Make openglunderqml functional with and without the rhi

It has to be written following the new split approach (beforeRendering
for resource setup, beforeRenderPassRecording to issue the actual
underlay draw calls), but it will then work both with and
without QSG_RHI=1.

Change-Id: I9b7b35434aa0caec543cae268064b2684256382d
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

3 files changed, 37 insertions, 29 deletions

diff --git a/examples/quick/scenegraph/openglunderqml/doc/src/openglunderqml.qdoc b/examples/quick/scenegraph/openglunderqml/doc/src/openglunderqml.qdoc
index 778b754869..69a9d2ce4b 100644
--- a/examples/quick/scenegraph/openglunderqml/doc/src/openglunderqml.qdoc
+++ b/examples/quick/scenegraph/openglunderqml/doc/src/openglunderqml.qdoc
@@ -94,17 +94,18 @@
 
     \snippet scenegraph/openglunderqml/squircle.cpp 3
 
-    The default behavior of the scene graph is to clear the
-    framebuffer before rendering. Since we render before the scene
-    graph, we need to turn this clearing off. This means that we need
-    to clear ourselves in the \c paint() function.
+    The default behavior of the scene graph is to clear the framebuffer before
+    rendering. This is fine since we will insert our own rendering code after
+    this clear is enqueued. Make sure however that we clear to the desired
+    color (black).
 
     \snippet scenegraph/openglunderqml/squircle.cpp 9
 
-    We use the \c sync() function to initialize the renderer and to
-    copy the state in our item into the renderer. When the renderer is
-    created, we also connect the \l QQuickWindow::beforeRendering() to
-    the renderer's \c paint() slot.
+    We use the \c sync() function to initialize the renderer and to copy the
+    state in our item into the renderer. When the renderer is created, we also
+    connect the \l QQuickWindow::beforeRendering() and \l
+    QQuickWindow::beforeRenderPassRecording() to the renderer's \c init() and
+    \c paint() slots.
 
     \note The \l QQuickWindow::beforeSynchronizing() signal is emitted
     on the rendering thread while the GUI thread is blocked, so it is
@@ -127,22 +128,13 @@
 
     \snippet scenegraph/openglunderqml/squircle.cpp 4
 
-    In the SquircleRenderer's \c paint() function we start by
-    initializing the shader program. By initializing the shader
-    program here, we make sure that the OpenGL context is bound and
-    that we are on the correct thread.
+    In the SquircleRenderer's \c init() function we start by initializing the
+    shader program if not yet done. The OpenGL context is current on the thread
+    when the slot is invoked.
 
     \snippet scenegraph/openglunderqml/squircle.cpp 5
 
-    We use the shader program to draw the squircle. At the end of the
-    \c paint function we release the program and disable the
-    attributes we used so that the OpenGL context is in a "clean"
-    state for the scene graph to pick it up.
-
-    \note If tracking the changes in the OpenGL context's state is not
-    feasible, one can use the function \l
-    QQuickWindow::resetOpenGLState() which will reset all state that
-    the scene graph relies on.
+    We use the shader program to draw the squircle in \c paint().
 
     \snippet scenegraph/openglunderqml/main.cpp 1
 
diff --git a/examples/quick/scenegraph/openglunderqml/squircle.cpp b/examples/quick/scenegraph/openglunderqml/squircle.cpp
index 4e5848be2a..828857fe24 100644
--- a/examples/quick/scenegraph/openglunderqml/squircle.cpp
+++ b/examples/quick/scenegraph/openglunderqml/squircle.cpp
@@ -83,10 +83,9 @@ void Squircle::handleWindowChanged(QQuickWindow *win)
         connect(win, &QQuickWindow::beforeSynchronizing, this, &Squircle::sync, Qt::DirectConnection);
         connect(win, &QQuickWindow::sceneGraphInvalidated, this, &Squircle::cleanup, Qt::DirectConnection);
 //! [1]
-        // If we allow QML to do the clearing, they would clear what we paint
-        // and nothing would show.
 //! [3]
-        win->setClearBeforeRendering(false);
+        // Ensure we start with cleared to black. The squircle's blend mode relies on this.
+        win->setColor(Qt::black);
     }
 }
 //! [3]
@@ -124,7 +123,8 @@ void Squircle::sync()
 {
     if (!m_renderer) {
         m_renderer = new SquircleRenderer();
-        connect(window(), &QQuickWindow::beforeRendering, m_renderer, &SquircleRenderer::paint, Qt::DirectConnection);
+        connect(window(), &QQuickWindow::beforeRendering, m_renderer, &SquircleRenderer::init, Qt::DirectConnection);
+        connect(window(), &QQuickWindow::beforeRenderPassRecording, m_renderer, &SquircleRenderer::paint, Qt::DirectConnection);
     }
     m_renderer->setViewportSize(window()->size() * window()->devicePixelRatio());
     m_renderer->setT(m_t);
@@ -133,9 +133,12 @@ void Squircle::sync()
 //! [9]
 
 //! [4]
-void SquircleRenderer::paint()
+void SquircleRenderer::init()
 {
     if (!m_program) {
+        QSGRendererInterface *rif = m_window->rendererInterface();
+        Q_ASSERT(rif->graphicsApi() == QSGRendererInterface::OpenGL || rif->graphicsApi() == QSGRendererInterface::OpenGLRhi);
+
         initializeOpenGLFunctions();
 
         m_program = new QOpenGLShaderProgram();
@@ -160,7 +163,15 @@ void SquircleRenderer::paint()
         m_program->link();
 
     }
+}
+
 //! [4] //! [5]
+void SquircleRenderer::paint()
+{
+    // Play nice with the RHI. Not strictly needed when the scenegraph uses
+    // OpenGL directly.
+    m_window->beginExternalCommands();
+
     m_program->bind();
 
     m_program->enableAttributeArray(0);
@@ -171,6 +182,11 @@ void SquircleRenderer::paint()
         -1, 1,
         1, 1
     };
+
+    // This example relies on (deprecated) client-side pointers for the vertex
+    // input. Therefore, we have to make sure no vertex buffer is bound.
+    glBindBuffer(GL_ARRAY_BUFFER, 0);
+
     m_program->setAttributeArray(0, GL_FLOAT, values, 2);
     m_program->setUniformValue("t", (float) m_t);
 
@@ -178,9 +194,6 @@ void SquircleRenderer::paint()
 
     glDisable(GL_DEPTH_TEST);
 
-    glClearColor(0, 0, 0, 1);
-    glClear(GL_COLOR_BUFFER_BIT);
-
     glEnable(GL_BLEND);
     glBlendFunc(GL_SRC_ALPHA, GL_ONE);
 
@@ -192,5 +205,7 @@ void SquircleRenderer::paint()
     // Not strictly needed for this example, but generally useful for when
     // mixing with raw OpenGL.
     m_window->resetOpenGLState();
+
+    m_window->endExternalCommands();
 }
 //! [5]
diff --git a/examples/quick/scenegraph/openglunderqml/squircle.h b/examples/quick/scenegraph/openglunderqml/squircle.h
index 962ba21101..1b9995bc1e 100644
--- a/examples/quick/scenegraph/openglunderqml/squircle.h
+++ b/examples/quick/scenegraph/openglunderqml/squircle.h
@@ -70,6 +70,7 @@ public:
     void setWindow(QQuickWindow *window) { m_window = window; }
 
 public slots:
+    void init();
     void paint();
 
 private: