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