tablet example: enable high-DPI rendering

Change-Id: Ie03867208902fdc9cf794de16d7249c8994351f4
Reviewed-by: Venugopal Shivashankar <Venugopal.Shivashankar@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

2 files changed, 25 insertions, 16 deletions

diff --git a/examples/widgets/doc/src/tablet.qdoc b/examples/widgets/doc/src/tablet.qdoc
index 88fdefa68f..2b11020c07 100644
--- a/examples/widgets/doc/src/tablet.qdoc
+++ b/examples/widgets/doc/src/tablet.qdoc
@@ -206,8 +206,7 @@
 
     \snippet widgets/tablet/tabletcanvas.cpp 0
 
-    In the constructor we initialize our class variables. We need
-    to draw the background of our pixmap, as the default is gray.
+    In the constructor we initialize most of our class variables.
 
     Here is the implementation of \c saveImage():
 
@@ -247,7 +246,15 @@
 
     \snippet widgets/tablet/tabletcanvas.cpp 4
 
-    We simply draw the pixmap to the top left of the widget.
+    The first time Qt calls paintEvent(), m_pixmap is default-constructed, so
+    QPixmap::isNull() returns \c true. Now that we know which screen we will be
+    rendering to, we can create a pixmap with the appropriate resolution.
+    The size of the pixmap with which we fill the window depends on the screen
+    resolution, as the example does not support zoom; and it may be that one
+    screen is \l {High DPI Displays}{high DPI} while another is not. We need to
+    draw the background too, as the default is gray.
+
+    After that, we simply draw the pixmap to the top left of the widget.
 
     Here is the implementation of \c paintPixmap():
 
diff --git a/examples/widgets/widgets/tablet/tabletcanvas.cpp b/examples/widgets/widgets/tablet/tabletcanvas.cpp
index 9812c29554..778722cc41 100644
--- a/examples/widgets/widgets/tablet/tabletcanvas.cpp
+++ b/examples/widgets/widgets/tablet/tabletcanvas.cpp
@@ -65,21 +65,9 @@ TabletCanvas::TabletCanvas()
   , m_deviceDown(false)
 {
     resize(500, 500);
-    initPixmap();
     setAutoFillBackground(true);
     setAttribute(Qt::WA_TabletTracking);
 }
-
-void TabletCanvas::initPixmap()
-{
-    QPixmap newPixmap = QPixmap(width(), height());
-    newPixmap.fill(Qt::white);
-    QPainter painter(&newPixmap);
-    if (!m_pixmap.isNull())
-        painter.drawPixmap(0, 0, m_pixmap);
-    painter.end();
-    m_pixmap = newPixmap;
-}
 //! [0]
 
 //! [1]
@@ -105,7 +93,6 @@ bool TabletCanvas::loadImage(const QString &file)
 //! [3]
 void TabletCanvas::tabletEvent(QTabletEvent *event)
 {
-
     switch (event->type()) {
         case QEvent::TabletPress:
             if (!m_deviceDown) {
@@ -140,8 +127,23 @@ void TabletCanvas::tabletEvent(QTabletEvent *event)
 //! [3]
 
 //! [4]
+void TabletCanvas::initPixmap()
+{
+    qreal dpr = devicePixelRatioF();
+    QPixmap newPixmap = QPixmap(width() * dpr, height() * dpr);
+    newPixmap.setDevicePixelRatio(dpr);
+    newPixmap.fill(Qt::white);
+    QPainter painter(&newPixmap);
+    if (!m_pixmap.isNull())
+        painter.drawPixmap(0, 0, m_pixmap);
+    painter.end();
+    m_pixmap = newPixmap;
+}
+
 void TabletCanvas::paintEvent(QPaintEvent *)
 {
+    if (m_pixmap.isNull())
+        initPixmap();
     QPainter painter(this);
     painter.drawPixmap(0, 0, m_pixmap);
 }