rhiwindow: Set DPR on image texture to ensure DPR-agnostic drawing

Otherwise the 20x20 margins will produce different layouts depending
on the DPR, which is not what we want.

Pick-to: 6.7
Change-Id: I4153d0843ef51c8e0f60d7d5166b153d45201c95
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

2 files changed, 10 insertions, 4 deletions

diff --git a/examples/gui/doc/src/rhiwindow.qdoc b/examples/gui/doc/src/rhiwindow.qdoc
index b6fc279816..3ee33c8002 100644
--- a/examples/gui/doc/src/rhiwindow.qdoc
+++ b/examples/gui/doc/src/rhiwindow.qdoc
@@ -427,6 +427,11 @@
     QRhiShaderResourceBindings, continue to be valid all the time. It is only
     the underlying native resources that come and go over time.
 
+    Note also that we set a device pixel ratio on the image that matches
+    the window that we're drawing into. This ensures that the drawing code
+    can be DPR-agnostic, producing the same layout regardless of the DPR,
+    while also taking advantage of the additional pixels for improved fidelity.
+
     \snippet rhiwindow/rhiwindow.cpp ensure-texture
 
     Once a QImage is generated and the QPainter-based drawing into it has
diff --git a/examples/gui/rhiwindow/rhiwindow.cpp b/examples/gui/rhiwindow/rhiwindow.cpp
index 345286b635..3ae6faa802 100644
--- a/examples/gui/rhiwindow/rhiwindow.cpp
+++ b/examples/gui/rhiwindow/rhiwindow.cpp
@@ -281,17 +281,18 @@ void HelloWindow::ensureFullscreenTexture(const QSize &pixelSize, QRhiResourceUp
     m_texture->create();
 
     QImage image(pixelSize, QImage::Format_RGBA8888_Premultiplied);
+    image.setDevicePixelRatio(devicePixelRatio());
 //! [ensure-texture]
     QPainter painter(&image);
-    painter.fillRect(QRectF(QPointF(0, 0), pixelSize), QColor::fromRgbF(0.4f, 0.7f, 0.0f, 1.0f));
+    painter.fillRect(QRectF(QPointF(0, 0), size()), QColor::fromRgbF(0.4f, 0.7f, 0.0f, 1.0f));
     painter.setPen(Qt::transparent);
     painter.setBrush({ QGradient(QGradient::DeepBlue) });
-    painter.drawRoundedRect(QRectF(QPointF(20, 20), pixelSize - QSize(40, 40)), 16, 16);
+    painter.drawRoundedRect(QRectF(QPointF(20, 20), size() - QSize(40, 40)), 16, 16);
     painter.setPen(Qt::black);
     QFont font;
-    font.setPixelSize(0.05 * qMin(pixelSize.width(), pixelSize.height()));
+    font.setPixelSize(0.05 * qMin(width(), height()));
     painter.setFont(font);
-    painter.drawText(QRectF(QPointF(60, 60), pixelSize - QSize(120, 120)), 0,
+    painter.drawText(QRectF(QPointF(60, 60), size() - QSize(120, 120)), 0,
                      QLatin1String("Rendering with QRhi to a resizable QWindow.\nThe 3D API is %1.\nUse the command-line options to choose a different API.")
                      .arg(graphicsApiName()));
     painter.end();