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>

4 files changed, 16 insertions, 4 deletions

diff --git a/src/gui/image/qimage.cpp b/src/gui/image/qimage.cpp
index 7287f54e74..0efb9c2646 100644
--- a/src/gui/image/qimage.cpp
+++ b/src/gui/image/qimage.cpp
@@ -4994,12 +4994,17 @@ int QImage::metric(PaintDeviceMetric metric) const
         break;
 
     case PdmPhysicalDpiX:
-        return qRound(d->dpmx * 0.0254 * d->devicePixelRatio);
+        return qRound(d->dpmx * 0.0254);
         break;
 
     case PdmPhysicalDpiY:
-        return qRound(d->dpmy * 0.0254 * d->devicePixelRatio);
+        return qRound(d->dpmy * 0.0254);
         break;
+
+    case PdmDevicePixelRatio:
+        return d->devicePixelRatio;
+        break;
+
     default:
         qWarning("QImage::metric(): Unhandled metric type %d", metric);
         break;
diff --git a/src/gui/image/qpicture.cpp b/src/gui/image/qpicture.cpp
index f6de22851d..1071ed754b 100644
--- a/src/gui/image/qpicture.cpp
+++ b/src/gui/image/qpicture.cpp
@@ -956,6 +956,9 @@ int QPicture::metric(PaintDeviceMetric m) const
         case PdmDepth:
             val = 24;
             break;
+        case PdmDevicePixelRatio:
+            val = 1;
+            break;
         default:
             val = 0;
             qWarning("QPicture::metric: Invalid metric command");
diff --git a/src/gui/image/qpixmap_blitter.cpp b/src/gui/image/qpixmap_blitter.cpp
index 75b7a4ba06..4c1b30a6d8 100644
--- a/src/gui/image/qpixmap_blitter.cpp
+++ b/src/gui/image/qpixmap_blitter.cpp
@@ -120,6 +120,8 @@ int QBlittablePlatformPixmap::metric(QPaintDevice::PaintDeviceMetric metric) con
     case QPaintDevice::PdmDpiY: // fall-through
     case QPaintDevice::PdmPhysicalDpiY:
         return qt_defaultDpiY();
+    case QPaintDevice::PdmDevicePixelRatio:
+        return 1;
     default:
         qWarning("QRasterPlatformPixmap::metric(): Unhandled metric type %d", metric);
         break;
diff --git a/src/gui/image/qpixmap_raster.cpp b/src/gui/image/qpixmap_raster.cpp
index 302945dbf8..c80ccd8b1d 100644
--- a/src/gui/image/qpixmap_raster.cpp
+++ b/src/gui/image/qpixmap_raster.cpp
@@ -278,11 +278,13 @@ int QRasterPlatformPixmap::metric(QPaintDevice::PaintDeviceMetric metric) const
     case QPaintDevice::PdmDpiX:
         return qt_defaultDpiX();
     case QPaintDevice::PdmPhysicalDpiX:
-        return qt_defaultDpiX() * image.devicePixelRatio();
+        return qt_defaultDpiX();
     case QPaintDevice::PdmDpiY:
         return qt_defaultDpiX();
     case QPaintDevice::PdmPhysicalDpiY:
-        return qt_defaultDpiY() * image.devicePixelRatio();
+        return qt_defaultDpiY();
+    case QPaintDevice::PdmDevicePixelRatio:
+        return image.devicePixelRatio();
     default:
         qWarning("QRasterPlatformPixmap::metric(): Unhandled metric type %d", metric);
         break;