Add devicePixelRatio metric to QPaintDevice.

Previously QPainter computed the devicePixelRatio
based on the physical and logical dpi, and expected
that the ratio between them would be either 1x or
2x.

This was problematic for paint devices like printers
where the physical dpi can be much higher than the
logical dpi, and also for QScreen where the physical
dpi would have to be defined as a multiple of the
logical dpi.

Add QPaintDevice::PdmDevicePixelRatio and QPaintDevice::
devicePixelRatio() getter and implement it for the
QPaintDevice subclasses. Use it when calculating the
highdpi scale transform in qpainter.cpp and when scaling
the clip rect in qwidget.cpp.

Remove physical dpi scaling for QImage, QPixmap and
QOpenGLPaintDevice, reverting to the old behavior.

Change-Id: I6c97510613196d4536ff39d08e9750b8782283d4
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Gunnar Sletta <gunnar.sletta@digia.com>

6 files changed, 38 insertions, 17 deletions

diff --git a/src/gui/painting/qpaintdevice.cpp b/src/gui/painting/qpaintdevice.cpp
index 6ac288607d..81d2063039 100644
--- a/src/gui/painting/qpaintdevice.cpp
+++ b/src/gui/painting/qpaintdevice.cpp
@@ -95,6 +95,8 @@ int QPaintDevice::metric(PaintDeviceMetric m) const
     } else if (m == PdmNumColors) {
         // FIXME: does this need to be a real value?
         return 256;
+    } else if (m == PdmDevicePixelRatio) {
+        return 1;
     } else {
         qDebug("Unrecognised metric %d!",m);
         return 0;
diff --git a/src/gui/painting/qpaintdevice.h b/src/gui/painting/qpaintdevice.h
index 1529b701cf..bb66f32b7f 100644
--- a/src/gui/painting/qpaintdevice.h
+++ b/src/gui/painting/qpaintdevice.h
@@ -65,7 +65,8 @@ public:
         PdmDpiX,
         PdmDpiY,
         PdmPhysicalDpiX,
-        PdmPhysicalDpiY
+        PdmPhysicalDpiY,
+        PdmDevicePixelRatio
     };
 
     virtual ~QPaintDevice();
@@ -82,6 +83,7 @@ public:
     int logicalDpiY() const { return metric(PdmDpiY); }
     int physicalDpiX() const { return metric(PdmPhysicalDpiX); }
     int physicalDpiY() const { return metric(PdmPhysicalDpiY); }
+    int devicePixelRatio() const { return metric(PdmDevicePixelRatio); }
     int colorCount() const { return metric(PdmNumColors); }
     int depth() const { return metric(PdmDepth); }
 
diff --git a/src/gui/painting/qpaintdevice.qdoc b/src/gui/painting/qpaintdevice.qdoc
index 7397dc7fc2..993b23850e 100644
--- a/src/gui/painting/qpaintdevice.qdoc
+++ b/src/gui/painting/qpaintdevice.qdoc
@@ -114,6 +114,10 @@
     \value PdmPhysicalDpiY The vertical resolution of the device in
     dots per inch. See also physicalDpiY().
 
+    \value PdmDevicePixelRatio The device pixel ratio for device. Common
+    values are 1 for normal-dpi displays and 2 for high-dpi "retina"
+    displays.
+
     \sa metric()
 */
 
@@ -273,3 +277,12 @@
 
     \sa physicalDpiX(), logicalDpiY()
 */
+
+/*!
+    \fn int QPaintDevice::devicePixelRatio() const
+
+    Returns the device pixel ratio for device.
+
+    Common values are 1 for normal-dpi displays and 2 for high-dpi
+    "retina" displays.
+*/
diff --git a/src/gui/painting/qpainter.cpp b/src/gui/painting/qpainter.cpp
index be77fffc7c..e42b70427c 100644
--- a/src/gui/painting/qpainter.cpp
+++ b/src/gui/painting/qpainter.cpp
@@ -225,17 +225,24 @@ QTransform QPainterPrivate::viewTransform() const
     return QTransform();
 }
 
-QTransform QPainterPrivate::hidpiScaleTransform() const
+int QPainterPrivate::effectiveDevicePixelRatio() const
 {
-#ifdef Q_OS_MAC
     // Limited feature introduction for Qt 5.0.0, remove ifdef in a later release.
-    if (device->devType() == QInternal::Printer || device->physicalDpiX() == 0 || device->logicalDpiX() == 0)
-        return QTransform();
-    const qreal deviceScale = (device->physicalDpiX() / device->logicalDpiX());
-    if (deviceScale > 1.0)
-        return QTransform::fromScale(deviceScale, deviceScale);
+#ifdef Q_OS_MAC
+    // Special cases for devices that does not support PdmDevicePixelRatio go here:
+    if (device->devType() == QInternal::Printer)
+        return 1;
+
+    return qMax(1, device->metric(QPaintDevice::PdmDevicePixelRatio));
+#else
+    return 1;
 #endif
-    return QTransform();
+}
+
+QTransform QPainterPrivate::hidpiScaleTransform() const
+{
+    int devicePixelRatio = effectiveDevicePixelRatio();
+    return QTransform::fromScale(devicePixelRatio, devicePixelRatio);
 }
 
 /*
@@ -1837,14 +1844,7 @@ bool QPainter::begin(QPaintDevice *pd)
 
     Q_ASSERT(d->engine->isActive());
 
-#ifdef Q_OS_MAC
-    // Limited feature introduction for Qt 5.0.0, remove ifdef in a later release.
-    const bool isHighDpi = (pd->devType() == QInternal::Printer || d->device->physicalDpiX() == 0 || d->device->logicalDpiX() == 0) ?
-                           false : (d->device->physicalDpiX() / d->device->logicalDpiX() > 1);
-#else
-    const bool isHighDpi = false;
-#endif
-    if (!d->state->redirectionMatrix.isIdentity() || isHighDpi)
+    if (!d->state->redirectionMatrix.isIdentity() || d->effectiveDevicePixelRatio() > 1)
         d->updateMatrix();
 
     Q_ASSERT(d->engine->isActive());
diff --git a/src/gui/painting/qpainter_p.h b/src/gui/painting/qpainter_p.h
index 36a73866e7..04772b3ec9 100644
--- a/src/gui/painting/qpainter_p.h
+++ b/src/gui/painting/qpainter_p.h
@@ -249,6 +249,7 @@ public:
     }
 
     QTransform viewTransform() const;
+    int effectiveDevicePixelRatio() const;
     QTransform hidpiScaleTransform() const;
     static bool attachPainterPrivate(QPainter *q, QPaintDevice *pdev);
     void detachPainterPrivate(QPainter *q);
diff --git a/src/gui/painting/qpdf.cpp b/src/gui/painting/qpdf.cpp
index eed64180e5..5d9a743dac 100644
--- a/src/gui/painting/qpdf.cpp
+++ b/src/gui/painting/qpdf.cpp
@@ -1455,6 +1455,9 @@ int QPdfEngine::metric(QPaintDevice::PaintDeviceMetric metricType) const
     case QPaintDevice::PdmDepth:
         val = 32;
         break;
+    case QPaintDevice::PdmDevicePixelRatio:
+        val = 1;
+        break;
     default:
         qWarning("QPdfWriter::metric: Invalid metric command");
         return 0;