Basic high-dpi "retina" support for Qt 5.

Bring Qt 5 on par with Qt 4, prepare for more comprehensive
support later on.

Introduce device independent pixels (dips), device pixels,
and devicePixelRatio. Add high-dpi support to QPainter,
QGLWidget, the cocoa platform plugin, mac and fusion styles.

Dips are similar to CSS pixels, Apple points and
Android density-independent pixels. Device pixels
are pixels in the backing store/physical pixels on screen.
devicePixelRatio is the ratio between them, which is
1.0 on standard displays and 2.0 on "retina" displays.

New API:
QImage::devicePixelRatio() and setDevicePixelRatio()
QPixmap::devicePixelRatio() and setDevicePixelRatio()
QWindow::devicePixelRatio()
QScreen::devicePixelRatio()
QGuiApplicaiton::devicePixelRatio()

Change-Id: If98c3ca9bfdf0e1bdbcf7574cd5b912c9ff63856
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

2 files changed, 24 insertions, 6 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index e167e646c0..ace25fa78b 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -1794,6 +1794,23 @@ QRegion QWidgetPrivate::clipRegion() const
     return r;
 }
 
+void QWidgetPrivate::setSystemClip(QPaintDevice *paintDevice, const QRegion &region)
+{
+// Transform the system clip region from device-independent pixels to device pixels
+// Qt 5.0.0: This is a Mac-only code path for now, can be made cross-platform once
+// it has been tested.
+    QPaintEngine *paintEngine = paintDevice->paintEngine();
+#ifdef Q_OS_MAC
+    const qreal devicePixelRatio = (paintDevice->physicalDpiX() == 0 || paintDevice->logicalDpiX() == 0) ?
+                                    1.0 : (paintDevice->physicalDpiX() / paintDevice->logicalDpiX());
+    QTransform scaleTransform;
+    scaleTransform.scale(devicePixelRatio, devicePixelRatio);
+    paintEngine->d_func()->systemClip = scaleTransform.map(region);
+#else
+    paintEngine->d_func()->systemClip = region;
+#endif
+}
+
 #ifndef QT_NO_GRAPHICSEFFECT
 void QWidgetPrivate::invalidateGraphicsEffectsRecursively()
 {
@@ -4998,13 +5015,12 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
             QWidgetPaintContext context(pdev, rgn, offset, flags, sharedPainter, backingStore);
             sourced->context = &context;
             if (!sharedPainter) {
-                QPaintEngine *paintEngine = pdev->paintEngine();
-                paintEngine->d_func()->systemClip = rgn.translated(offset);
+                setSystemClip(pdev, rgn.translated(offset));
                 QPainter p(pdev);
                 p.translate(offset);
                 context.painter = &p;
                 graphicsEffect->draw(&p);
-                paintEngine->d_func()->systemClip = QRegion();
+                setSystemClip(pdev, QRegion());
             } else {
                 context.painter = sharedPainter;
                 if (sharedPainter->worldTransform() != sourced->lastEffectTransform) {
@@ -5061,7 +5077,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
 
 #endif
                 if (sharedPainter)
-                    paintEngine->d_func()->systemClip = toBePainted;
+                    setSystemClip(pdev, toBePainted);
                 else
                     paintEngine->d_func()->systemRect = q->data->crect;
 
@@ -5073,7 +5089,7 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                 }
 
                 if (!sharedPainter)
-                    paintEngine->d_func()->systemClip = toBePainted.translated(offset);
+                    setSystemClip(pdev, toBePainted.translated(offset));
 
                 if (!onScreen && !asRoot && !isOpaque && q->testAttribute(Qt::WA_TintedBackground)) {
                     QPainter p(q);
@@ -5108,7 +5124,8 @@ void QWidgetPrivate::drawWidget(QPaintDevice *pdev, const QRegion &rgn, const QP
                     paintEngine->d_func()->systemRect = QRect();
                 else
                     paintEngine->d_func()->currentClipDevice = 0;
-                paintEngine->d_func()->systemClip = QRegion();
+
+                setSystemClip(pdev, QRegion());
             }
             q->setAttribute(Qt::WA_WState_InPaintEvent, false);
             if (q->paintingActive())
diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index 8aba276966..1d183e41f1 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -402,6 +402,7 @@ public:
 
     QRect clipRect() const;
     QRegion clipRegion() const;
+    void setSystemClip(QPaintDevice *paintDevice, const QRegion &region);
     void subtractOpaqueChildren(QRegion &rgn, const QRect &clipRect) const;
     void subtractOpaqueSiblings(QRegion &source, bool *hasDirtySiblingsAbove = 0,
                                 bool alsoNonOpaque = false) const;