Add support for painting at integer DPR with downscale

Enable by setting QT_WIDGETS_HIGHDPI_DOWNSCALE=1 and QT_WIDGETS_RHI=1.

This will make the backing store and painter operate at the next
highest integer DPR in cases where QWindow::devicePixelRatio() returns
a fractional value. The backing store image will then be downscaled
to the target DPR at flush time, using the RHI flush pipeline.

[ChangeLog][QWidgets] Added experimental support for always
painting at an integer device pixel ratio (rounding the DPR up if
necessary), followed by a downscale to the target DPR.Enable by setting
QT_WIDGETS_HIGHDPI_DOWNSCALE=1 and QT_WIDGETS_RHI=1.

Pick-to: 6.4
Task-number: QTBUG-86344
Change-Id: Id5b834a0e3499818b0b656161f5e0c38a6caa340
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 13 insertions, 2 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index 01a8905c79..9c1cec5d8d 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -12698,6 +12698,17 @@ int QWidget::metric(PaintDeviceMetric m) const
         return QPaintDevice::metric(m);
     }
 
+    auto resolveDevicePixelRatio = [this, screen]() -> qreal {
+
+        // Note: keep in sync with QBackingStorePrivate::backingStoreDevicePixelRatio()!
+        static bool downscale = qEnvironmentVariableIntValue("QT_WIDGETS_HIGHDPI_DOWNSCALE") > 0;
+        QWindow *window = this->window()->windowHandle();
+        if (downscale && window)
+            return std::ceil(window->devicePixelRatio());
+
+        return screen->devicePixelRatio();
+    };
+
     switch (m) {
     case PdmWidth:
         return data->crect.width();
@@ -12726,9 +12737,9 @@ int QWidget::metric(PaintDeviceMetric m) const
     case PdmPhysicalDpiY:
         return qRound(screen->physicalDotsPerInchY());
     case PdmDevicePixelRatio:
-        return screen->devicePixelRatio();
+        return resolveDevicePixelRatio();
     case PdmDevicePixelRatioScaled:
-        return QPaintDevice::devicePixelRatioFScale() * screen->devicePixelRatio();
+        return QPaintDevice::devicePixelRatioFScale() * resolveDevicePixelRatio();
     default:
         break;
     }