Performance issue with QGraphicsItem::ItemClipsChildrenToShape.

If the child rect is bigger than the parent rect and parent has the
ItemClipsChildrenToShape flag set, then by updating the child, the whole
child rect is marked as dirty, resulting in a much larger update area
than required. This has a major impact on performance in Orbit/HB,
where e.g. item-views typically consist of a container item that clips
its children/items to shape. See attached video in QTBUG-9024.

Auto test included.

Task-number: QTBUG-9024

1 files changed, 57 insertions, 0 deletions

diff --git a/tests/auto/qgraphicsview/tst_qgraphicsview.cpp b/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
index 1df9a37..b8df7f6 100644
--- a/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
+++ b/tests/auto/qgraphicsview/tst_qgraphicsview.cpp
@@ -218,6 +218,7 @@ private slots:
     void update();
     void update2_data();
     void update2();
+    void update_ancestorClipsChildrenToShape();
     void inputMethodSensitivity();
     void inputContextReset();
     void indirectPainting();
@@ -3758,6 +3759,62 @@ void tst_QGraphicsView::update2()
 #endif
 }
 
+void tst_QGraphicsView::update_ancestorClipsChildrenToShape()
+{
+    QGraphicsScene scene(-150, -150, 300, 300);
+
+    /*
+    Add three rects:
+
+    +------------------+
+    | child            |
+    | +--------------+ |
+    | | parent       | |
+    | |  +-----------+ |
+    | |  |grandParent| |
+    | |  +-----------+ |
+    | +--------------+ |
+    +------------------+
+
+    ... where both the parent and the grand parent clips children to shape.
+    */
+    QApplication::processEvents(); // Get rid of pending update.
+
+    QGraphicsRectItem *grandParent = static_cast<QGraphicsRectItem *>(scene.addRect(0, 0, 50, 50));
+    grandParent->setBrush(Qt::black);
+    grandParent->setFlag(QGraphicsItem::ItemClipsChildrenToShape);
+
+    QGraphicsRectItem *parent = static_cast<QGraphicsRectItem *>(scene.addRect(-50, -50, 100, 100));
+    parent->setBrush(QColor(0, 0, 255, 125));
+    parent->setParentItem(grandParent);
+    parent->setFlag(QGraphicsItem::ItemClipsChildrenToShape);
+
+    QGraphicsRectItem *child = static_cast<QGraphicsRectItem *>(scene.addRect(-100, -100, 200, 200));
+    child->setBrush(QColor(255, 0, 0, 125));
+    child->setParentItem(parent);
+
+    CustomView view(&scene);
+    view.show();
+    QTest::qWaitForWindowShown(&view);
+    QTRY_VERIFY(view.painted);
+
+    view.lastUpdateRegions.clear();
+    view.painted = false;
+
+    // Call child->update() and make sure the updated area is within the ancestors' clip.
+    QRectF expected = child->deviceTransform(view.viewportTransform()).mapRect(child->boundingRect());
+    expected &= grandParent->deviceTransform(view.viewportTransform()).mapRect(grandParent->boundingRect());
+
+    child->update();
+    QTRY_VERIFY(view.painted);
+
+#ifndef QT_MAC_USE_COCOA //cocoa doesn't support drawing regions
+    QTRY_VERIFY(view.painted);
+    QCOMPARE(view.lastUpdateRegions.size(), 1);
+    QCOMPARE(view.lastUpdateRegions.at(0), QRegion(expected.toAlignedRect()));
+#endif
+}
+
 class FocusItem : public QGraphicsRectItem
 {
 public: