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+
+/*!
+ \example opengl/2dpainting
+ \title 2D Painting Example
+
+ The 2D Painting example shows how QPainter and QGLWidget can be used
+ together to display accelerated 2D graphics on supported hardware.
+
+ \image 2dpainting-example.png
+
+ The QPainter class is used to draw 2D graphics primitives onto
+ paint devices provided by QPaintDevice subclasses, such as QWidget
+ and QImage.
+
+ Since QGLWidget is a subclass of QWidget, it is possible
+ to reimplement its \l{QWidget::paintEvent()}{paintEvent()} and use
+ QPainter to draw on the device, just as you would with a QWidget.
+ The only difference is that the painting operations will be accelerated
+ in hardware if it is supported by your system's OpenGL drivers.
+
+ In this example, we perform the same painting operations on a
+ QWidget and a QGLWidget. The QWidget is shown with anti-aliasing
+ enabled, and the QGLWidget will also use anti-aliasing if the
+ required extensions are supported by your system's OpenGL driver.
+
+ \section1 Overview
+
+ To be able to compare the results of painting onto a QGLWidget subclass
+ with native drawing in a QWidget subclass, we want to show both kinds
+ of widget side by side. To do this, we derive subclasses of QWidget and
+ QGLWidget, using a separate \c Helper class to perform the same painting
+ operations for each, and lay them out in a top-level widget, itself
+ provided a the \c Window class.
+
+ \section1 Helper Class Definition
+
+ In this example, the painting operations are performed by a helper class.
+ We do this because we want the same painting operations to be performed
+ for both our QWidget subclass and the QGLWidget subclass.
+
+ The \c Helper class is minimal:
+
+ \snippet examples/opengl/2dpainting/helper.h 0
+
+ Apart from the constructor, it only provides a \c paint() function to paint
+ using a painter supplied by one of our widget subclasses.
+
+ \section1 Helper Class Implementation
+
+ The constructor of the class sets up the resources it needs to paint
+ content onto a widget:
+
+ \snippet examples/opengl/2dpainting/helper.cpp 0
+
+ The actual painting is performed in the \c paint() function. This takes
+ a QPainter that has already been set up to paint onto a paint device
+ (either a QWidget or a QGLWidget), a QPaintEvent that provides information
+ about the region to be painted, and a measure of the elapsed time (in
+ milliseconds) since the paint device was last updated.
+
+ \snippet examples/opengl/2dpainting/helper.cpp 1
+
+ We begin painting by filling in the region contained in the paint event
+ before translating the origin of the coordinate system so that the rest
+ of the painting operations will be displaced towards the center of the
+ paint device.
+
+ We draw a spiral pattern of circles, using the elapsed time specified to
+ animate them so that they appear to move outward and around the coordinate
+ system's origin:
+
+ \snippet examples/opengl/2dpainting/helper.cpp 2
+
+ Since the coordinate system is rotated many times during
+ this process, we \l{QPainter::save()}{save()} the QPainter's state
+ beforehand and \l{QPainter::restore()}{restore()} it afterwards.
+
+ \snippet examples/opengl/2dpainting/helper.cpp 3
+
+ We draw some text at the origin to complete the effect.
+
+ \section1 Widget Class Definition
+
+ The \c Widget class provides a basic custom widget that we use to
+ display the simple animation painted by the \c Helper class.
+
+ \snippet examples/opengl/2dpainting/widget.h 0
+
+ Apart from the constructor, it only contains a
+ \l{QWidget::paintEvent()}{paintEvent()} function, that lets us draw
+ customized content, and a slot that is used to animate its contents.
+ One member variable keeps track of the \c Helper that the widget uses
+ to paint its contents, and the other records the elapsed time since
+ it was last updated.
+
+ \section1 Widget Class Implementation
+
+ The constructor only initializes the member variables, storing the
+ \c Helper object supplied and calling the base class's constructor,
+ and enforces a fixed size for the widget:
+
+ \snippet examples/opengl/2dpainting/widget.cpp 0
+
+ The \c animate() slot is called whenever a timer, which we define later, times
+ out:
+
+ \snippet examples/opengl/2dpainting/widget.cpp 1
+
+ Here, we determine the interval that has elapsed since the timer last
+ timed out, and we add it to any existing value before repainting the
+ widget. Since the animation used in the \c Helper class loops every second,
+ we can use the modulo operator to ensure that the \c elapsed variable is
+ always less than 1000.
+
+ Since the \c Helper class does all of the actual painting, we only have
+ to implement a paint event that sets up a QPainter for the widget and calls
+ the helper's \c paint() function:
+
+ \snippet examples/opengl/2dpainting/widget.cpp 2
+
+ \section1 GLWidget Class Definition
+
+ The \c GLWidget class definition is basically the same as the \c Widget
+ class except that it is derived from QGLWidget.
+
+ \snippet examples/opengl/2dpainting/glwidget.h 0
+
+ Again, the member variables record the \c Helper used to paint the
+ widget and the elapsed time since the previous update.
+
+ \section1 GLWidget Class Implementation
+
+ The constructor differs a little from the \c Widget class's constructor:
+
+ \snippet examples/opengl/2dpainting/glwidget.cpp 0
+
+ As well as initializing the \c elapsed member variable and storing the
+ \c Helper object used to paint the widget, the base class's constructor
+ is called with the format that specifies the \l QGL::SampleBuffers flag.
+ This enables anti-aliasing if it is supported by your system's OpenGL
+ driver.
+
+ The \c animate() slot is exactly the same as that provided by the \c Widget
+ class:
+
+ \snippet examples/opengl/2dpainting/glwidget.cpp 1
+
+ The \c paintEvent() is almost the same as that found in the \c Widget class:
+
+ \snippet examples/opengl/2dpainting/glwidget.cpp 2
+
+ Since anti-aliasing will be enabled if available, we only need to set up
+ a QPainter on the widget and call the helper's \c paint() function to display
+ the widget's contents.
+
+ \section1 Window Class Definition
+
+ The \c Window class has a basic, minimal definition:
+
+ \snippet examples/opengl/2dpainting/window.h 0
+
+ It contains a single \c Helper object that will be shared between all
+ widgets.
+
+ \section1 Window Class Implementation
+
+ The constructor does all the work, creating a widget of each type and
+ inserting them with labels into a layout:
+
+ \snippet examples/opengl/2dpainting/window.cpp 0
+
+ A timer with a 50 millisecond time out is constructed for animation purposes,
+ and connected to the \c animate() slots of the \c Widget and \c GLWidget objects.
+ Once started, the widgets should be updated at around 20 frames per second.
+
+ \section1 Running the Example
+
+ The example shows the same painting operations performed at the same time
+ in a \c Widget and a \c GLWidget. The quality and speed of rendering in the
+ \c GLWidget depends on the level of support for multisampling and hardware
+ acceleration that your system's OpenGL driver provides. If support for either
+ of these is lacking, the driver may fall back on a software renderer that
+ may trade quality for speed.
+*/