QGraphicsItem device coordinate cache unefficient in portrait mode

Problem was that we always invalidated the cache whenever the item was
rotated. This is however not required for simple rotations such as
90, 180 and 270 degrees. This commit also removes the somewhat arbitrary
logic which takes the desktop size into account. We now use the viewport
size instead.

Auto test included.

Task-number: QT-3779
Reviewed-by: yoann
(cherry picked from commit e84ab1fee7f44a28ee82793f83b0b27d04d28c09)

1 files changed, 58 insertions, 18 deletions

diff --git a/src/gui/graphicsview/qgraphicsscene.cpp b/src/gui/graphicsview/qgraphicsscene.cpp
index c166c30..83fe56a 100644
--- a/src/gui/graphicsview/qgraphicsscene.cpp
+++ b/src/gui/graphicsview/qgraphicsscene.cpp
@@ -4362,6 +4362,50 @@ static void _q_paintIntoCache(QPixmap *pix, QGraphicsItem *item, const QRegion &
     }
 }
 
+// Copied from qpaintengine_vg.cpp
+// Returns true for 90, 180, and 270 degree rotations.
+static inline bool transformIsSimple(const QTransform& transform)
+{
+    QTransform::TransformationType type = transform.type();
+    if (type == QTransform::TxNone || type == QTransform::TxTranslate) {
+        return true;
+    } else if (type == QTransform::TxScale) {
+        // Check for 0 and 180 degree rotations.
+        // (0 might happen after 4 rotations of 90 degrees).
+        qreal m11 = transform.m11();
+        qreal m12 = transform.m12();
+        qreal m21 = transform.m21();
+        qreal m22 = transform.m22();
+        if (m12 == 0.0f && m21 == 0.0f) {
+            if (m11 == 1.0f && m22 == 1.0f)
+                return true; // 0 degrees
+            else if (m11 == -1.0f && m22 == -1.0f)
+                return true; // 180 degrees.
+            if(m11 == 1.0f && m22 == -1.0f)
+                return true; // 0 degrees inverted y.
+            else if(m11 == -1.0f && m22 == 1.0f)
+                return true; // 180 degrees inverted y.
+        }
+    } else if (type == QTransform::TxRotate) {
+        // Check for 90, and 270 degree rotations.
+        qreal m11 = transform.m11();
+        qreal m12 = transform.m12();
+        qreal m21 = transform.m21();
+        qreal m22 = transform.m22();
+        if (m11 == 0.0f && m22 == 0.0f) {
+            if (m12 == 1.0f && m21 == -1.0f)
+                return true; // 90 degrees.
+            else if (m12 == -1.0f && m21 == 1.0f)
+                return true; // 270 degrees.
+            else if (m12 == -1.0f && m21 == -1.0f)
+                return true; // 90 degrees inverted y.
+            else if (m12 == 1.0f && m21 == 1.0f)
+                return true; // 270 degrees inverted y.
+        }
+    }
+    return false;
+}
+
 /*!
     \internal
 
@@ -4530,32 +4574,28 @@ void QGraphicsScenePrivate::drawItemHelper(QGraphicsItem *item, QPainter *painte
         if (invertable)
             diff *= painter->worldTransform();
         deviceData->lastTransform = painter->worldTransform();
-        if (!invertable
-            || diff.type() > QTransform::TxTranslate
-            || painter->worldTransform().type() > QTransform::TxScale) {
+        bool allowPartialCacheExposure = false;
+        bool simpleTransform = invertable && diff.type() <= QTransform::TxTranslate
+                               && transformIsSimple(painter->worldTransform());
+        if (!simpleTransform) {
             pixModified = true;
             itemCache->allExposed = true;
             itemCache->exposed.clear();
+            deviceData->cacheIndent = QPoint();
             pix = QPixmap();
+        } else {
+            allowPartialCacheExposure = deviceData->cacheIndent != QPoint();
         }
 
-        // ### This is a pretty bad way to determine when to start partial
-        // exposure for DeviceCoordinateCache but it's the least intrusive
-        // approach for now.
-#if 0
-        // Only if the device rect isn't fully contained.
-        bool allowPartialCacheExposure = !viewRect.contains(deviceRect);
-#else
-        // Only if deviceRect is 20% taller or wider than the desktop.
-        bool allowPartialCacheExposure = false;
-        if (widget) {
-            QRect desktopRect = QApplication::desktop()->availableGeometry(widget);
-            allowPartialCacheExposure = (desktopRect.width() * 1.2 < deviceRect.width()
-                                         || desktopRect.height() * 1.2 < deviceRect.height());
+        // Allow partial cache exposure if the device rect isn't fully contained and
+        // deviceRect is 20% taller or wider than the viewRect.
+        if (!allowPartialCacheExposure && !viewRect.contains(deviceRect)) {
+            allowPartialCacheExposure = (viewRect.width() * 1.2 < deviceRect.width())
+                                         || (viewRect.height() * 1.2 < deviceRect.height());
         }
-#endif
+
         QRegion scrollExposure;
-        if (deviceData->cacheIndent != QPoint() || allowPartialCacheExposure) {
+        if (allowPartialCacheExposure) {
             // Part of pixmap is drawn. Either device contains viewrect (big
             // item covers whole screen) or parts of device are outside the
             // viewport. In either case the device rect must be the intersect