WIP: Add platform independent high-dpi support to QtGui

Add coordinate scaling support to the QWindow/
QWindowSystemInterface layer. The scale factor can
be set with the QT_HIGHDPI_SCALE_FACTOR environment
variable.

Setting a scale factor different than the default (1)
now has the following effects:

QWindow::devicePixelRatio is set accordingly, enabling
the high-dpi code paths.

QWindow and related classes now return geometry in
device independent pixels. This includes screen,
desktop and window geometry as well as event coordinates.

The platform plugins continue to operate in device
pixels, unaware of the scaling.

Task-number: QTBUG-38858
Change-Id: I85b0d1bc682b25196f6db286e672a64f8da0ae5c

1 files changed, 18 insertions, 6 deletions

diff --git a/src/gui/painting/qbackingstore.cpp b/src/gui/painting/qbackingstore.cpp
index 19074e4c47..88e9aaac2c 100644
--- a/src/gui/painting/qbackingstore.cpp
+++ b/src/gui/painting/qbackingstore.cpp
@@ -42,6 +42,8 @@
 #include <private/qguiapplication_p.h>
 #include <private/qwindow_p.h>
 
+#include <private/qhighdpiscaling_p.h>
+
 QT_BEGIN_NAMESPACE
 
 class QBackingStorePrivate
@@ -102,7 +104,7 @@ void QBackingStore::flush(const QRegion &region, QWindow *win, const QPoint &off
     }
 #endif
 
-    d_ptr->platformBackingStore->flush(win, region, offset);
+    d_ptr->platformBackingStore->flush(win, qHighDpiToDevicePixels(region), offset);
 }
 
 /*!
@@ -112,7 +114,17 @@ void QBackingStore::flush(const QRegion &region, QWindow *win, const QPoint &off
 */
 QPaintDevice *QBackingStore::paintDevice()
 {
-    return d_ptr->platformBackingStore->paintDevice();
+    QPaintDevice *device = d_ptr->platformBackingStore->paintDevice();
+    // When QtGui is applying a high-dpi scale factor we are asking
+    // the platform backing store to create a "large" backing store
+    // image. This image needs to be converted into a high-dpi image by
+    // setting the scale factor on the image:
+    if (QHighDpiScaling::isActive() && device->devType() == QInternal::Image) {
+        QImage *image = reinterpret_cast<QImage *>(device);
+        image->setDevicePixelRatio(d_ptr->window->devicePixelRatio());
+    }
+
+    return device;
 }
 
 /*!
@@ -150,7 +162,7 @@ QWindow* QBackingStore::window() const
 
 void QBackingStore::beginPaint(const QRegion &region)
 {
-    d_ptr->platformBackingStore->beginPaint(region);
+    d_ptr->platformBackingStore->beginPaint(qHighDpiToDevicePixels(region));
 }
 
 /*!
@@ -170,8 +182,8 @@ void QBackingStore::endPaint()
 */
 void QBackingStore::resize(const QSize &size)
 {
-    d_ptr->size = size;
-    d_ptr->platformBackingStore->resize(size, d_ptr->staticContents);
+    d_ptr->size = size; // QBackingStore stores size in point, QPlatformBackingStore gets it in pixel.
+    d_ptr->platformBackingStore->resize(size * QHighDpiScaling::factor(), d_ptr->staticContents);
 }
 
 /*!
@@ -194,7 +206,7 @@ bool QBackingStore::scroll(const QRegion &area, int dx, int dy)
     Q_UNUSED(dx);
     Q_UNUSED(dy);
 
-    return d_ptr->platformBackingStore->scroll(area, dx, dy);
+    return d_ptr->platformBackingStore->scroll(qHighDpiToDevicePixels(area), qHighDpiToDevicePixels(dx), qHighDpiToDevicePixels(dy));
 }
 
 void QBackingStore::setStaticContents(const QRegion &region)