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, 44 insertions, 0 deletions

diff --git a/src/gui/image/qpixmap.cpp b/src/gui/image/qpixmap.cpp
index 113369fd75..8782119b7a 100644
--- a/src/gui/image/qpixmap.cpp
+++ b/src/gui/image/qpixmap.cpp
@@ -638,6 +638,50 @@ void QPixmap::setMask(const QBitmap &mask)
     data->fromImage(image, Qt::AutoColor);
 }
 
+/*!
+    Returns the device pixel ratio for the pixmap. This is the
+    ratio between pixmap pixels and device-independent pixels.
+
+    Use this function when calculating layout geometry based on
+    the pixmap size: QSize layoutSize = image.size() / image.devicePixelRatio()
+
+    The default value is 1.0.
+
+    \sa setDevicePixelRatio()
+*/
+qreal QPixmap::devicePixelRatio() const
+{
+    if (!data)
+        return qreal(1.0);
+    return data->devicePixelRatio();
+}
+
+/*!
+    Sets the the device pixel ratio for the pixmap. This is the
+    ratio between image pixels and device-independent pixels.
+
+    The default value is 1.0. Setting it to something else has
+    two effects:
+
+    QPainters that are opened on the pixmap will be scaled. For
+    example, painting on a 200x200 image if with a ratio of 2.0
+    will result in effective (device-independent) painting bounds
+    of 100x100.
+
+    Code paths in Qt that calculate layout geometry based on the
+    pixmap size will take the ratio into account:
+    QSize layoutSize = pixmap.size() / pixmap.devicePixelRatio()
+    The net effect of this is that the pixmap is displayed as
+    high-dpi pixmap rather than a large pixmap.
+
+        \sa devicePixelRatio()
+*/
+void QPixmap::setDevicePixelRatio(qreal scaleFactor)
+{
+    detach();
+    data->setDevicePixelRatio(scaleFactor);
+}
+
 #ifndef QT_NO_IMAGE_HEURISTIC_MASK
 /*!
     Creates and returns a heuristic mask for this pixmap.