when the item
is rotated and when it uses DeviceCoordinateMode cache mode, the entire
cache is always repainted in that case.

Task-number: QTBUG-7863
Reviewed-by: trustme

2 files changed, 17 insertions, 14 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index 6934abc..6581727 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -4285,12 +4285,7 @@ static void _q_paintIntoCache(QPixmap *pix, QGraphicsItem *item, const QRegion &
     if (!subPix.isNull()) {
         // Blit the subpixmap into the main pixmap.
         pixmapPainter.begin(pix);
-        if (item->cacheMode() == QGraphicsItem::DeviceCoordinateCache
-            && itemToPixmap.type() > QTransform::TxTranslate) {
-            pixmapPainter.setCompositionMode(QPainter::CompositionMode_SourceAtop);
-        } else {
-            pixmapPainter.setCompositionMode(QPainter::CompositionMode_Source);
-        }
+        pixmapPainter.setCompositionMode(QPainter::CompositionMode_Source);
         pixmapPainter.setClipRegion(pixmapExposed);
         pixmapPainter.drawPixmap(br.topLeft(), subPix);
         pixmapPainter.end();
@@ -4456,6 +4451,8 @@ void QGraphicsScenePrivate::drawItemHelper(QGraphicsItem *item, QPainter *painte
         }
 
         // Create or reuse offscreen pixmap, possibly scroll/blit from the old one.
+        // If the world transform is rotated we always recreate the cache to avoid
+        // wrong blending.
         bool pixModified = false;
         QGraphicsItemCache::DeviceData *deviceData = &itemCache->deviceData[widget];
         bool invertable = true;
@@ -4463,7 +4460,9 @@ void QGraphicsScenePrivate::drawItemHelper(QGraphicsItem *item, QPainter *painte
         if (invertable)
             diff *= painter->worldTransform();
         deviceData->lastTransform = painter->worldTransform();
-        if (!invertable || diff.type() > QTransform::TxTranslate) {
+        if (!invertable
+            || diff.type() > QTransform::TxTranslate
+            || painter->worldTransform().type() > QTransform::TxScale) {
             pixModified = true;
             itemCache->allExposed = true;
             itemCache->exposed.clear();
diff --git a/tests/auto/qgraphicsitem/tst_qgraphicsitem.cpp b/tests/auto/qgraphicsitem/tst_qgraphicsitem.cpp
index 2b507cb..6725159 100644
--- a/tests/auto/qgraphicsitem/tst_qgraphicsitem.cpp
+++ b/tests/auto/qgraphicsitem/tst_qgraphicsitem.cpp
@@ -6825,6 +6825,9 @@ void tst_QGraphicsItem::cacheMode()
     QTRY_COMPARE(tester->repaints, 4);
     QCOMPARE(testerChild->repaints, 4);
     QCOMPARE(testerChild2->repaints, 3);
+    tester->resetTransform();
+    testerChild->resetTransform();
+    testerChild2->resetTransform();
 
     // Explicit update causes a repaint.
     tester->update(0, 0, 5, 5);
@@ -6898,23 +6901,24 @@ void tst_QGraphicsItem::cacheMode()
     // because the parent is rotated with a perspective.
     testerChild->setPos(1, 1);
     QTest::qWait(25);
-    QTRY_COMPARE(tester->repaints, 10);
+    QTRY_COMPARE(tester->repaints, 11);
     QCOMPARE(testerChild->repaints, 10);
     QCOMPARE(testerChild2->repaints, 5);
+    tester->resetTransform();
 
     // Make a huge item
     tester->setGeometry(QRectF(-4000, -4000, 8000, 8000));
     QTest::qWait(25);
-    QTRY_COMPARE(tester->repaints, 11);
-    QCOMPARE(testerChild->repaints, 10);
+    QTRY_COMPARE(tester->repaints, 12);
+    QCOMPARE(testerChild->repaints, 11);
     QCOMPARE(testerChild2->repaints, 5);
 
     // Move the large item - will cause a repaint as the
     // cache is clipped.
     tester->setPos(5, 0);
     QTest::qWait(25);
-    QTRY_COMPARE(tester->repaints, 12);
-    QCOMPARE(testerChild->repaints, 10);
+    QTRY_COMPARE(tester->repaints, 13);
+    QCOMPARE(testerChild->repaints, 11);
     QCOMPARE(testerChild2->repaints, 5);
 
     // Hiding and showing should invalidate the cache
@@ -6922,8 +6926,8 @@ void tst_QGraphicsItem::cacheMode()
     QTest::qWait(25);
     tester->show();
     QTest::qWait(25);
-    QTRY_COMPARE(tester->repaints, 13);
-    QCOMPARE(testerChild->repaints, 11);
+    QTRY_COMPARE(tester->repaints, 14);
+    QCOMPARE(testerChild->repaints, 12);
     QCOMPARE(testerChild2->repaints, 6);
 }