Fix disfunctional QQuickRenderControl with multiple screens

Having a retina and non-retina screen connected resulted in getting
no output from QQuickRenderControl and QQuickWidget on the
non-retina screen.

This is caused by the fact that Quick is blindly calling
QWindow::devicePixelRatio(). This approach is wrong when using
QQuickRenderControl since the QQuickWindow does not have an actual
native window and so devicePixelRatio() merely returns some default value
which will definitely be wrong for one of the screens.

The patch fixes the problem by introducing
QQuickWindow::effectiveDevicePixelRatio(), which, via
QQuickRenderControl::renderWindowFor, supports the redirected case too.

Task-number: QTBUG-42114
Change-Id: I057e01f0a00758dde438fc9b10af3a435b06bb0b
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

5 files changed, 26 insertions, 7 deletions

diff --git a/src/quick/items/qquickimagebase.cpp b/src/quick/items/qquickimagebase.cpp
index 5ec3e81727..1a5d115a3c 100644
--- a/src/quick/items/qquickimagebase.cpp
+++ b/src/quick/items/qquickimagebase.cpp
@@ -206,7 +206,7 @@ void QQuickImageBase::load()
             options |= QQuickPixmap::Cache;
         d->pix.clear(this);
 
-        const qreal targetDevicePixelRatio = (window() ? window()->devicePixelRatio() : qApp->devicePixelRatio());
+        const qreal targetDevicePixelRatio = (window() ? window()->effectiveDevicePixelRatio() : qApp->devicePixelRatio());
         d->devicePixelRatio = 1.0;
 
         QUrl loadUrl = d->url;
diff --git a/src/quick/items/qquickrendercontrol.cpp b/src/quick/items/qquickrendercontrol.cpp
index 79db80595b..7ccd3a0fb4 100644
--- a/src/quick/items/qquickrendercontrol.cpp
+++ b/src/quick/items/qquickrendercontrol.cpp
@@ -318,7 +318,7 @@ QImage QQuickRenderControl::grab()
         return QImage();
 
     render();
-    QImage grabContent = qt_gl_read_framebuffer(d->window->size() * d->window->devicePixelRatio(), false, false);
+    QImage grabContent = qt_gl_read_framebuffer(d->window->size() * d->window->effectiveDevicePixelRatio(), false, false);
     return grabContent;
 }
 
diff --git a/src/quick/items/qquickshadereffectsource.cpp b/src/quick/items/qquickshadereffectsource.cpp
index 52e5ba2464..ccff98828c 100644
--- a/src/quick/items/qquickshadereffectsource.cpp
+++ b/src/quick/items/qquickshadereffectsource.cpp
@@ -628,7 +628,7 @@ QSGNode *QQuickShaderEffectSource::updatePaintNode(QSGNode *oldNode, UpdatePaint
 
     // Crate large textures on high-dpi displays.
     if (sourceItem())
-        textureSize *= d->window->devicePixelRatio();
+        textureSize *= d->window->effectiveDevicePixelRatio();
 
     const QSize minTextureSize = d->sceneGraphContext()->minimumFBOSize();
     // Keep power-of-two by doubling the size.
@@ -637,7 +637,7 @@ QSGNode *QQuickShaderEffectSource::updatePaintNode(QSGNode *oldNode, UpdatePaint
     while (textureSize.height() < minTextureSize.height())
         textureSize.rheight() *= 2;
 
-    m_texture->setDevicePixelRatio(d->window->devicePixelRatio());
+    m_texture->setDevicePixelRatio(d->window->effectiveDevicePixelRatio());
     m_texture->setSize(textureSize);
     m_texture->setRecursive(m_recursive);
     m_texture->setFormat(GLenum(m_format));
diff --git a/src/quick/items/qquickwindow.cpp b/src/quick/items/qquickwindow.cpp
index 4ec901ceb4..f6c1412aff 100644
--- a/src/quick/items/qquickwindow.cpp
+++ b/src/quick/items/qquickwindow.cpp
@@ -365,7 +365,7 @@ void QQuickWindowPrivate::renderSceneGraph(const QSize &size)
     emit q->beforeRendering();
     runAndClearJobs(&beforeRenderingJobs);
     int fboId = 0;
-    const qreal devicePixelRatio = q->devicePixelRatio();
+    const qreal devicePixelRatio = q->effectiveDevicePixelRatio();
     renderer->setDeviceRect(QRect(QPoint(0, 0), size * devicePixelRatio));
     if (renderTargetId) {
         fboId = renderTargetId;
@@ -374,7 +374,7 @@ void QQuickWindowPrivate::renderSceneGraph(const QSize &size)
         renderer->setViewportRect(QRect(QPoint(0, 0), size * devicePixelRatio));
     }
     renderer->setProjectionMatrixToRect(QRect(QPoint(0, 0), size));
-    renderer->setDevicePixelRatio(q->devicePixelRatio());
+    renderer->setDevicePixelRatio(devicePixelRatio);
 
     context->renderNextFrame(renderer, fboId);
     emit q->afterRendering();
@@ -3160,7 +3160,7 @@ QImage QQuickWindow::grabWindow()
         d->syncSceneGraph();
         d->renderSceneGraph(size());
 
-        QImage image = qt_gl_read_framebuffer(size() * devicePixelRatio(), false, false);
+        QImage image = qt_gl_read_framebuffer(size() * effectiveDevicePixelRatio(), false, false);
         d->cleanupNodesOnShutdown();
         d->context->invalidate();
         context.doneCurrent();
@@ -3923,6 +3923,23 @@ void QQuickWindow::runJobsAfterSwap()
     d->runAndClearJobs(&d->afterSwapJobs);
 }
 
+/*!
+ * Returns the device pixel ratio for this window.
+ *
+ * This is different from QWindow::devicePixelRatio() in that it supports
+ * redirected rendering via QQuickRenderControl. When using a
+ * QQuickRenderControl, the QQuickWindow is often not created, meaning it is
+ * never shown and there is no underlying native window created in the
+ * windowing system. As a result, querying properties like the device pixel
+ * ratio cannot give correct results. Use this function instead.
+ *
+ * \sa QWindow::devicePixelRatio()
+ */
+int QQuickWindow::effectiveDevicePixelRatio() const
+{
+    QWindow *w = QQuickRenderControl::renderWindowFor(const_cast<QQuickWindow *>(this));
+    return w ? w->devicePixelRatio() : devicePixelRatio();
+}
 
 #include "moc_qquickwindow.cpp"
 
diff --git a/src/quick/items/qquickwindow.h b/src/quick/items/qquickwindow.h
index b52d6b807e..3cac691963 100644
--- a/src/quick/items/qquickwindow.h
+++ b/src/quick/items/qquickwindow.h
@@ -142,6 +142,8 @@ public:
 
     void scheduleRenderJob(QRunnable *job, RenderStage schedule);
 
+    int effectiveDevicePixelRatio() const;
+
 Q_SIGNALS:
     void frameSwapped();
     Q_REVISION(2) void openglContextCreated(QOpenGLContext *context);