Paint Devices and Backends¶
Creating a Paint Device¶
The
QPaintDevice
class is the base class of objects that can be painted, i.e.QPainter
can draw on anyQPaintDevice
subclass.QPaintDevice
‘s drawing capabilities are among others implemented byQWidget
,QImage
,QPixmap
,QPicture
,QPrinter
, andQOpenGLPaintDevice
.
Widget
The
QWidget
class 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.Image
The
QImage
class provides a hardware-independent image representation which is designed and optimized for I/O, and for direct pixel access and manipulation.QImage
supports several image formats including monochrome, 8-bit, 32-bit and alpha-blended images.One advantage of using
QImage
as 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.Pixmap
The
QPixmap
class is an off-screen image representation which is designed and optimized for showing images on screen. UnlikeQImage
, the pixel data in a pixmap is internal and is managed by the underlying window system, i.e. pixels can only be accessed throughQPainter
functions or by converting theQPixmap
to aQImage
.To optimize drawing with
QPixmap
, Qt provides theQPixmapCache
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, inheritingQPixmap
.QBitmap
guarantees monochrome (1-bit depth) pixmaps, and is mainly used for creating customQCursor
andQBrush
objects, constructingQRegion
objects.OpenGL Paint Device
As mentioned previously, Qt is offering classes that makes it easy to use OpenGL in Qt applications. For example, the
QOpenGLPaintDevice
enables the OpenGL API for rendering withQPainter
.Picture
The
QPicture
class is a paint device that records and replaysQPainter
commands. A picture serializes painter commands to an IO device in a platform-independent format.QPicture
is also resolution independent, i.e. aQPicture
can be displayed on different devices (for example svg, pdf, ps, printer and screen) looking the same.Qt provides the
load()
andsave()
functions as well as streaming operators for loading and saving pictures.Custom Backends
Support for a new backend can be implemented by deriving from the
QPaintDevice
class and reimplementing the virtualpaintEngine()
function to tellQPainter
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 theQPaintEngine
class.
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