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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
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** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\example scenegraph/openglunderqml
\title Scene Graph - OpenGL Under QML
\ingroup qtquickexamples
\brief Shows how to render OpenGL under a Qt Quick scene.
\image openglunderqml-example.jpg
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
QQuickWindow::afterRendering() signal.
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 scenegraph/openglunderqml/squircle.h 2
First of all, we need an object we can expose to QML. This is a
subclass of QQuickItem so we can easily access \l QQuickItem::window().
\snippet scenegraph/openglunderqml/squircle.h 1
Then we need an object to take care of the rendering. This
instance needs to be separated from the QQuickItem because the
item lives in the GUI thread and the rendering potentially happens
on the render thread. Since we want to connect to \l
QQuickWindow::beforeRendering(), we make the renderer a QObject.
The renderer contains a copy of all the state it needs,
independent of the GUI thread.
\note Don't be tempted to merge the two objects into
one. QQuickItems may be deleted on the GUI thread while the render
thread is rendering.
Lets move on to the implementation.
\snippet scenegraph/openglunderqml/squircle.cpp 7
The constructor of the \c Squircle class simply initializes the
values and connects to the window changed signal which we will use
to prepare our renderer.
\snippet scenegraph/openglunderqml/squircle.cpp 1
Once we have a window, we attach to the \l
QQuickWindow::beforeSynchronizing() signal which we will use to
create the renderer and to copy state into it safely. We also
connect to the \l QQuickWindow::sceneGraphInvalidated() signal to
handle the cleanup of the renderer.
\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
present.
\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.
\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.
\note The \l QQuickWindow::beforeSynchronizing() 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 scenegraph/openglunderqml/squircle.cpp 6
In the \c cleanup() function we delete the renderer which in turn
cleans up its own resources.
\snippet scenegraph/openglunderqml/squircle.cpp 8
When the value of \c t changes, we call \l QQuickWindow::update()
rather than \l QQuickItem::update() because the former will force
the entire window to be redrawn, even when the scene graph has not
changed since the last frame.
\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.
\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.
\snippet 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
qmlRegisterType() macro.
\snippet 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 its \c t property.
\snippet 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
scene.
*/
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