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/*!
    \example opengl/hellogl
    \title Hello GL Example

    The Hello GL example demonstrates the basic use of the OpenGL-related classes
    provided with Qt.

    \image hellogl-example.png

    Qt provides the QGLWidget class to enable OpenGL graphics to be rendered within
    a standard application user interface. By subclassing this class, and providing
    reimplementations of event handler functions, 3D scenes can be displayed on
    widgets that can be placed in layouts, connected to other objects using signals
    and slots, and manipulated like any other widget.

    \tableofcontents

    \section1 GLWidget Class Definition

    The \c GLWidget class contains some standard public definitions for the
    constructor, destructor, \l{QWidget::sizeHint()}{sizeHint()}, and
    \l{QWidget::minimumSizeHint()}{minimumSizeHint()} functions:

    \snippet examples/opengl/hellogl/glwidget.h 0

    We use a destructor to ensure that any OpenGL-specific data structures
    are deleted when the widget is no longer needed.

    \snippet examples/opengl/hellogl/glwidget.h 1

    The signals and slots are used to allow other objects to interact with the
    3D scene.

    \snippet examples/opengl/hellogl/glwidget.h 2

    OpenGL initialization, viewport resizing, and painting are handled by
    reimplementing the QGLWidget::initializeGL(), QGLWidget::resizeGL(), and
    QGLWidget::paintGL() handler functions. To enable the user to interact
    directly with the scene using the mouse, we reimplement
    QWidget::mousePressEvent() and QWidget::mouseMoveEvent().

    \snippet examples/opengl/hellogl/glwidget.h 3

    The rest of the class contains utility functions and variables that are
    used to construct and hold orientation information for the scene. The
    \c object variable will be used to hold an identifier for an OpenGL
    display list.

    \section1 GLWidget Class Implementation

    In this example, we split the class into groups of functions and describe
    them separately. This helps to illustrate the differences between subclasses
    of native widgets (such as QWidget and QFrame) and QGLWidget subclasses.

    \section2 Widget Construction and Sizing

    The constructor provides default rotation angles for the scene, initializes
    the variable used for the display list, and sets up some colors for later use.

    \snippet examples/opengl/hellogl/glwidget.cpp 0

    We also implement a destructor to release OpenGL-related resources when the
    widget is deleted:

    \snippet examples/opengl/hellogl/glwidget.cpp 1

    The destructor ensures that the display list is deleted properly.

    We provide size hint functions to ensure that the widget is shown at a
    reasonable size:

    \snippet examples/opengl/hellogl/glwidget.cpp 2
    \codeline
    \snippet examples/opengl/hellogl/glwidget.cpp 3
    \snippet examples/opengl/hellogl/glwidget.cpp 4

    The widget provides three slots that enable other components in the
    example to change the orientation of the scene:

    \snippet examples/opengl/hellogl/glwidget.cpp 5

    In the above slot, the \c xRot variable is updated only if the new angle
    is different to the old one, the \c xRotationChanged() signal is emitted to
    allow other components to be updated, and the widget's
    \l{QGLWidget::updateGL()}{updateGL()} handler function is called.

    The \c setYRotation() and \c setZRotation() slots perform the same task for
    rotations measured by the \c yRot and \c zRot variables.

    \section2 OpenGL Initialization

    The \l{QGLWidget::initializeGL()}{initializeGL()} function is used to
    perform useful initialization tasks that are needed to render the 3D scene.
    These often involve defining colors and materials, enabling and disabling
    certain rendering flags, and setting other properties used to customize the
    rendering process.

    \snippet examples/opengl/hellogl/glwidget.cpp 6

    In this example, we reimplement the function to set the background color,
    create a display list containing information about the object we want to
    display, and set up the rendering process to use a particular shading model
    and rendering flags:

    \section2 Resizing the Viewport

    The \l{QGLWidget::resizeGL()}{resizeGL()} function is used to ensure that
    the OpenGL implementation renders the scene onto a viewport that matches the
    size of the widget, using the correct transformation from 3D coordinates to
    2D viewport coordinates.

    The function is called whenever the widget's dimensions change, and is
    supplied with the new width and height. Here, we define a square viewport
    based on the length of the smallest side of the widget to ensure that
    the scene is not distorted if the widget has sides of unequal length:

    \snippet examples/opengl/hellogl/glwidget.cpp 8

    A discussion of the projection transformation used is outside the scope of
    this example. Please consult the OpenGL reference documentation for an
    explanation of projection matrices.

    \section2 Painting the Scene

    The \l{QGLWidget::paintGL()}{paintGL()} function is used to paint the
    contents of the scene onto the widget. For widgets that only need to be
    decorated with pure OpenGL content, we reimplement QGLWidget::paintGL()
    \e instead of reimplementing QWidget::paintEvent():

    \snippet examples/opengl/hellogl/glwidget.cpp 7

    In this example, we clear the widget using the background color that
    we defined in the \l{QGLWidget::initializeGL()}{initializeGL()} function,
    set up the frame of reference for the object we want to display, and call
    the display list containing the rendering commands for the object.

    \section2 Mouse Handling

    Just as in subclasses of native widgets, mouse events are handled by
    reimplementing functions such as QWidget::mousePressEvent() and
    QWidget::mouseMoveEvent().

    The \l{QWidget::mousePressEvent()}{mousePressEvent()} function simply
    records the position of the mouse when a button is initially pressed:

    \snippet examples/opengl/hellogl/glwidget.cpp 9

    The \l{QWidget::mouseMoveEvent()}{mouseMoveEvent()} function uses the
    previous location of the mouse cursor to determine how much the object
    in the scene should be rotated, and in which direction:

    \snippet examples/opengl/hellogl/glwidget.cpp 10

    Since the user is expected to hold down the mouse button and drag the
    cursor to rotate the object, the cursor's position is updated every time
    a move event is received.

    \section2 Utility Functions

    We have omitted the utility functions, \c makeObject(), \c quad(),
    \c extrude(), and \c normalizeAngle() from our discussion. These can be
    viewed in the quoted source for \c glwidget.cpp via the link at the
    start of this document.

    \section1 Window Class Definition

    The \c Window class is used as a container for the \c GLWidget used to
    display the scene:

    \snippet examples/opengl/hellogl/window.h 0

    In addition, it contains sliders that are used to change the orientation
    of the object in the scene.

    \section1 Window Class Implementation

    The constructor constructs an instance of the \c GLWidget class and some
    sliders to manipulate its contents.

    \snippet examples/opengl/hellogl/window.cpp 0

    We connect the \l{QAbstractSlider::valueChanged()}{valueChanged()} signal
    from each of the sliders to the appropriate slots in \c{glWidget}.
    This allows the user to change the orientation of the object by dragging
    the sliders.

    We also connect the \c xRotationChanged(), \c yRotationChanged(), and
    \c zRotationChanged() signals from \c glWidget to the
    \l{QAbstractSlider::setValue()}{setValue()} slots in the
    corresponding sliders.

    \snippet examples/opengl/hellogl/window.cpp 1

    The sliders are placed horizontally in a layout alongside the \c GLWidget,
    and initialized with suitable default values.

    The \c createSlider() utility function constructs a QSlider, and ensures
    that it is set up with a suitable range, step value, tick interval, and
    page step value before returning it to the calling function:

    \snippet examples/opengl/hellogl/window.cpp 2

    \section1 Summary

    The \c GLWidget class implementation shows how to subclass QGLWidget for
    the purposes of rendering a 3D scene using OpenGL calls. Since QGLWidget
    is a subclass of QWidget, subclasses of QGLWidget can be placed in layouts
    and provided with interactive features just like normal custom widgets.

    We ensure that the widget is able to correctly render the scene using OpenGL
    by reimplementing the following functions:

    \list
    \o QGLWidget::initializeGL() sets up resources needed by the OpenGL implementation
       to render the scene.
    \o QGLWidget::resizeGL() resizes the viewport so that the rendered scene fits onto
       the widget, and sets up a projection matrix to map 3D coordinates to 2D viewport
       coordinates.
    \o QGLWidget::paintGL() performs painting operations using OpenGL calls.
    \endlist

    Since QGLWidget is a subclass of QWidget, it can also be used
    as a normal paint device, allowing 2D graphics to be drawn with QPainter.
    This use of QGLWidget is discussed in the \l{2D Painting Example}{2D Painting}
    example.

    More advanced users may want to paint over parts of a scene rendered using
    OpenGL. QGLWidget allows pure OpenGL rendering to be mixed with QPainter
    calls, but care must be taken to maintain the state of the OpenGL implementation.
    See the \l{Overpainting Example}{Overpainting} example for more information.
*/
