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/*!
    \example opengl/overpainting
    \title Overpainting Example

    The Overpainting example shows how QPainter can be used
    to overpaint a scene rendered using OpenGL in a QGLWidget.

    \image overpainting-example.png

    QGLWidget provides a widget with integrated OpenGL graphics support
    that enables 3D graphics to be displayed using normal OpenGL calls,
    yet also behaves like any other standard Qt widget with support for
    signals and slots, properties, and Qt's action system.

    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 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
*/
