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>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/widgets/styles/qstyle_p.h b/src/widgets/styles/qstyle_p.h
index 85e8e54b16..78dfc6fed9 100644
--- a/src/widgets/styles/qstyle_p.h
+++ b/src/widgets/styles/qstyle_p.h
@@ -43,6 +43,7 @@
 #define QSTYLE_P_H
 
 #include "private/qobject_p.h"
+#include <QtGui/qguiapplication.h>
 #include <QtWidgets/qstyle.h>
 
 QT_BEGIN_NAMESPACE
@@ -74,12 +75,18 @@ public:
 
 inline QImage styleCacheImage(const QSize &size)
 {
-    return QImage(size, QImage::Format_ARGB32_Premultiplied);
+    const qreal pixelRatio = qApp->devicePixelRatio();
+    QImage cacheImage = QImage(size * pixelRatio, QImage::Format_ARGB32_Premultiplied);
+    cacheImage.setDevicePixelRatio(pixelRatio);
+    return cacheImage;
 }
 
 inline QPixmap styleCachePixmap(const QSize &size)
 {
-    return QPixmap(size);
+    const qreal pixelRatio = qApp->devicePixelRatio();
+    QPixmap cachePixmap = QPixmap(size * pixelRatio);
+    cachePixmap.setDevicePixelRatio(pixelRatio);
+    return cachePixmap;
 }
 
 #define BEGIN_STYLE_PIXMAPCACHE(a) \