Fix Scene3D initialization in QQuickWidget

Scene3D can now be used inside a QQuickWidget. There are some
remaining problems, most notably an assert when exiting, but at
least the scene shows up now.

Task-number: QTBUG-52132
Change-Id: Ifc05d859711f5f42342e111c1d7cbea7ba373552
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>

2 files changed, 23 insertions, 2 deletions

diff --git a/src/quick3d/imports/scene3d/scene3ditem.cpp b/src/quick3d/imports/scene3d/scene3ditem.cpp
index a156b9ef3..6c285e133 100644
--- a/src/quick3d/imports/scene3d/scene3ditem.cpp
+++ b/src/quick3d/imports/scene3d/scene3ditem.cpp
@@ -47,7 +47,11 @@
 #include <Qt3DRender/QRenderAspect>
 #include <Qt3DRender/qcamera.h>
 #include <Qt3DRender/qrendersurfaceselector.h>
+
+#include <QtGui/qguiapplication.h>
+#include <QtGui/qoffscreensurface.h>
 #include <QtQuick/qquickwindow.h>
+#include <QtQuick/qquickrendercontrol.h>
 
 #include <Qt3DRender/private/qrendersurfaceselector_p.h>
 #include <scene3dcleaner_p.h>
@@ -201,8 +205,22 @@ void Scene3DItem::setWindowSurface(QObject *rootObject)
 
     // Set the item's window surface if it appears
     // the surface wasn't set on the surfaceSelector
-    if (surfaceSelector && !surfaceSelector->surface())
-        surfaceSelector->setSurface(this->window());
+    if (surfaceSelector && !surfaceSelector->surface()) {
+        // We may not have a real, exposed QQuickWindow when the Quick rendering
+        // is redirected via QQuickRenderControl (f.ex. QQuickWidget).
+        if (QWindow *rw = QQuickRenderControl::renderWindowFor(this->window())) {
+            // rw is the top-level window that is backed by a native window. Do
+            // not use that though since we must not clash with e.g. the widget
+            // backingstore compositor in the gui thread.
+            m_dummySurface = new QOffscreenSurface;
+            m_dummySurface->setParent(qGuiApp); // parent to something suitably long-living
+            m_dummySurface->setFormat(rw->format());
+            m_dummySurface->create();
+            surfaceSelector->setSurface(m_dummySurface);
+        } else {
+            surfaceSelector->setSurface(this->window());
+        }
+    }
 }
 
 void Scene3DItem::setItemArea(const QSize &area)
diff --git a/src/quick3d/imports/scene3d/scene3ditem_p.h b/src/quick3d/imports/scene3d/scene3ditem_p.h
index f3d452d3d..eb14645c3 100644
--- a/src/quick3d/imports/scene3d/scene3ditem_p.h
+++ b/src/quick3d/imports/scene3d/scene3ditem_p.h
@@ -56,6 +56,8 @@
 
 QT_BEGIN_NAMESPACE
 
+class QOffscreenSurface;
+
 namespace Qt3DCore {
 class QAspectEngine;
 class QEntity;
@@ -131,6 +133,7 @@ private:
 
     QPointer<Qt3DRender::QCamera> m_camera;
     CameraAspectRatioMode m_cameraAspectRatioMode;
+    QOffscreenSurface *m_dummySurface;
 };
 
 } // Qt3DRender