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\title Scene Graph - OpenGL Under QML
\brief Shows how to render OpenGL under a Qt Quick scene.
The OpenGL under QML example shows how an application can make use
of the \l QQuickWindow::beforeRendering() signal to draw custom
OpenGL content under a Qt Quick scene. This signal is emitted at
the start of every frame, before the scene graph starts its
rendering, thus any OpenGL draw calls that are made as a response
to this signal, will stack under the Qt Quick items.
As an alternative, applications that wish to render OpenGL content
on top of the Qt Quick scene, can do so by connecting to the \l
In this example, we will also see how it is possible to have
values that are exposed to QML which affect the OpenGL
rendering. We animate the threshold value using a NumberAnimation
in the QML file and this value is used by the OpenGL shader
program that draws the squircles.
\snippet quick/scenegraph/openglunderqml/squircle.h 1
First of all, we need a QObject with a slot to connect the signals
to. We subclass QQuickItem in order to use the \l
QQuickItem::window() which holds the window instance we want to
We use two values of \c t. The variable \c m_t is the property
value as it exists in the GUI thread. The \c m_thread_t value is a
copy of \c m_t for use in the rendering thread. We need an
explicit copy because the scene graph can render in one thread
while updating properties on the GUI thread in preparation for the
next frame. If we had used only one value, the animation could
have updated the value to that of the next frame before we got a
chance to render it.
\note In this example, a wrong value for \c t will have minimal
consequences, but we emphasize that rendering and GUI thread
objects and values must stay separate to avoid race conditions,
undesired behavior and in the worst case, crashes.
Lets move on to the implementation.
\snippet quick/scenegraph/openglunderqml/squircle.cpp 7
The constructor of the \c Squircle class simply initializes the
values. The shader program will be initialized during rendering
\snippet quick/scenegraph/openglunderqml/squircle.cpp 8
The property setter checks that the value has indeed changed
before updating its internal variable. It then calls \l
QQuickWindow::update() which will trigger another frame to be
rendered. Note that the setter might be called during
initialization, before the object has been entered into the scene
and before it has a window.
\snippet quick/scenegraph/openglunderqml/squircle.cpp 1
\snippet quick/scenegraph/openglunderqml/squircle.cpp 2
For our paint function to be called, we need to connect to the
window's signals. When Squircle object is populated into the
scene, the itemChange function is called with the change type \c
ItemSceneChange. We connect \l QQuickWindow::beforeRendering() to
\c paint() to do the rendering, and \l
QQuickWindow::beforeSynchronizing() to \c sync() to copy the state
of the \c t property for the upcoming frame.
\note Since the Squircle object has affinity to the GUI thread and
the signals are emitted from the rendering thread, it is crucial
that the connections are made with \l
Qt::DirectConnection. Failing to do so, will result in that the
slots are invoked on the wrong thread with no OpenGL context
\snippet quick/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.
\snippet quick/scenegraph/openglunderqml/squircle.cpp 4
The first thing we do in the \c paint() function is to
initialize 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.
We also connect to the QOpenGLContext::aboutToBeDestroyed()
signal, so that we can clean up the shader program when the
context is destroyed. Again, this is a \l Qt::DirectConnection as
all rendering related operations must happen on the rendering
\snippet quick/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.
\snippet quick/scenegraph/openglunderqml/squircle.cpp 6
In the \c cleanup() function we delete the program.
\snippet quick/scenegraph/openglunderqml/squircle.cpp 9
We use the \c sync() function to copy the state of the
object in the GUI thread into the rendering thread.
The signal is emitted on the rendering thread while the GUI
thread is blocked, so it is safe to simply copy the value without
any additional protection.
\snippet quick/scenegraph/openglunderqml/main.cpp 1
The application's \c main() function instantiates a QQuickView and
launches the \c main.qml file. The only thing worth noting is that
we export the \c Squircle class to QML using the \l
\snippet quick/scenegraph/openglunderqml/main.qml 1
We import the Squircle QML type with the name we registered in the
\c main() function. We then instantiate it and create a running
NumberAnimation on the its \c t property.
\snippet quick/scenegraph/openglunderqml/main.qml 2
Then we overlay a short descriptive text, so that it is clearly
visible that we are in fact rendering OpenGL under our Qt Quick