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/****************************************************************************
**
** Copyright (C) 2009 Nokia Corporation and/or its subsidiary(-ies).
** All rights reserved.
** Contact: Nokia Corporation (qt-info@nokia.com)
**
** This file is part of the documentation of the Qt Toolkit.
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** $QT_BEGIN_LICENSE:LGPL$
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** GNU General Public License Usage
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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.
*/
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