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<td >Qt 5.12</td><td ><a href="qtgui-index.html">Qt GUI</a></td><td >Paint Devices and Backends</td></tr></table><table class="buildversion"><tr>
<td id="buildversion" width="100%" align="right"><a href="qtgui-index.html">Qt 5.12.6 Reference Documentation</a></td>
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<li class="level1"><a href="#creating-a-paint-device">Creating a Paint Device</a></li>
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<h1 class="title">Paint Devices and Backends</h1>
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<h2 id="creating-a-paint-device">Creating a Paint Device</h2>
<p>The <a href="qpaintdevice.html">QPaintDevice</a> class is the base class of objects that can be painted, i.e. <a href="qpainter.html">QPainter</a> can draw on any <a href="qpaintdevice.html">QPaintDevice</a> subclass. <a href="qpaintdevice.html">QPaintDevice</a>'s drawing capabilities are among others implemented by <a href="../qtwidgets/qwidget.html">QWidget</a>, <a href="qimage.html">QImage</a>, <a href="qpixmap.html">QPixmap</a>, <a href="qpicture.html">QPicture</a>, QPrinter, and <a href="qopenglpaintdevice.html">QOpenGLPaintDevice</a>.</p>
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<tr valign="top" class="odd"><td ><b>Widget</b><p>The <a href="../qtwidgets/qwidget.html">QWidget</a> class is the base class of user interface elements in the <a href="../qtwidgets/qtwidgets-index.html">Qt Widgets</a> module. It receives mouse, keyboard and other events from the window system, and paints a representation of itself on the screen.</p>
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<tr valign="top" class="even"><td ><b>Image</b><p>The <a href="qimage.html">QImage</a> class provides a hardware-independent image representation which is designed and optimized for I/O, and for direct pixel access and manipulation. <a href="qimage.html">QImage</a> supports several image formats including monochrome, 8-bit, 32-bit and alpha-blended images.</p>
<p>One advantage of using <a href="qimage.html">QImage</a> 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.</p>
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<tr valign="top" class="odd"><td ><b>Pixmap</b><p>The <a href="qpixmap.html">QPixmap</a> class is an off-screen image representation which is designed and optimized for showing images on screen. Unlike <a href="qimage.html">QImage</a>, the pixel data in a pixmap is internal and is managed by the underlying window system, i.e. pixels can only be accessed through <a href="qpainter.html">QPainter</a> functions or by converting the <a href="qpixmap.html">QPixmap</a> to a <a href="qimage.html">QImage</a>.</p>
<p>To optimize drawing with <a href="qpixmap.html">QPixmap</a>, Qt provides the <a href="qpixmapcache.html">QPixmapCache</a> class which can be used to store temporary pixmaps that are expensive to generate without using more storage space than the cache limit.</p>
<p>Qt also provides the <a href="qbitmap.html">QBitmap</a> convenience class, inheriting <a href="qpixmap.html">QPixmap</a>. <a href="qbitmap.html">QBitmap</a> guarantees monochrome (1-bit depth) pixmaps, and is mainly used for creating custom <a href="qcursor.html">QCursor</a> and <a href="qbrush.html">QBrush</a> objects, constructing <a href="qregion.html">QRegion</a> objects.</p>
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<tr valign="top" class="even"><td ><b>OpenGL Paint Device</b><p>As mentioned previously, Qt is offering classes that makes it easy to use OpenGL in Qt applications. For example, the <a href="qopenglpaintdevice.html">QOpenGLPaintDevice</a> enables the OpenGL API for rendering with <a href="qpainter.html">QPainter</a>.</p>
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<tr valign="top" class="odd"><td ><b>Picture</b><p>The <a href="qpicture.html">QPicture</a> class is a paint device that records and replays <a href="qpainter.html">QPainter</a> commands. A picture serializes painter commands to an IO device in a platform-independent format. <a href="qpicture.html">QPicture</a> is also resolution independent, i.e. a <a href="qpicture.html">QPicture</a> can be displayed on different devices (for example svg, pdf, ps, printer and screen) looking the same.</p>
<p>Qt provides the <a href="qpicture.html#load">QPicture::load</a>() and <a href="qpicture.html#save">QPicture::save</a>() functions as well as streaming operators for loading and saving pictures.</p>
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<tr valign="top" class="even"><td ><b>Custom Backends</b><p>Support for a new backend can be implemented by deriving from the <a href="qpaintdevice.html">QPaintDevice</a> class and reimplementing the virtual <a href="qpaintdevice.html#paintEngine">QPaintDevice::paintEngine</a>() function to tell <a href="qpainter.html">QPainter</a> 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 <a href="qpaintengine.html">QPaintEngine</a> class.</p>
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