Document and demo that fboId can be 0

...in QQuickWindow::setRenderTarget().

The rendercontrol example is extended with a --onscreen
command line argument that can be used to request rendering
to the default framebuffer of the window.

Change-Id: I7a500d1585dee8334b902fb1dddcb1cb21a2e038
Task-number: QTBUG-59340
Reviewed-by: Gunnar Sletta <gunnar@crimson.no>

examples/quick/rendercontrol/window_singlethreaded.cpp

@@ -82,6 +82,10 @@ WindowSingleThreaded::WindowSingleThreaded()
 {
     setSurfaceType(QSurface::OpenGLSurface);
 
+    // The rendercontrol does not necessarily need an FBO. Demonstrate this
+    // when requested.
+    m_onscreen = QCoreApplication::arguments().contains(QStringLiteral("--onscreen"));
+
     QSurfaceFormat format;
     // Qt Quick may need a depth and stencil buffer. Always make sure these are available.
     format.setDepthBufferSize(16);
@@ -164,8 +168,14 @@ void WindowSingleThreaded::createFbo()
     // The scene graph has been initialized. It is now time to create an FBO and associate
     // it with the QQuickWindow.
     m_dpr = devicePixelRatio();
-    m_fbo = new QOpenGLFramebufferObject(size() * m_dpr, QOpenGLFramebufferObject::CombinedDepthStencil);
+    if (!m_onscreen) {
-    m_quickWindow->setRenderTarget(m_fbo);
+        m_fbo = new QOpenGLFramebufferObject(size() * m_dpr, QOpenGLFramebufferObject::CombinedDepthStencil);
+        m_quickWindow->setRenderTarget(m_fbo);
+    } else {
+        // Special case: No FBO. Render directly to the window's default framebuffer.
+        m_onscreenSize = size() * m_dpr;
+        m_quickWindow->setRenderTarget(0, m_onscreenSize);
+    }
 }
 
 void WindowSingleThreaded::destroyFbo()
@@ -176,7 +186,10 @@ void WindowSingleThreaded::destroyFbo()
 
 void WindowSingleThreaded::render()
 {
-    if (!m_context->makeCurrent(m_offscreenSurface))
+    QSurface *surface = m_offscreenSurface;
+    if (m_onscreen)
+        surface = this;
+    if (!m_context->makeCurrent(surface))
         return;
 
     // Polish, synchronize and render the next frame (into our fbo).  In this example
@@ -195,7 +208,10 @@ void WindowSingleThreaded::render()
     m_quickReady = true;
 
     // Get something onto the screen.
-    m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
+    if (!m_onscreen)
+        m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
+    else
+        m_context->swapBuffers(this);
 }
 
 void WindowSingleThreaded::requestUpdate()
@@ -237,7 +253,10 @@ void WindowSingleThreaded::run()
     updateSizes();
 
     // Initialize the render control and our OpenGL resources.
-    m_context->makeCurrent(m_offscreenSurface);
+    QSurface *surface = m_offscreenSurface;
+    if (m_onscreen)
+        surface = this;
+    m_context->makeCurrent(surface);
     m_renderControl->initialize(m_context);
     m_quickInitialized = true;
 }
@@ -266,7 +285,8 @@ void WindowSingleThreaded::exposeEvent(QExposeEvent *)
 {
     if (isExposed()) {
         if (!m_quickInitialized) {
-            m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
+            if (!m_onscreen)
+                m_cubeRenderer->render(this, m_context, m_quickReady ? m_fbo->texture() : 0);
             startQuick(QStringLiteral("qrc:/rendercontrol/demo.qml"));
         }
     }
@@ -274,7 +294,10 @@ void WindowSingleThreaded::exposeEvent(QExposeEvent *)
 
 void WindowSingleThreaded::resizeFbo()
 {
-    if (m_rootItem && m_context->makeCurrent(m_offscreenSurface)) {
+    QSurface *surface = m_offscreenSurface;
+    if (m_onscreen)
+        surface = this;
+    if (m_rootItem && m_context->makeCurrent(surface)) {
         delete m_fbo;
         createFbo();
         m_context->doneCurrent();
@@ -287,8 +310,13 @@ void WindowSingleThreaded::resizeEvent(QResizeEvent *)
 {
     // If this is a resize after the scene is up and running, recreate the fbo and the
     // Quick item and scene.
-    if (m_fbo && m_fbo->size() != size() * devicePixelRatio())
+    if (!m_onscreen) {
-        resizeFbo();
+        if (m_fbo && m_fbo->size() != size() * devicePixelRatio())
+            resizeFbo();
+    } else {
+        if (m_onscreenSize != size() * devicePixelRatio())
+            resizeFbo();
+    }
 }
 
 void WindowSingleThreaded::handleScreenChange()

examples/quick/rendercontrol/window_singlethreaded.h

@@ -97,6 +97,8 @@ private:
     QTimer m_updateTimer;
     CubeRenderer *m_cubeRenderer;
     qreal m_dpr;
+    bool m_onscreen;
+    QSize m_onscreenSize;
 };
 
 #endif

src/quick/items/qquickwindow.cpp

@@ -3399,6 +3399,11 @@ void QQuickWindow::setRenderTarget(QOpenGLFramebufferObject *fbo)
     The specified FBO must be created in the context of the window
     or one that shares with it.
 
+    \note \a fboId can also be set to 0. In this case rendering will target the
+    default framebuffer of whichever surface is current when the scenegraph
+    renders. \a size must still be valid, specifying the dimensions of the
+    surface.
+
     \note
     This function only has an effect when using the default OpenGL scene
     graph adaptation.