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, 28 insertions, 1 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 80e13227ee..fb14490a24 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -703,7 +703,34 @@ QList<QScreen *> QGuiApplication::screens()
 
 
 /*!
-    Returns the top level window at the given position \a pos, if any.
+    Returns the highest screen device pixel ratio found on
+    the system. This is the ratio between physical pixels and
+    device-independent pixels.
+
+    Use this function only when you don't know which window you are targeting.
+    If you do know the target window use QWindow::devicePixelRatio() instead.
+
+    \sa QWindow::devicePixelRatio();
+    \sa QGuiApplicaiton::devicePixelRatio();
+*/
+qreal QGuiApplication::devicePixelRatio() const
+{
+    // Cache topDevicePixelRatio, iterate through the screen list once only.
+    static qreal topDevicePixelRatio = 0.0;
+    if (!qFuzzyIsNull(topDevicePixelRatio)) {
+        return topDevicePixelRatio;
+    }
+
+    topDevicePixelRatio = 1.0; // make sure we never return 0.
+    foreach (QScreen *screen, QGuiApplicationPrivate::screen_list) {
+        topDevicePixelRatio = qMax(topDevicePixelRatio, screen->devicePixelRatio());
+    }
+
+    return topDevicePixelRatio;
+}
+
+/*!
+    Returns the top level window at the given position, if any.
 */
 QWindow *QGuiApplication::topLevelAt(const QPoint &pos)
 {