Paint Devices and Backends¶
Creating a Paint Device¶
QPaintDevice class is the base class of objects that can be painted, i.e.
QPainter can draw on any
QPaintDevice ‘s drawing capabilities are among others implemented by
QPrinter , and
QWidgetclass is the base class of user interface elements in the Qt Widgets module. It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen.
QImageclass provides a hardware-independent image representation which is designed and optimized for I/O, and for direct pixel access and manipulation.
QImagesupports several image formats including monochrome, 8-bit, 32-bit and alpha-blended images.
One advantage of using
QImageas a paint device is that it is possible to guarantee the pixel exactness of any drawing operation in a platform-independent way. Another benefit is that the painting can be performed in another thread than the current GUI thread.
QPixmapclass 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
QPainterfunctions or by converting the
To optimize drawing with
QPixmap, Qt provides the
QPixmapCacheclass 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
QBitmapconvenience class, inheriting
QBitmapguarantees monochrome (1-bit depth) pixmaps, and is mainly used for creating custom
OpenGL Paint Device
As mentioned previously, Qt is offering classes that makes it easy to use OpenGL in Qt applications. For example, the
QOpenGLPaintDeviceenables the OpenGL API for rendering with
QPictureclass is a paint device that records and replays
QPaintercommands. A picture serializes painter commands to an IO device in a platform-independent format.
QPictureis also resolution independent, i.e. a
QPicturecan be displayed on different devices (for example svg, pdf, ps, printer and screen) looking the same.
Qt provides the
save()functions as well as streaming operators for loading and saving pictures.
Support for a new backend can be implemented by deriving from the
QPaintDeviceclass and reimplementing the virtual
paintEngine()function to tell
QPainterwhich 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
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