Paint Devices and Backends

The QWidget class is the base class of all user interface objects. The widget is the atom of the user interface: it receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen.

The QPixmap class is an off-screen image representation which is designed and optimized for showing images on screen. Unlike QImage, the pixel data in a pixmap is internal and is managed by the underlying window system, i.e. pixels can only be accessed through QPainter functions or by converting the QPixmap to a QImage.

To optimize drawing with QPixmap, Qt provides the QPixmapCache class which can be used to store temporary pixmaps that are expensive to generate without using more storage space than the cache limit.

Qt also provides the QBitmap convenience class, inheriting QPixmap. QBitmap guarantees monochrome (1-bit depth) pixmaps, and is mainly used for creating custom QCursor and QBrush objects, constructing QRegion objects, and for setting masks for pixmaps and widgets.

The QtOpenGL module also provides the QGLPixelBuffer class which inherits QPaintDevice directly.

QGLPixelBuffer encapsulates an OpenGL pbuffer. Rendering into a pbuffer is normally done using full hardware acceleration which can be significantly faster than rendering into a QPixmap.

The QPicture class is a paint device that records and replays QPainter commands. A picture serializes painter commands to an IO device in a platform-independent format. QPicture is also resolution independent, i.e. a QPicture can be displayed on different devices (for example svg, pdf, ps, printer and screen) looking the same.

Qt provides the QPicture::load() and QPicture::save() functions as well as streaming operators for loading and saving pictures.

Support for a new backend can be implemented by deriving from the QPaintDevice class and reimplementing the virtual QPaintDevice::paintEngine() function to tell QPainter which paint engine should be used to draw on this particular device. To actually be able to draw on the device, this paint engine must be a custom paint engine created by deriving from the QPaintEngine class.