Support stereoscopic rendering with QGraphicsView

This patch adds a manual test and the required work in graphicsview and
qwidget private apis to support stereoscopic rendeing. Basically it
works by doing the drawing in QGraphicsView::paintEvent twice, once for
each buffer. This way the scene items are rendered to both buffers.
There's also an update to resolvement in QOpenGLWidgetPrivate
so that multisampling works correctly.

[ChangeLog][Widgets][QGraphicsView] Added support for
stereoscopic rendering.

Task-number: QTBUG-64587
Change-Id: I20650682daa805b64fe7f0d2ba086917d3f12229
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

13 files changed, 763 insertions, 131 deletions

diff --git a/src/openglwidgets/qopenglwidget.cpp b/src/openglwidgets/qopenglwidget.cpp
index 7a0ac05cee..4d8c181c22 100644
--- a/src/openglwidgets/qopenglwidget.cpp
+++ b/src/openglwidgets/qopenglwidget.cpp
@@ -552,7 +552,7 @@ public:
 
     void destroyFbos();
 
-    void setCurrentTargetBuffer(QOpenGLWidget::TargetBuffer targetBuffer);
+    bool setCurrentTargetBuffer(QOpenGLWidget::TargetBuffer targetBuffer);
     QImage grabFramebuffer(QOpenGLWidget::TargetBuffer targetBuffer);
     QImage grabFramebuffer() override;
     void beginBackingStorePainting() override { inBackingStorePaint = true; }
@@ -560,9 +560,13 @@ public:
     void beginCompose() override;
     void endCompose() override;
     void initializeViewportFramebuffer() override;
+    bool isStereoEnabled() override;
+    bool toggleStereoTargetBuffer() override;
     void resizeViewportFramebuffer() override;
     void resolveSamples() override;
 
+    void resolveSamplesForBuffer(QOpenGLWidget::TargetBuffer targetBuffer);
+
     QOpenGLContext *context = nullptr;
     QRhiTexture *wrapperTextures[2] = {};
     QOpenGLFramebufferObject *fbos[2] = {};
@@ -697,8 +701,6 @@ void QOpenGLWidgetPrivate::reset()
 
 void QOpenGLWidgetPrivate::resetRhiDependentResources()
 {
-    Q_Q(QOpenGLWidget);
-
     // QRhi resource created from the QRhi. These must be released whenever the
     // widget gets associated with a different QRhi, even when all OpenGL
     // contexts share resources.
@@ -706,7 +708,7 @@ void QOpenGLWidgetPrivate::resetRhiDependentResources()
     delete wrapperTextures[0];
     wrapperTextures[0] = nullptr;
 
-    if (q->format().stereo()) {
+    if (isStereoEnabled()) {
         delete wrapperTextures[1];
         wrapperTextures[1] = nullptr;
     }
@@ -738,9 +740,9 @@ void QOpenGLWidgetPrivate::recreateFbos()
     if (samples > 0)
         resolvedFbos[QOpenGLWidget::LeftBuffer] = new QOpenGLFramebufferObject(deviceSize);
 
-    const bool stereoEnabled = q->format().stereo();
+    const bool stereo = isStereoEnabled();
 
-    if (stereoEnabled) {
+    if (stereo) {
         fbos[QOpenGLWidget::RightBuffer] = new QOpenGLFramebufferObject(deviceSize, format);
         if (samples > 0)
             resolvedFbos[QOpenGLWidget::RightBuffer] = new QOpenGLFramebufferObject(deviceSize);
@@ -753,11 +755,12 @@ void QOpenGLWidgetPrivate::recreateFbos()
     context->functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
     ensureRhiDependentResources();
 
-    if (stereoEnabled) {
+    if (stereo) {
         currentTargetBuffer = QOpenGLWidget::RightBuffer;
         fbos[currentTargetBuffer]->bind();
         context->functions()->glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
         ensureRhiDependentResources();
+        currentTargetBuffer = QOpenGLWidget::LeftBuffer;
     }
 
     flushPending = true; // Make sure the FBO is initialized before use
@@ -895,11 +898,17 @@ void QOpenGLWidgetPrivate::initialize()
 
 void QOpenGLWidgetPrivate::resolveSamples()
 {
+    resolveSamplesForBuffer(QOpenGLWidget::LeftBuffer);
+    resolveSamplesForBuffer(QOpenGLWidget::RightBuffer);
+}
+
+void QOpenGLWidgetPrivate::resolveSamplesForBuffer(QOpenGLWidget::TargetBuffer targetBuffer)
+{
     Q_Q(QOpenGLWidget);
-    if (resolvedFbos[currentTargetBuffer]) {
-        q->makeCurrent();
-        QRect rect(QPoint(0, 0), fbos[currentTargetBuffer]->size());
-        QOpenGLFramebufferObject::blitFramebuffer(resolvedFbos[currentTargetBuffer], rect, fbos[currentTargetBuffer], rect);
+    if (resolvedFbos[targetBuffer]) {
+        q->makeCurrent(targetBuffer);
+        QRect rect(QPoint(0, 0), fbos[targetBuffer]->size());
+        QOpenGLFramebufferObject::blitFramebuffer(resolvedFbos[targetBuffer], rect, fbos[targetBuffer], rect);
         flushPending = true;
     }
 }
@@ -924,8 +933,8 @@ void QOpenGLWidgetPrivate::render()
         return;
     }
 
-    const bool stereoEnabled = q->format().stereo();
-    if (stereoEnabled) {
+    const bool stereo = isStereoEnabled();
+    if (stereo) {
         static bool warningGiven = false;
         if (!fbos[QOpenGLWidget::RightBuffer] && !warningGiven) {
             qWarning("QOpenGLWidget: Stereo is enabled, but no right buffer. Using only left buffer");
@@ -936,7 +945,7 @@ void QOpenGLWidgetPrivate::render()
     if (updateBehavior == QOpenGLWidget::NoPartialUpdate && hasBeenComposed) {
         invalidateFbo();
 
-        if (stereoEnabled && fbos[QOpenGLWidget::RightBuffer]) {
+        if (stereo && fbos[QOpenGLWidget::RightBuffer]) {
             setCurrentTargetBuffer(QOpenGLWidget::RightBuffer);
             invalidateFbo();
             setCurrentTargetBuffer(QOpenGLWidget::LeftBuffer);
@@ -952,7 +961,7 @@ void QOpenGLWidgetPrivate::render()
     QOpenGLContextPrivate::get(ctx)->defaultFboRedirect = fbos[currentTargetBuffer]->handle();
     q->paintGL();
 
-    if (stereoEnabled && fbos[QOpenGLWidget::RightBuffer]) {
+    if (stereo && fbos[QOpenGLWidget::RightBuffer]) {
         setCurrentTargetBuffer(QOpenGLWidget::RightBuffer);
         QOpenGLContextPrivate::get(ctx)->defaultFboRedirect = fbos[currentTargetBuffer]->handle();
         q->paintGL();
@@ -1017,7 +1026,7 @@ QImage QOpenGLWidgetPrivate::grabFramebuffer(QOpenGLWidget::TargetBuffer targetB
 
     // The second fbo is only created when stereoscopic rendering is enabled
     // Just use the default one if not.
-    if (targetBuffer == QOpenGLWidget::RightBuffer && !q->format().stereo())
+    if (targetBuffer == QOpenGLWidget::RightBuffer && !isStereoEnabled())
         targetBuffer = QOpenGLWidget::LeftBuffer;
 
     if (!fbos[targetBuffer]) // could be completely offscreen, without ever getting a resize event
@@ -1028,7 +1037,7 @@ QImage QOpenGLWidgetPrivate::grabFramebuffer(QOpenGLWidget::TargetBuffer targetB
 
     setCurrentTargetBuffer(targetBuffer);
     if (resolvedFbos[targetBuffer]) {
-        resolveSamples();
+        resolveSamplesForBuffer(targetBuffer);
         resolvedFbos[targetBuffer]->bind();
     }
 
@@ -1054,6 +1063,22 @@ void QOpenGLWidgetPrivate::initializeViewportFramebuffer()
     q->setAutoFillBackground(true);
 }
 
+bool QOpenGLWidgetPrivate::isStereoEnabled()
+{
+    Q_Q(QOpenGLWidget);
+    // Note that because this internally might use the requested format,
+    // then this can return a false positive on hardware where
+    // steroscopic rendering is not supported.
+    return q->format().stereo();
+}
+
+bool QOpenGLWidgetPrivate::toggleStereoTargetBuffer()
+{
+    return setCurrentTargetBuffer(currentTargetBuffer == QOpenGLWidget::LeftBuffer ?
+                               QOpenGLWidget::RightBuffer :
+                               QOpenGLWidget::LeftBuffer);
+}
+
 void QOpenGLWidgetPrivate::resizeViewportFramebuffer()
 {
     Q_Q(QOpenGLWidget);
@@ -1254,6 +1279,34 @@ void QOpenGLWidget::makeCurrent()
 }
 
 /*!
+  Prepares for rendering OpenGL content for this widget by making the
+  context for the passed in buffer current and binding the framebuffer object in that
+  context.
+
+  \note This only makes sense to call when stereoscopic rendering is enabled.
+  Nothing will happen if the right buffer is requested when it's disabled.
+
+  It is not necessary to call this function in most cases, because it
+  is called automatically before invoking paintGL().
+
+  \since 6.5
+
+  \sa context(), paintGL(), doneCurrent()
+ */
+void QOpenGLWidget::makeCurrent(TargetBuffer targetBuffer)
+{
+    Q_D(QOpenGLWidget);
+    if (!d->initialized)
+        return;
+
+    // The FBO for the right buffer is only initialized when stereo is set
+   if (targetBuffer == TargetBuffer::RightBuffer && !format().stereo())
+       return;
+
+    d->setCurrentTargetBuffer(targetBuffer); // calls makeCurrent
+}
+
+/*!
   Releases the context.
 
   It is not necessary to call this function in most cases, since the
@@ -1585,11 +1638,18 @@ QPaintEngine *QOpenGLWidget::paintEngine() const
     return d->paintDevice->paintEngine();
 }
 
-void QOpenGLWidgetPrivate::setCurrentTargetBuffer(QOpenGLWidget::TargetBuffer targetBuffer)
+
+bool QOpenGLWidgetPrivate::setCurrentTargetBuffer(QOpenGLWidget::TargetBuffer targetBuffer)
 {
     Q_Q(QOpenGLWidget);
+
+    if (targetBuffer == QOpenGLWidget::RightBuffer && !isStereoEnabled())
+        return false;
+
     currentTargetBuffer = targetBuffer;
     q->makeCurrent();
+
+    return true;
 }
 
 /*!
diff --git a/src/openglwidgets/qopenglwidget.h b/src/openglwidgets/qopenglwidget.h
index 84097854e5..847e684cd9 100644
--- a/src/openglwidgets/qopenglwidget.h
+++ b/src/openglwidgets/qopenglwidget.h
@@ -45,6 +45,7 @@ public:
     bool isValid() const;
 
     void makeCurrent();
+    void makeCurrent(TargetBuffer targetBuffer);
     void doneCurrent();
 
     QOpenGLContext *context() const;
diff --git a/src/widgets/graphicsview/qgraphicsview.cpp b/src/widgets/graphicsview/qgraphicsview.cpp
index 8e362ce1e0..f9502b938a 100644
--- a/src/widgets/graphicsview/qgraphicsview.cpp
+++ b/src/widgets/graphicsview/qgraphicsview.cpp
@@ -85,6 +85,16 @@ static const int QGRAPHICSVIEW_PREALLOC_STYLE_OPTIONS = 503; // largest prime <
     view coordinates and scene coordinates, and to find items on the scene
     using view coordinates.
 
+    When using a QOpenGLWidget as a viewport, stereoscopic rendering is
+    supported. This is done using the same pattern as QOpenGLWidget::paintGL.
+    To enable it, enable the QSurfaceFormat::StereoBuffers flag. Because of
+    how the flag is handled internally, set QSurfaceFormat::StereoBuffers flag
+    globally before the window is created using QSurfaceFormat::setDefaultFormat().
+    If the flag is enabled and there is hardware support for stereoscopic
+    rendering, then drawBackground() and drawForeground() will be triggered twice
+    each frame. Call QOpenGLWidget::currentTargetBuffer() to query which buffer
+    is currently being drawn to.
+
     \image graphicsview-view.png
 
     \note Using an OpenGL viewport limits the ability to use QGraphicsProxyWidget.
@@ -2724,6 +2734,9 @@ void QGraphicsView::setupViewport(QWidget *widget)
 
     widget->setFocusPolicy(Qt::StrongFocus);
 
+    if (isGLWidget)
+        d->stereoEnabled = QWidgetPrivate::get(widget)->isStereoEnabled();
+
     if (!isGLWidget) {
         // autoFillBackground enables scroll acceleration.
         widget->setAutoFillBackground(true);
@@ -3425,132 +3438,146 @@ void QGraphicsView::paintEvent(QPaintEvent *event)
         painter.setWorldTransform(viewportTransform());
     const QTransform viewTransform = painter.worldTransform();
 
-    // Draw background
-    if (d->cacheMode & CacheBackground) {
-        // Recreate the background pixmap, and flag the whole background as
-        // exposed.
-        if (d->mustResizeBackgroundPixmap) {
-            const qreal dpr = d->viewport->devicePixelRatio();
-            d->backgroundPixmap = QPixmap(viewport()->size() * dpr);
-            d->backgroundPixmap.setDevicePixelRatio(dpr);
-            QBrush bgBrush = viewport()->palette().brush(viewport()->backgroundRole());
-            if (!bgBrush.isOpaque())
-                d->backgroundPixmap.fill(Qt::transparent);
-            QPainter p(&d->backgroundPixmap);
-            p.fillRect(0, 0, d->backgroundPixmap.width(), d->backgroundPixmap.height(), bgBrush);
-            d->backgroundPixmapExposed = QRegion(viewport()->rect());
-            d->mustResizeBackgroundPixmap = false;
-        }
+    const auto actuallyDraw = [&]() {
+        // Draw background
+        if (d->cacheMode & CacheBackground) {
+            // Recreate the background pixmap, and flag the whole background as
+            // exposed.
+            if (d->mustResizeBackgroundPixmap) {
+                const qreal dpr = d->viewport->devicePixelRatio();
+                d->backgroundPixmap = QPixmap(viewport()->size() * dpr);
+                d->backgroundPixmap.setDevicePixelRatio(dpr);
+                QBrush bgBrush = viewport()->palette().brush(viewport()->backgroundRole());
+                if (!bgBrush.isOpaque())
+                    d->backgroundPixmap.fill(Qt::transparent);
+                QPainter p(&d->backgroundPixmap);
+                p.fillRect(0, 0, d->backgroundPixmap.width(), d->backgroundPixmap.height(), bgBrush);
+                d->backgroundPixmapExposed = QRegion(viewport()->rect());
+                d->mustResizeBackgroundPixmap = false;
+            }
 
-        // Redraw exposed areas
-        if (!d->backgroundPixmapExposed.isEmpty()) {
-            QPainter backgroundPainter(&d->backgroundPixmap);
-            backgroundPainter.setClipRegion(d->backgroundPixmapExposed, Qt::ReplaceClip);
-            if (viewTransformed)
-                backgroundPainter.setTransform(viewTransform);
-            QRectF backgroundExposedSceneRect = mapToScene(d->backgroundPixmapExposed.boundingRect()).boundingRect();
-            drawBackground(&backgroundPainter, backgroundExposedSceneRect);
-            d->backgroundPixmapExposed = QRegion();
-        }
+            // Redraw exposed areas
+            if (!d->backgroundPixmapExposed.isEmpty()) {
+                QPainter backgroundPainter(&d->backgroundPixmap);
+                backgroundPainter.setClipRegion(d->backgroundPixmapExposed, Qt::ReplaceClip);
+                if (viewTransformed)
+                    backgroundPainter.setTransform(viewTransform);
+                QRectF backgroundExposedSceneRect = mapToScene(d->backgroundPixmapExposed.boundingRect()).boundingRect();
+                drawBackground(&backgroundPainter, backgroundExposedSceneRect);
+                d->backgroundPixmapExposed = QRegion();
+            }
 
-        // Blit the background from the background pixmap
-        if (viewTransformed) {
-            painter.setWorldTransform(QTransform());
-            painter.drawPixmap(QPoint(), d->backgroundPixmap);
-            painter.setWorldTransform(viewTransform);
+            // Blit the background from the background pixmap
+            if (viewTransformed) {
+                painter.setWorldTransform(QTransform());
+                painter.drawPixmap(QPoint(), d->backgroundPixmap);
+                painter.setWorldTransform(viewTransform);
+            } else {
+                painter.drawPixmap(QPoint(), d->backgroundPixmap);
+            }
         } else {
-            painter.drawPixmap(QPoint(), d->backgroundPixmap);
+            if (!(d->optimizationFlags & DontSavePainterState))
+                painter.save();
+
+            drawBackground(&painter, exposedSceneRect);
+            if (!(d->optimizationFlags & DontSavePainterState))
+                painter.restore();
         }
-    } else {
-        if (!(d->optimizationFlags & DontSavePainterState))
-            painter.save();
-        drawBackground(&painter, exposedSceneRect);
-        if (!(d->optimizationFlags & DontSavePainterState))
-            painter.restore();
-    }
 
-    // Items
-    if (!(d->optimizationFlags & IndirectPainting)) {
-        const quint32 oldRectAdjust = d->scene->d_func()->rectAdjust;
-        if (d->optimizationFlags & QGraphicsView::DontAdjustForAntialiasing)
-            d->scene->d_func()->rectAdjust = 1;
-        else
-            d->scene->d_func()->rectAdjust = 2;
-        d->scene->d_func()->drawItems(&painter, viewTransformed ? &viewTransform : nullptr,
-                                      &d->exposedRegion, viewport());
-        d->scene->d_func()->rectAdjust = oldRectAdjust;
-        // Make sure the painter's world transform is restored correctly when
-        // drawing without painter state protection (DontSavePainterState).
-        // We only change the worldTransform() so there's no need to do a full-blown
-        // save() and restore(). Also note that we don't have to do this in case of
-        // IndirectPainting (the else branch), because in that case we always save()
-        // and restore() in QGraphicsScene::drawItems().
-        if (!d->scene->d_func()->painterStateProtection)
-            painter.setOpacity(1.0);
-        painter.setWorldTransform(viewTransform);
-    } else {
-        // Make sure we don't have unpolished items before we draw
-        if (!d->scene->d_func()->unpolishedItems.isEmpty())
-            d->scene->d_func()->_q_polishItems();
-        // We reset updateAll here (after we've issued polish events)
-        // so that we can discard update requests coming from polishEvent().
-        d->scene->d_func()->updateAll = false;
-
-        // Find all exposed items
-        bool allItems = false;
-        QList<QGraphicsItem *> itemList = d->findItems(d->exposedRegion, &allItems, viewTransform);
-        if (!itemList.isEmpty()) {
-            // Generate the style options.
-            const int numItems = itemList.size();
-            QGraphicsItem **itemArray = &itemList[0]; // Relies on QList internals, but is perfectly valid.
-            QStyleOptionGraphicsItem *styleOptionArray = d->allocStyleOptionsArray(numItems);
-            QTransform transform(Qt::Uninitialized);
-            for (int i = 0; i < numItems; ++i) {
-                QGraphicsItem *item = itemArray[i];
-                QGraphicsItemPrivate *itemd = item->d_ptr.data();
-                itemd->initStyleOption(&styleOptionArray[i], viewTransform, d->exposedRegion, allItems);
-                // Cache the item's area in view coordinates.
-                // Note that we have to do this here in case the base class implementation
-                // (QGraphicsScene::drawItems) is not called. If it is, we'll do this
-                // operation twice, but that's the price one has to pay for using indirect
-                // painting :-/.
-                const QRectF brect = adjustedItemEffectiveBoundingRect(item);
-                if (!itemd->itemIsUntransformable()) {
-                    transform = item->sceneTransform();
-                    if (viewTransformed)
-                        transform *= viewTransform;
-                } else {
-                    transform = item->deviceTransform(viewTransform);
+        // Items
+        if (!(d->optimizationFlags & IndirectPainting)) {
+            const quint32 oldRectAdjust = d->scene->d_func()->rectAdjust;
+            if (d->optimizationFlags & QGraphicsView::DontAdjustForAntialiasing)
+                d->scene->d_func()->rectAdjust = 1;
+            else
+                d->scene->d_func()->rectAdjust = 2;
+            d->scene->d_func()->drawItems(&painter, viewTransformed ? &viewTransform : nullptr,
+                                          &d->exposedRegion, viewport());
+            d->scene->d_func()->rectAdjust = oldRectAdjust;
+            // Make sure the painter's world transform is restored correctly when
+            // drawing without painter state protection (DontSavePainterState).
+            // We only change the worldTransform() so there's no need to do a full-blown
+            // save() and restore(). Also note that we don't have to do this in case of
+            // IndirectPainting (the else branch), because in that case we always save()
+            // and restore() in QGraphicsScene::drawItems().
+            if (!d->scene->d_func()->painterStateProtection)
+                painter.setOpacity(1.0);
+            painter.setWorldTransform(viewTransform);
+        } else {
+            // Make sure we don't have unpolished items before we draw
+            if (!d->scene->d_func()->unpolishedItems.isEmpty())
+                d->scene->d_func()->_q_polishItems();
+            // We reset updateAll here (after we've issued polish events)
+            // so that we can discard update requests coming from polishEvent().
+            d->scene->d_func()->updateAll = false;
+
+            // Find all exposed items
+            bool allItems = false;
+            QList<QGraphicsItem *> itemList = d->findItems(d->exposedRegion, &allItems, viewTransform);
+            if (!itemList.isEmpty()) {
+                // Generate the style options.
+                const int numItems = itemList.size();
+                QGraphicsItem **itemArray = &itemList[0]; // Relies on QList internals, but is perfectly valid.
+                QStyleOptionGraphicsItem *styleOptionArray = d->allocStyleOptionsArray(numItems);
+                QTransform transform(Qt::Uninitialized);
+                for (int i = 0; i < numItems; ++i) {
+                    QGraphicsItem *item = itemArray[i];
+                    QGraphicsItemPrivate *itemd = item->d_ptr.data();
+                    itemd->initStyleOption(&styleOptionArray[i], viewTransform, d->exposedRegion, allItems);
+                    // Cache the item's area in view coordinates.
+                    // Note that we have to do this here in case the base class implementation
+                    // (QGraphicsScene::drawItems) is not called. If it is, we'll do this
+                    // operation twice, but that's the price one has to pay for using indirect
+                    // painting :-/.
+                    const QRectF brect = adjustedItemEffectiveBoundingRect(item);
+                    if (!itemd->itemIsUntransformable()) {
+                        transform = item->sceneTransform();
+                        if (viewTransformed)
+                            transform *= viewTransform;
+                    } else {
+                        transform = item->deviceTransform(viewTransform);
+                    }
+                    itemd->paintedViewBoundingRects.insert(d->viewport, transform.mapRect(brect).toRect());
                 }
-                itemd->paintedViewBoundingRects.insert(d->viewport, transform.mapRect(brect).toRect());
+                // Draw the items.
+                drawItems(&painter, numItems, itemArray, styleOptionArray);
+                d->freeStyleOptionsArray(styleOptionArray);
             }
-            // Draw the items.
-            drawItems(&painter, numItems, itemArray, styleOptionArray);
-            d->freeStyleOptionsArray(styleOptionArray);
         }
-    }
 
-    // Foreground
-    drawForeground(&painter, exposedSceneRect);
+        // Foreground
+        drawForeground(&painter, exposedSceneRect);
 
-#if QT_CONFIG(rubberband)
-    // Rubberband
-    if (d->rubberBanding && !d->rubberBandRect.isEmpty()) {
-        painter.restore();
-        QStyleOptionRubberBand option;
-        option.initFrom(viewport());
-        option.rect = d->rubberBandRect;
-        option.shape = QRubberBand::Rectangle;
-
-        QStyleHintReturnMask mask;
-        if (viewport()->style()->styleHint(QStyle::SH_RubberBand_Mask, &option, viewport(), &mask)) {
-            // painter clipping for masked rubberbands
-            painter.setClipRegion(mask.region, Qt::IntersectClip);
+    #if QT_CONFIG(rubberband)
+        // Rubberband
+        if (d->rubberBanding && !d->rubberBandRect.isEmpty()) {
+            painter.restore();
+            QStyleOptionRubberBand option;
+            option.initFrom(viewport());
+            option.rect = d->rubberBandRect;
+            option.shape = QRubberBand::Rectangle;
+
+            QStyleHintReturnMask mask;
+            if (viewport()->style()->styleHint(QStyle::SH_RubberBand_Mask, &option, viewport(), &mask)) {
+                // painter clipping for masked rubberbands
+                painter.setClipRegion(mask.region, Qt::IntersectClip);
+            }
+
+            viewport()->style()->drawControl(QStyle::CE_RubberBand, &option, &painter, viewport());
         }
+    #endif
+    };
 
-        viewport()->style()->drawControl(QStyle::CE_RubberBand, &option, &painter, viewport());
+    actuallyDraw();
+
+    // For stereo we want to draw everything twice, once to each buffer
+    if (d->stereoEnabled) {
+        QWidgetPrivate* w = QWidgetPrivate::get(viewport());
+        if (w->toggleStereoTargetBuffer()) {
+            actuallyDraw();
+            w->toggleStereoTargetBuffer();
+        }
     }
-#endif
 
     painter.end();
 
diff --git a/src/widgets/graphicsview/qgraphicsview_p.h b/src/widgets/graphicsview/qgraphicsview_p.h
index f3388764b5..a15defe621 100644
--- a/src/widgets/graphicsview/qgraphicsview_p.h
+++ b/src/widgets/graphicsview/qgraphicsview_p.h
@@ -100,6 +100,8 @@ public:
     QGraphicsView::ViewportUpdateMode viewportUpdateMode;
     QGraphicsView::OptimizationFlags optimizationFlags;
 
+    bool stereoEnabled = false; // Set in setupViewport()
+
     QPointer<QGraphicsScene> scene;
 #if QT_CONFIG(rubberband)
     QRect rubberBandRect;
diff --git a/src/widgets/kernel/qwidget_p.h b/src/widgets/kernel/qwidget_p.h
index ea03e9c708..25a9c75701 100644
--- a/src/widgets/kernel/qwidget_p.h
+++ b/src/widgets/kernel/qwidget_p.h
@@ -621,6 +621,11 @@ public:
     // Called after each paint event.
     virtual void resolveSamples() { }
 
+    // These two are used in QGraphicsView for supporting stereoscopic rendering with a
+    // QOpenGLWidget viewport.
+    virtual bool isStereoEnabled() { return false; } // Called in QGraphicsView::setupViewport
+    virtual bool toggleStereoTargetBuffer() { return false; } // Called in QGraphicsView::paintEvent
+
     static void setWidgetParentHelper(QObject *widgetAsObject, QObject *newParent);
 
     std::string flagsForDumping() const override;
diff --git a/tests/manual/stereographicsview/CMakeLists.txt b/tests/manual/stereographicsview/CMakeLists.txt
new file mode 100644
index 0000000000..937e10de91
--- /dev/null
+++ b/tests/manual/stereographicsview/CMakeLists.txt
@@ -0,0 +1,32 @@
+# Copyright (C) 2022 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+# Generated from stereographicsview.pro.
+
+#####################################################################
+## stereographicsview Binary:
+#####################################################################
+
+set(CMAKE_AUTOUIC ON)
+
+qt_internal_add_manual_test(stereographicsview
+    GUI
+    SOURCES
+        main.cpp
+        mainwindow.cpp
+        mainwindow.h
+        mainwindow.ui
+        mygraphicsview.h
+        mygraphicsview.cpp
+    LIBRARIES
+        Qt::CorePrivate
+        Qt::Gui
+        Qt::GuiPrivate
+        Qt::OpenGL
+        Qt::OpenGLWidgets
+        Qt::Widgets
+        Qt::WidgetsPrivate
+)
+
+#### Keys ignored in scope 1:.:.:stereographicsview.pro:<TRUE>:
+# TEMPLATE = "app"
diff --git a/tests/manual/stereographicsview/main.cpp b/tests/manual/stereographicsview/main.cpp
new file mode 100644
index 0000000000..6564e8efc4
--- /dev/null
+++ b/tests/manual/stereographicsview/main.cpp
@@ -0,0 +1,26 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#include "mainwindow.h"
+
+#include <QApplication>
+#include <QSurfaceFormat>
+
+int main(int argc, char *argv[])
+{
+    QApplication a(argc, argv);
+
+    QSurfaceFormat format;
+    format.setDepthBufferSize(24);
+    format.setStencilBufferSize(8);
+    format.setSamples(16);
+    format.setStereo(true);
+    QSurfaceFormat::setDefaultFormat(format);
+
+    MainWindow w;
+    w.show();
+    return a.exec();
+}
+
+
+#include "main.moc"
diff --git a/tests/manual/stereographicsview/mainwindow.cpp b/tests/manual/stereographicsview/mainwindow.cpp
new file mode 100644
index 0000000000..003b4da8c3
--- /dev/null
+++ b/tests/manual/stereographicsview/mainwindow.cpp
@@ -0,0 +1,79 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#include "mainwindow.h"
+#include "./ui_mainwindow.h"
+#include <QPushButton>
+#include <QSlider>
+#include <QLabel>
+#include <QGraphicsEllipseItem>
+#include <QRandomGenerator>
+#include <QOpenGLWidget>
+
+MainWindow::MainWindow(QWidget *parent)
+    : QMainWindow(parent)
+    , ui(new Ui::MainWindow)
+{
+    ui->setupUi(this);
+
+    ui->graphicsView->setViewport(new QOpenGLWidget);
+    ui->graphicsView->setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
+
+    QRect rect = ui->graphicsView->rect();
+    QGraphicsScene *m_scene = new QGraphicsScene(rect);
+    ui->graphicsView->setScene(m_scene);
+
+    QSize initialSize(150, 150);
+    m_ellipse = m_scene->addEllipse(
+        rect.width() / 2 - initialSize.width() / 2,
+        rect.height() / 2 - initialSize.height() / 2,
+        initialSize.width(),
+        initialSize.height(),
+        QPen(Qt::magenta, 5),
+        Qt::blue);
+
+    QPushButton *button = new QPushButton();
+    button->setGeometry(QRect(rect.width() / 2 - 75, 100, 150, 50));
+    button->setText("Random ellipse color");
+    QObject::connect(button, &QPushButton::pressed, [&]() {
+        m_ellipse->setBrush(QColor::fromRgb(QRandomGenerator::global()->generate()));
+        m_ellipse->setPen(QPen(QColor::fromRgb(QRandomGenerator::global()->generate()), 5));
+    });
+    m_scene->addWidget(button);
+
+    m_label = new QLabel();
+    m_label->setGeometry(QRect(rect.width() / 2 - 50, rect.height() - 100, 100, 50));
+    m_label->setAlignment(Qt::AlignCenter);
+    m_scene->addWidget(m_label);
+
+    QSlider *slider = new QSlider(Qt::Horizontal);
+    slider->setGeometry(QRect(rect.width() / 2 - 100, rect.height() - 50, 200, 50));
+    slider->setMinimum(10);
+    slider->setMaximum(300);
+    slider->setValue(initialSize.width());
+    QObject::connect(slider, &QAbstractSlider::valueChanged, [this](int value){
+        QRectF rect = m_ellipse->rect();
+        rect.setWidth(value);
+        rect.setHeight(value);
+        m_ellipse->setRect(rect);
+        m_label->setText("Ellipse size: " + QString::number(value));
+    });
+    emit slider->valueChanged(initialSize.width());
+    m_scene->addWidget(slider);
+
+    QMenu *screenShotMenu = menuBar()->addMenu("&Screenshot");
+    screenShotMenu->addAction("Left buffer", this, [this](){
+        ui->graphicsView->saveImage(QOpenGLWidget::LeftBuffer);
+    });
+
+    screenShotMenu->addAction("Right buffer", this, [this](){
+        ui->graphicsView->saveImage(QOpenGLWidget::RightBuffer);
+    });
+
+}
+
+MainWindow::~MainWindow()
+{
+    delete ui;
+}
+
diff --git a/tests/manual/stereographicsview/mainwindow.h b/tests/manual/stereographicsview/mainwindow.h
new file mode 100644
index 0000000000..4a65dad68d
--- /dev/null
+++ b/tests/manual/stereographicsview/mainwindow.h
@@ -0,0 +1,30 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#ifndef MAINWINDOW_H
+#define MAINWINDOW_H
+
+#include <QMainWindow>
+
+QT_BEGIN_NAMESPACE
+namespace Ui { class MainWindow; }
+QT_END_NAMESPACE
+
+class QGraphicsEllipseItem;
+class QLabel;
+
+class MainWindow : public QMainWindow
+{
+    Q_OBJECT
+
+public:
+    MainWindow(QWidget *parent = nullptr);
+    ~MainWindow();
+
+private:
+    QGraphicsEllipseItem *m_ellipse;
+    QLabel *m_label;
+
+    Ui::MainWindow *ui;
+};
+#endif // MAINWINDOW_H
diff --git a/tests/manual/stereographicsview/mainwindow.ui b/tests/manual/stereographicsview/mainwindow.ui
new file mode 100644
index 0000000000..2ebb6b8caa
--- /dev/null
+++ b/tests/manual/stereographicsview/mainwindow.ui
@@ -0,0 +1,101 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<ui version="4.0">
+ <class>MainWindow</class>
+ <widget class="QMainWindow" name="MainWindow">
+  <property name="geometry">
+   <rect>
+    <x>0</x>
+    <y>0</y>
+    <width>1120</width>
+    <height>695</height>
+   </rect>
+  </property>
+  <property name="windowTitle">
+   <string>MainWindow</string>
+  </property>
+  <widget class="QWidget" name="centralwidget">
+   <layout class="QHBoxLayout" name="horizontalLayout">
+    <item>
+     <widget class="MyGraphicsView" name="graphicsView">
+      <property name="sizePolicy">
+       <sizepolicy hsizetype="MinimumExpanding" vsizetype="MinimumExpanding">
+        <horstretch>0</horstretch>
+        <verstretch>0</verstretch>
+       </sizepolicy>
+      </property>
+      <property name="minimumSize">
+       <size>
+        <width>0</width>
+        <height>0</height>
+       </size>
+      </property>
+      <property name="sizeAdjustPolicy">
+       <enum>QAbstractScrollArea::AdjustToContentsOnFirstShow</enum>
+      </property>
+     </widget>
+    </item>
+    <item>
+     <widget class="QFrame" name="frame">
+      <property name="sizePolicy">
+       <sizepolicy hsizetype="Preferred" vsizetype="Preferred">
+        <horstretch>0</horstretch>
+        <verstretch>0</verstretch>
+       </sizepolicy>
+      </property>
+      <property name="frameShape">
+       <enum>QFrame::StyledPanel</enum>
+      </property>
+      <property name="frameShadow">
+       <enum>QFrame::Raised</enum>
+      </property>
+      <layout class="QVBoxLayout" name="verticalLayout">
+       <item>
+        <widget class="QLabel" name="label">
+         <property name="text">
+          <string>QGraphicsView Stereoscopic Example</string>
+         </property>
+         <property name="alignment">
+          <set>Qt::AlignCenter</set>
+         </property>
+        </widget>
+       </item>
+       <item>
+        <spacer name="verticalSpacer">
+         <property name="orientation">
+          <enum>Qt::Vertical</enum>
+         </property>
+         <property name="sizeHint" stdset="0">
+          <size>
+           <width>20</width>
+           <height>40</height>
+          </size>
+         </property>
+        </spacer>
+       </item>
+      </layout>
+     </widget>
+    </item>
+   </layout>
+  </widget>
+  <widget class="QMenuBar" name="menubar">
+   <property name="geometry">
+    <rect>
+     <x>0</x>
+     <y>0</y>
+     <width>1120</width>
+     <height>21</height>
+    </rect>
+   </property>
+  </widget>
+  <widget class="QStatusBar" name="statusbar"/>
+ </widget>
+ <customwidgets>
+  <customwidget>
+   <class>MyGraphicsView</class>
+   <extends>QGraphicsView</extends>
+   <header>mygraphicsview.h</header>
+  </customwidget>
+ </customwidgets>
+ <resources/>
+ <connections/>
+</ui>
diff --git a/tests/manual/stereographicsview/mygraphicsview.cpp b/tests/manual/stereographicsview/mygraphicsview.cpp
new file mode 100644
index 0000000000..dc6e39f4a0
--- /dev/null
+++ b/tests/manual/stereographicsview/mygraphicsview.cpp
@@ -0,0 +1,209 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#include "mygraphicsview.h"
+#include <QResizeEvent>
+#include <QFileDialog>
+
+Q_OPENGL_EXPORT QImage qt_gl_read_framebuffer(const QSize &size, bool alpha_format, bool include_alpha);
+
+MyGraphicsView::MyGraphicsView(QWidget *parent) :
+    QGraphicsView(parent),
+    m_indexBuf(QOpenGLBuffer::IndexBuffer)
+{
+
+}
+
+MyGraphicsView::MyGraphicsView(QGraphicsScene *scene, QWidget *parent) :
+    QGraphicsView(scene, parent)
+{
+
+}
+
+void MyGraphicsView::saveImage(QOpenGLWidget::TargetBuffer targetBuffer)
+{
+    QOpenGLWidget *w = static_cast<QOpenGLWidget*>(viewport());
+    Q_ASSERT(w);
+
+    w->makeCurrent(targetBuffer);
+    draw(targetBuffer);
+
+    QImage img = qt_gl_read_framebuffer(w->size() * w->devicePixelRatio(), true, true);
+
+    if (img.isNull()) {
+        qFatal("Failed to grab framebuffer");
+    }
+
+    const char *fn =
+            targetBuffer == QOpenGLWidget::LeftBuffer
+            ? "leftBuffer.png" : "rightBuffer.png";
+
+    QFileDialog fd(this);
+    fd.setAcceptMode(QFileDialog::AcceptSave);
+    fd.setDefaultSuffix("png");
+    fd.selectFile(fn);
+    if (fd.exec() == QDialog::Accepted)
+        img.save(fd.selectedFiles().first());
+}
+
+void MyGraphicsView::drawBackground(QPainter *painter, const QRectF &rect)
+{
+    if (!m_initialized)
+        init();
+
+    QOpenGLWidget *w = static_cast<QOpenGLWidget*>(viewport());
+
+    painter->beginNativePainting();
+
+    draw(w->currentTargetBuffer());
+
+    painter->endNativePainting();
+
+
+    m_yaw += 0.5;
+    if (m_yaw > 360)
+        m_yaw = 0;
+}
+
+void MyGraphicsView::resizeEvent(QResizeEvent *event)
+{
+    QGraphicsView::resizeEvent(event);
+
+    resize(event->size().width(), event->size().height());
+}
+
+void MyGraphicsView::init()
+{
+    if (m_initialized)
+        return;
+
+    initializeOpenGLFunctions();
+
+    initShaders();
+    initCube();
+
+    resize(viewport()->width(), viewport()->height());
+
+    m_initialized = true;
+}
+
+void MyGraphicsView::initShaders()
+{
+    static const char *vertexShaderSource =
+            "#ifdef GL_ES\n"
+            "// Set default precision to medium\n"
+            "precision mediump int;\n"
+            "precision mediump float;\n"
+            "#endif\n"
+            "attribute vec4 a_position;\n"
+            "uniform mat4 u_mvp;\n"
+            "void main() {\n"
+               "gl_Position = u_mvp * a_position;\n"
+            "}\n";
+
+    static const char *fragmentShaderSource =
+            "#ifdef GL_ES\n"
+            "// Set default precision to medium\n"
+            "precision mediump int;\n"
+            "precision mediump float;\n"
+            "#endif\n"
+            "void main() {\n"
+               "gl_FragColor = vec4(0.7, 0.1, 0.0, 1.0);\n"
+            "}\n";
+
+    if (!m_program.addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSource))
+        close();
+
+    if (!m_program.addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSource))
+        close();
+
+    if (!m_program.link())
+        close();
+
+    if (!m_program.bind())
+        close();
+}
+
+void MyGraphicsView::initCube()
+{
+    m_arrayBuf.create();
+    m_indexBuf.create();
+
+    QVector3D vertices[] = {
+        QVector3D(-1.0f, -1.0f,  1.0f),
+        QVector3D( 1.0f, -1.0f,  1.0f),
+        QVector3D(-1.0f,  1.0f,  1.0f),
+        QVector3D( 1.0f,  1.0f,  1.0f),
+        QVector3D( 1.0f, -1.0f,  1.0f),
+        QVector3D( 1.0f, -1.0f, -1.0f),
+        QVector3D( 1.0f,  1.0f,  1.0f),
+        QVector3D( 1.0f,  1.0f, -1.0f),
+        QVector3D( 1.0f, -1.0f, -1.0f),
+        QVector3D(-1.0f, -1.0f, -1.0f),
+        QVector3D( 1.0f,  1.0f, -1.0f),
+        QVector3D(-1.0f,  1.0f, -1.0f),
+        QVector3D(-1.0f, -1.0f, -1.0f),
+        QVector3D(-1.0f, -1.0f,  1.0f),
+        QVector3D(-1.0f,  1.0f, -1.0f),
+        QVector3D(-1.0f,  1.0f,  1.0f),
+        QVector3D(-1.0f, -1.0f, -1.0f),
+        QVector3D( 1.0f, -1.0f, -1.0f),
+        QVector3D(-1.0f, -1.0f,  1.0f),
+        QVector3D( 1.0f, -1.0f,  1.0f),
+        QVector3D(-1.0f,  1.0f,  1.0f),
+        QVector3D( 1.0f,  1.0f,  1.0f),
+        QVector3D(-1.0f,  1.0f, -1.0f),
+        QVector3D( 1.0f,  1.0f, -1.0f)
+    };
+
+    GLushort indices[] = {
+        0,  1,  2,  3,  3,
+        4,  4,  5,  6,  7,  7,
+        8,  8,  9, 10, 11, 11,
+        12, 12, 13, 14, 15, 15,
+        16, 16, 17, 18, 19, 19,
+        20, 20, 21, 22, 23
+    };
+
+
+    m_arrayBuf.bind();
+    m_arrayBuf.allocate(vertices, 24 * sizeof(QVector3D));
+
+    m_indexBuf.bind();
+    m_indexBuf.allocate(indices, 34 * sizeof(GLushort));
+}
+
+void MyGraphicsView::draw(QOpenGLWidget::TargetBuffer targetBuffer)
+{
+    glClearColor(0.0f, 0.7f, 0.0f, 1.0f);
+    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+
+    m_program.bind();
+    m_arrayBuf.bind();
+    m_indexBuf.bind();
+
+    QMatrix4x4 matrix;
+    matrix.translate(targetBuffer == QOpenGLWidget::LeftBuffer ? -2.0 : 2.0,
+                     0.0,
+                     -10.0);
+    matrix.scale(2.0);
+    matrix.rotate(QQuaternion::fromEulerAngles(10, m_yaw, 0));
+    m_program.setUniformValue("u_mvp", m_projection * matrix);
+
+    int vertexLocation = m_program.attributeLocation("a_position");
+    m_program.enableAttributeArray(vertexLocation);
+    m_program.setAttributeBuffer(vertexLocation, GL_FLOAT, 0, 3, sizeof(QVector3D));
+
+    glDrawElements(GL_TRIANGLE_STRIP, 34, GL_UNSIGNED_SHORT, nullptr);
+}
+
+void MyGraphicsView::resize(int w, int h)
+{
+    qreal aspect = qreal(w) / qreal(h ? h : 1);
+    const qreal zNear = 0.0, zFar = 100.0, fov = 45.0;
+    m_projection.setToIdentity();
+    m_projection.perspective(fov, aspect, zNear, zFar);
+
+    if (m_initialized)
+        glViewport(0, 0, w, h);
+}
diff --git a/tests/manual/stereographicsview/mygraphicsview.h b/tests/manual/stereographicsview/mygraphicsview.h
new file mode 100644
index 0000000000..03204cb316
--- /dev/null
+++ b/tests/manual/stereographicsview/mygraphicsview.h
@@ -0,0 +1,47 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR LGPL-3.0-only OR GPL-2.0-only OR GPL-3.0-only
+
+#ifndef MYGRAPHICSVIEW_H
+#define MYGRAPHICSVIEW_H
+
+#include <QOpenGLFunctions>
+#include <QGraphicsView>
+#include <QOpenGLShaderProgram>
+#include <QOpenGLBuffer>
+#include <QMatrix4x4>
+#include <QOpenGLWidget>
+
+class MyGraphicsView : public QGraphicsView, protected QOpenGLFunctions
+{
+public:
+    MyGraphicsView(QWidget *parent = nullptr);
+    MyGraphicsView(QGraphicsScene *scene, QWidget *parent = nullptr);
+
+    void saveImage(QOpenGLWidget::TargetBuffer targetBuffer);
+
+protected:
+    void drawBackground(QPainter *painter, const QRectF &rect) override;
+
+    void resizeEvent(QResizeEvent *event) override;
+
+private:
+    void init();
+    void initShaders();
+    void initCube();
+
+    void draw(QOpenGLWidget::TargetBuffer targetBuffer);
+
+    void resize(int w, int h);
+
+
+    bool m_initialized = false;
+
+    QOpenGLShaderProgram m_program;
+    QMatrix4x4 m_projection;
+    QOpenGLBuffer m_arrayBuf;
+    QOpenGLBuffer m_indexBuf;
+
+    qreal m_yaw = 0;
+};
+
+#endif // MYGRAPHICSVIEW_H
diff --git a/tests/manual/stereographicsview/stereographicsview.pro b/tests/manual/stereographicsview/stereographicsview.pro
new file mode 100644
index 0000000000..597166367c
--- /dev/null
+++ b/tests/manual/stereographicsview/stereographicsview.pro
@@ -0,0 +1,13 @@
+QT += widgets widgets-private gui-private core-private
+
+TARGET = stereographicsview
+TEMPLATE = app
+
+SOURCES += main.cpp \
+        mainwindow.cpp \
+        mainwindow.h \
+        mainwindow.ui \
+        mygraphicsview.h \
+        mygraphicsview.cpp \
+
+HEADERS  += openglwidget.h