WIP: Add PdmDevicePixelRatioScaled.

Work around QPainDevice::metric's int return type
by adding a new metric that returns a scaled devicePixelRatio.
Choose a scale factor that gives us more than enough
range.

The QPainDevice::devicePixelRatio() convenience accessor
is public API and can unfortunately not be changed
to return a qreal. Add devicePixelRatioF() which
returns the (unscaled) devicePixelRatio.

Change all call sites of QPainDevice::devicePixelRatio()
to use QPainDevice::devicePixelRatioF().

Task-number: QTBUG-38858
Change-Id: Ia97c24e1c7b10241d6425fab2fa513702bb750b8

4 files changed, 17 insertions, 4 deletions

diff --git a/src/gui/painting/qpaintdevice.cpp b/src/gui/painting/qpaintdevice.cpp
index 36e0bbe223..4ee3d91ea3 100644
--- a/src/gui/painting/qpaintdevice.cpp
+++ b/src/gui/painting/qpaintdevice.cpp
@@ -41,6 +41,8 @@ QPaintDevice::QPaintDevice()
     painters = 0;
 }
 
+qreal QPaintDevice::devicePixelRatioFScale = 10000000.0;
+
 QPaintDevice::~QPaintDevice()
 {
     if (paintingActive())
@@ -79,7 +81,13 @@ Q_GUI_EXPORT int qt_paint_device_metric(const QPaintDevice *device, QPaintDevice
 
 int QPaintDevice::metric(PaintDeviceMetric m) const
 {
+    // Fallback: A subclass has not implemented PdmDevicePixelRatioScaled but might
+    // have implemented PdmDevicePixelRatio.
+    if (m == PdmDevicePixelRatioScaled)
+        return this->metric(PdmDevicePixelRatio) * devicePixelRatioFScale;
+
     qWarning("QPaintDevice::metrics: Device has no metric information");
+
     if (m == PdmDpiX) {
         return 72;
     } else if (m == PdmDpiY) {
@@ -95,4 +103,5 @@ int QPaintDevice::metric(PaintDeviceMetric m) const
     }
 }
 
+
 QT_END_NAMESPACE
diff --git a/src/gui/painting/qpaintdevice.h b/src/gui/painting/qpaintdevice.h
index 7c756c66de..819116c585 100644
--- a/src/gui/painting/qpaintdevice.h
+++ b/src/gui/painting/qpaintdevice.h
@@ -58,7 +58,8 @@ public:
         PdmDpiY,
         PdmPhysicalDpiX,
         PdmPhysicalDpiY,
-        PdmDevicePixelRatio
+        PdmDevicePixelRatio,
+        PdmDevicePixelRatioScaled
     };
 
     virtual ~QPaintDevice();
@@ -76,9 +77,13 @@ public:
     int physicalDpiX() const { return metric(PdmPhysicalDpiX); }
     int physicalDpiY() const { return metric(PdmPhysicalDpiY); }
     int devicePixelRatio() const { return metric(PdmDevicePixelRatio); }
+    qreal devicePixelRatioF()  const { return metric(PdmDevicePixelRatioScaled) / devicePixelRatioFScale; }
     int colorCount() const { return metric(PdmNumColors); }
     int depth() const { return metric(PdmDepth); }
 
+    // ### Classes that are not QPaintDevice subclasses are implementing metric()
+    // ### There needs to be some kind of (semi)-public API.
+    static qreal devicePixelRatioFScale;
 protected:
     QPaintDevice();
     virtual int metric(PaintDeviceMetric metric) const;
@@ -87,7 +92,6 @@ protected:
     virtual QPainter *sharedPainter() const;
 
     ushort        painters;                        // refcount
-
 private:
     Q_DISABLE_COPY(QPaintDevice)
 
diff --git a/src/gui/painting/qpainter.cpp b/src/gui/painting/qpainter.cpp
index 6f00abfc5f..69cbb42923 100644
--- a/src/gui/painting/qpainter.cpp
+++ b/src/gui/painting/qpainter.cpp
@@ -219,7 +219,7 @@ QTransform QPainterPrivate::viewTransform() const
     return QTransform();
 }
 
-int QPainterPrivate::effectiveDevicePixelRatio() const
+qreal QPainterPrivate::effectiveDevicePixelRatio() const
 {
     // Special cases for devices that does not support PdmDevicePixelRatio go here:
     if (device->devType() == QInternal::Printer)
diff --git a/src/gui/painting/qpainter_p.h b/src/gui/painting/qpainter_p.h
index 7c32dc1694..fdad8e2460 100644
--- a/src/gui/painting/qpainter_p.h
+++ b/src/gui/painting/qpainter_p.h
@@ -241,7 +241,7 @@ public:
     }
 
     QTransform viewTransform() const;
-    int effectiveDevicePixelRatio() const;
+    qreal effectiveDevicePixelRatio() const;
     QTransform hidpiScaleTransform() const;
     static bool attachPainterPrivate(QPainter *q, QPaintDevice *pdev);
     void detachPainterPrivate(QPainter *q);