QGraphicsView: Create high-dpi background pixmap

Scale pixmap size by the viewport devicePixelRatio,
and make sure scrolling deltas and expose regions
are correctly scaled.

Change-Id: Ibeac34c5ecd531ca7c09802f0b5e1f45b8e31b65
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 11 insertions, 3 deletions

diff --git a/src/widgets/graphicsview/qgraphicsview.cpp b/src/widgets/graphicsview/qgraphicsview.cpp
index 705d5a0f4d..1320545748 100644
--- a/src/widgets/graphicsview/qgraphicsview.cpp
+++ b/src/widgets/graphicsview/qgraphicsview.cpp
@@ -3483,7 +3483,9 @@ void QGraphicsView::paintEvent(QPaintEvent *event)
         // Recreate the background pixmap, and flag the whole background as
         // exposed.
         if (d->mustResizeBackgroundPixmap) {
-            d->backgroundPixmap = QPixmap(viewport()->size());
+            const qreal dpr = d->viewport->devicePixelRatioF();
+            d->backgroundPixmap = QPixmap(viewport()->size() * dpr);
+            d->backgroundPixmap.setDevicePixelRatio(dpr);
             QBrush bgBrush = viewport()->palette().brush(viewport()->backgroundRole());
             if (!bgBrush.isOpaque())
                 d->backgroundPixmap.fill(Qt::transparent);
@@ -3679,14 +3681,20 @@ void QGraphicsView::scrollContentsBy(int dx, int dy)
         && X11->use_xrender
 #endif
         ) {
+        // Below, QPixmap::scroll() works in device pixels, while the delta values
+        // and backgroundPixmapExposed are in device independent pixels.
+        const qreal dpr = d->backgroundPixmap.devicePixelRatio();
+        const qreal inverseDpr = qreal(1) / dpr;
+
         // Scroll the background pixmap
         QRegion exposed;
         if (!d->backgroundPixmap.isNull())
-            d->backgroundPixmap.scroll(dx, dy, d->backgroundPixmap.rect(), &exposed);
+            d->backgroundPixmap.scroll(dx * dpr, dy * dpr, d->backgroundPixmap.rect(), &exposed);
 
         // Invalidate the background pixmap
         d->backgroundPixmapExposed.translate(dx, dy);
-        d->backgroundPixmapExposed += exposed;
+        const QRegion exposedScaled = QTransform::fromScale(inverseDpr, inverseDpr).map(exposed);
+        d->backgroundPixmapExposed += exposedScaled;
     }
 
     // Always replay on scroll.