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Usually, QGLWidget is subclassed to display a pure 3D scene; the developer reimplements \l{QGLWidget::initializeGL()}{initializeGL()} to initialize any required resources, \l{QGLWidget::resizeGL()}{resizeGL()} to set up the projection and viewport, and \l{QGLWidget::paintGL()}{paintGL()} to perform the OpenGL calls needed to render the scene. However, it is possible to subclass QGLWidget differently to allow 2D graphics, drawn using QPainter, to be painted over a scene rendered using OpenGL. In this example, we demonstrate how this is done by reusing the code from the \l{Hello GL Example}{Hello GL} example to provide a 3D scene, and painting over it with some translucent 2D graphics. Instead of examining each class in detail, we only cover the parts of the \c GLWidget class that enable overpainting, and provide more detailed discussion in the final section of this document. \section1 GLWidget Class Definition The \c GLWidget class is a subclass of QGLWidget, based on the one used in the \l{Hello GL Example}{Hello GL} example. Rather than describe the class as a whole, we show the first few lines of the class and only discuss the changes we have made to the rest of it: \snippet examples/opengl/overpainting/glwidget.h 0 \dots \snippet examples/opengl/overpainting/glwidget.h 1 \dots \snippet examples/opengl/overpainting/glwidget.h 4 As usual, the widget uses \l{QGLWidget::initializeGL()}{initializeGL()} to set up objects for our scene and perform other OpenGL initialization tasks. The \l{QGLWidget::resizeGL()}{resizeGL()} function is used to ensure that the 3D graphics in the scene are transformed correctly to the 2D viewport displayed in the widget. Instead of implementing \l{QGLWidget::paintGL()}{paintGL()} to handle updates to the widget, we implement a normal QWidget::paintEvent(). This allows us to mix OpenGL calls and QPainter operations in a controlled way. In this example, we also implement QWidget::showEvent() to help with the initialization of the 2D graphics used. The new private member functions and variables relate exclusively to the 2D graphics and animation. The \c animate() slot is called periodically by the \c animationTimer to update the widget; the \c createBubbles() function initializes the \c bubbles list with instances of a helper class used to draw the animation; the \c drawInstructions() function is responsible for a semi-transparent messages that is also overpainted onto the OpenGL scene. \section1 GLWidget Class Implementation Again, we only show the parts of the \c GLWidget implementation that are relevant to this example. In the constructor, we initialize a QTimer to control the animation: \snippet examples/opengl/overpainting/glwidget.cpp 0 We turn off the widget's \l{QWidget::autoFillBackground}{autoFillBackground} property to instruct OpenGL not to paint a background for the widget when \l{QPainter::begin()}{QPainter::begin()} is called. As in the \l{Hello GL Example}{Hello GL} example, the destructor is responsible for freeing any OpenGL-related resources: \snippet examples/opengl/overpainting/glwidget.cpp 1 The \c initializeGL() function is fairly minimal, only setting up the display list used in the scene. \snippet examples/opengl/overpainting/glwidget.cpp 2 To cooperate fully with QPainter, we defer matrix stack operations and attribute initialization until the widget needs to be updated. In this example, we implement \l{QWidget::paintEvent()}{paintEvent()} rather than \l{QGLWidget::paintGL()}{paintGL()} to render our scene. When drawing on a QGLWidget, the paint engine used by QPainter performs certain operations that change the states of the OpenGL implementation's matrix and property stacks. Therefore, it is necessary to make all the OpenGL calls to display the 3D graphics before we construct a QPainter to draw the 2D overlay. We render a 3D scene by setting up model and projection transformations and other attributes. We use an OpenGL stack operation to preserve the original matrix state, allowing us to recover it later: \snippet examples/opengl/overpainting/glwidget.cpp 4 We define a color to use for the widget's background, and set up various attributes that define how the scene will be rendered. \snippet examples/opengl/overpainting/glwidget.cpp 6 We call the \c setupViewport() private function to set up the projection used for the scene. This is unnecessary in OpenGL examples that implement the \l{QGLWidget::paintGL()}{paintGL()} function because the matrix stacks are usually unmodified between calls to \l{QGLWidget::resizeGL()}{resizeGL()} and \l{QGLWidget::paintGL()}{paintGL()}. Since the widget's background is not drawn by the system or by Qt, we use an OpenGL call to paint it before positioning the object defined earlier in the scene: \snippet examples/opengl/overpainting/glwidget.cpp 7 Once the list containing the object has been executed, the GL states we changed and the matrix stack needs to be restored to its original state at the start of this function before we can begin overpainting: \snippet examples/opengl/overpainting/glwidget.cpp 8 With the 3D graphics done, we construct a QPainter for use on the widget and simply overpaint the widget with 2D graphics; in this case, using a helper class to draw a number of translucent bubbles onto the widget, and calling \c drawInstructions() to overlay some instructions: \snippet examples/opengl/overpainting/glwidget.cpp 10 When QPainter::end() is called, suitable OpenGL-specific calls are made to write the scene, and its additional contents, onto the widget. The implementation of the \l{QGLWidget::resizeGL()}{resizeGL()} function sets up the dimensions of the viewport and defines a projection transformation: \snippet examples/opengl/overpainting/glwidget.cpp 11 Ideally, we want to arrange the 2D graphics to suit the widget's dimensions. To achieve this, we implement the \l{QWidget::showEvent()}{showEvent()} handler, creating new graphic elements (bubbles) if necessary at appropriate positions in the widget. \snippet examples/opengl/overpainting/glwidget.cpp 12 This function only has an effect if less than 20 bubbles have already been created. The \c animate() slot is called every time the widget's \c animationTimer emits the \l{QTimer::timeout()}{timeout()} signal. This keeps the bubbles moving around. \snippet examples/opengl/overpainting/glwidget.cpp 13 We simply iterate over the bubbles in the \c bubbles list, updating the widget before and after each of them is moved. The \c setupViewport() function is called from \c paintEvent() and \c resizeGL(). \snippet examples/opengl/overpainting/glwidget.cpp 14 The \c drawInstructions() function is used to prepare some basic instructions that will be painted with the other 2D graphics over the 3D scene. \snippet examples/opengl/overpainting/glwidget.cpp 15 \section1 Summary When overpainting 2D content onto 3D content, we need to use a QPainter \e and make OpenGL calls to achieve the desired effect. Since QPainter itself uses OpenGL calls when used on a QGLWidget subclass, we need to preserve the state of various OpenGL stacks when we perform our own calls, using the following approach: \list \o Reimplement QGLWidget::initializeGL(), but only perform minimal initialization. QPainter will perform its own initialization routines, modifying the matrix and property stacks, so it is better to defer certain initialization tasks until just before you render the 3D scene. \o Reimplement QGLWidget::resizeGL() as in the pure 3D case. \o Reimplement QWidget::paintEvent() to draw both 2D and 3D graphics. \endlist The \l{QWidget::paintEvent()}{paintEvent()} implementation performs the following tasks: \list \o Push the current OpenGL modelview matrix onto a stack. \o Perform initialization tasks usually done in the \l{QGLWidget::initializeGL()}{initializeGL()} function. \o Perform code that would normally be located in the widget's \l{QGLWidget::resizeGL()}{resizeGL()} function to set the correct perspective transformation and set up the viewport. \o Render the scene using OpenGL calls. \o Pop the OpenGL modelview matrix off the stack. \o Construct a QPainter object. \o Initialize it for use on the widget with the QPainter::begin() function. \o Draw primitives using QPainter's member functions. \o Call QPainter::end() to finish painting. \endlist */