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+
+/*!
+ \page openvg.html
+ \title OpenVG Rendering in Qt
+ \since 4.6
+ \ingroup best-practices
+
+ \brief Efficient rendering on embedded devices with OpenVG
+
+ OpenVG is a standard API from the
+ \l{http://www.khronos.org/openvg}{Khronos Group} for accelerated
+ 2D vector graphics that is appearing in an increasing number of
+ embedded devices. The QtOpenVG plugin provides support for OpenVG
+ painting.
+
+ OpenVG is optimized for 2D vector operations, and closely matches
+ the functionality in QPainter. It can therefore be an excellent
+ substitute for the default raster-based QPaintEngine on hardware
+ that supports OpenVG.
+
+ \tableofcontents
+
+ \section1 Building Qt with OpenVG support
+
+ OpenVG support can be enabled by passing the \c{-openvg} option
+ to configure. It is assumed that the following qmake variables
+ are set to appropriate values in the qmake.conf file for your
+ platform:
+
+ \list
+ \o QMAKE_INCDIR_OPENVG
+ \o QMAKE_LIBDIR_OPENVG
+ \o QMAKE_LIBS_OPENVG
+ \endlist
+
+ Most OpenVG implementations are based on EGL, so the following
+ variables may also need to be set:
+
+ \list
+ \o QMAKE_INCDIR_EGL
+ \o QMAKE_LIBDIR_EGL
+ \o QMAKE_LIBS_EGL
+ \endlist
+
+ See \l{qmake Variable Reference} for more information on these variables.
+
+ Two kinds of OpenVG engines are currently supported: EGL based,
+ and engines built on top of OpenGL such as
+ \l{http://sourceforge.net/projects/shivavg}{ShivaVG}.
+ EGL based engines are preferred.
+
+ It is assumed that the EGL implementation has some way to turn a
+ QWidget::winId() into an EGL rendering surface with
+ \c{eglCreateWindowSurface()}. If this is not the case, then
+ modifications may be needed to the code under \c{src/gui/egl} and
+ \c{src/plugins/graphicssystems/openvg} to accomodate the EGL
+ implementation.
+
+ The ShivaVG graphics system under \c{src/plugins/graphicssystems/shivavg}
+ is an example of how to integrate a non-EGL implementation of
+ OpenVG into Qt. It is currently only supported with Qt/X11
+ and being an example only, the resulting screen output may not
+ be as good as with other OpenVG engines.
+
+ \section1 Using the OpenVG graphics system
+
+ Once the graphics system plugin has been built and installed,
+ applications can be run as follows to use the plugin:
+
+ \code
+ app -graphicssystem OpenVG
+ \endcode
+
+ If ShivaVG is being used, then substitute \c ShivaVG instead of
+ \c OpenVG in the line above.
+
+ If the plugin fails to load, try setting the \c QT_DEBUG_PLUGINS
+ environment variable to 1 and try again. Usually the plugin
+ cannot be loaded because Qt cannot locate it in the directory
+ \c{plugins/graphicssystems} within the Qt installation, or the
+ dynamic library path does not include the directory containing
+ the system's \c libOpenVG.so library.
+
+ \section1 Supported features
+
+ \section2 Context modes
+
+ The default configuration is "single-context" mode, where a single
+ EGLContext object is used for all drawing, regardless of the surface.
+ Multiple EGLSurfaces are created, one for each window surface or pixmap.
+ eglMakeCurrent() is called with the same EGLContext every time, but a
+ different EGLSurface.
+
+ Single-context mode is necessary for QPixmapData to be implemented in
+ terms of a VGImage. If single-context mode is not enabled, then QPixmapData
+ will use the fallback QRasterPixmapData implementation, which is less
+ efficient performance-wise.
+
+ Single-context mode can be disabled with the QVG_NO_SINGLE_CONTEXT define
+ if the OpenVG engine does not support one context with multiple surfaces.
+
+ \section2 Transformation matrices
+
+ All affine and projective transformation matrices are supported.
+
+ QVGPaintEngine will use the engine to accelerate affine transformation
+ matrices only. When a projective transformation matrix is used,
+ QVGPaintEngine will transform the coordinates before passing them
+ to the engine. This will probably incur a performance penalty.
+
+ Pixmaps and images are always transformed by the engine, because
+ OpenVG specifies that projective transformations must work for images.
+
+ It is recommended that client applications should avoid using projective
+ transformations for non-image elements in performance critical code.
+
+ \section2 Composition modes
+
+ The following composition modes are supported:
+
+ \list
+ \o QPainter::CompositionMode_SourceOver
+ \o QPainter::CompositionMode_DestinationOver
+ \o QPainter::CompositionMode_Source
+ \o QPainter::CompositionMode_SourceIn
+ \o QPainter::CompositionMode_DestinationIn
+ \o QPainter::CompositionMode_Plus
+ \o QPainter::CompositionMode_Multiply
+ \o QPainter::CompositionMode_Screen
+ \o QPainter::CompositionMode_Darken
+ \o QPainter::CompositionMode_Lighten
+ \endlist
+
+ The other members of QPainter::CompositionMode are not supported
+ because OpenVG 1.1 does not have an equivalent in its \c VGBlendMode
+ enumeration. Any attempt to set an unsupported mode will result in
+ the actual mode being set to QPainter::CompositionMode_SourceOver.
+ Client applications should avoid using unsupported modes.
+
+ \section2 Pens and brushes
+
+ All pen styles are supported, including cosmetic pens.
+
+ All brush styles are supported except for conical gradients, which are
+ not supported by OpenVG 1.1. Conical gradients will be converted into a
+ solid color brush corresponding to the first color in the gradient's
+ color ramp.
+
+ Affine matrices are supported for brush transforms, but not projective
+ matrices.
+
+ \section2 Rectangles, lines, and points
+
+ Rectangles, lines, and rounded rectangles use cached VGPath objects
+ to try to accelerate drawing operations. vgModifyPathCoords() is used
+ to modify the co-ordinates in the cached VGPath object each time
+ fillRect(), drawRects(), drawLines(), or drawRoundedRect() is called.
+
+ If the engine does not implement vgModifyPathCoords() properly, then the
+ QVG_NO_MODIFY_PATH define can be set to disable path caching. This will
+ incur a performance penalty.
+
+ Points are implemented as lines from the point to itself. The cached
+ line drawing VGPath object is used when drawing points.
+
+ \section2 Polygons and Ellipses
+
+ Polygon and ellipse drawing creates a new VGPath object every time
+ drawPolygon() or drawEllipse() is called. If the client application is
+ making heavy use of these functions, the constant creation and destruction
+ of VGPath objects could have an impact on performance.
+
+ If a projective transformation is active, ellipses are converted into
+ cubic curves prior to transformation, which may further impact performance.
+
+ Client applications should avoid polygon and ellipse drawing in performance
+ critical code if possible.
+
+ \section2 Other Objects
+
+ Most other objects (arcs, pies, etc) use drawPath(), which takes a
+ QPainterPath argument. The default implementation in QPainterEngineEx
+ converts the QPainterPath into a QVectorPath and then calls draw(),
+ which in turn converts the QVectorPath into a VGPath for drawing.
+
+ To reduce the overhead, we have overridden drawPath() in QVGPaintEngine
+ to convert QPainterPath's directly into VGPath's. This should help improve
+ performance compared to the default implementation.
+
+ Client applications should try to avoid these types of objects in
+ performance critical code because of the QPainterPath to VGPath
+ conversion cost.
+
+ \section2 Clipping
+
+ Clipping with QRect, QRectF, and QRegion objects is supported on all
+ OpenVG engines with vgMask() if the transformation matrix is the identity
+ or a simple origin translation.
+
+ Clipping with an arbitrary QPainterPath, or setting the clip region when
+ the transformation matrix is simple, is supported only if the OpenVG engine
+ has the vgRenderToMask() function (OpenVG 1.1 and higher).
+
+ The QVG_NO_RENDER_TO_MASK define will disable the use of vgRenderToMask().
+
+ The QVG_SCISSOR_CLIP define will disable clipping with vgMask() or
+ vgRenderToMask() and instead use the scissor rectangle list to perform
+ clipping. Clipping with an arbitrary QPainterPath will not be supported.
+ The QVG_SCISSOR_CLIP define should only be used if the OpenVG engine
+ does not support vgMask() or vgRenderToMask().
+
+ \section2 Opacity
+
+ Opacity is supported for all drawing operations. Solid color pens,
+ solid color brushes, gradient brushes, and image drawing with drawPixmap()
+ and drawImage() will probably have the best performance compared to
+ other kinds of pens and brushes.
+
+ \section2 Text Drawing
+
+ If OpenVG 1.1 is used, the paint engine will use VG fonts to cache glyphs
+ while drawing. If the engine does not support VG fonts correctly,
+ QVG_NO_DRAW_GLYPHS can be defined to disable this mode. Text drawing
+ performance will suffer if VG fonts are not used.
+
+ By default, image-based glyphs are used. If QVG_NO_IMAGE_GLYPHS is defined,
+ then path-based glyphs will be used instead. QVG_NO_IMAGE_GLYPHS is ignored
+ if QVG_NO_DRAW_GLYPHS is defined.
+
+ If path-based glyphs are used, then the OpenVG engine will need to
+ support hinting to render text with good results. Image-based glyphs
+ avoids the need for hinting and will usually give better results than
+ path-based glyphs.
+
+ \section2 Pixmaps
+
+ In single-context mode, pixmaps will be implemented using VGImage
+ unless QVG_NO_PIXMAP_DATA is defined.
+
+ QVGPixmapData will convert QImage's into VGImage's when the application
+ calls drawPixmap(), and the pixmap will be kept in VGImage form for the
+ lifetime of the QVGPixmapData object. When the application tries to paint
+ into a QPixmap with QPainter, the data will be converted back into a
+ QImage and the raster paint engine will be used to render into the QImage.
+
+ This arrangement optimizes for the case of drawing the same static pixmap
+ over and over (e.g. for icons), but does not optimize the case of drawing
+ into pixmaps.
+
+ Bitmaps must use QRasterPixmapData. They are not accelerated with
+ VGImage at present.
+
+ \section2 Pixmap filters
+
+ Convolution, colorize, drop shadow, and blur filters are accelerated
+ using OpenVG operations.
+
+ \section1 Known issues
+
+ Performance of copying the contents of an OpenVG-rendered window to the
+ screen needs platform-specific work in the QVGWindowSurface class.
+
+ Clipping with arbitrary non-rectangular paths only works on engines
+ that support vgRenderToMask(). Simple rectangular paths are supported
+ on all engines that correctly implement vgMask().
+
+ The paint engine is not yet thread-safe, so it is not recommended for
+ use in threaded Qt applications that draw from multiple threads.
+ Drawing should be limited to the main GUI thread.
+
+ Performance of projective matrices for non-image drawing is not as good
+ as for affine matrices.
+
+ QPixmap's are implemented as VGImage objects so that they can be quickly
+ rendered with drawPixmap(). Rendering into a QPixmap using QPainter
+ will use the default Qt raster paint engine on a QImage copy of the
+ QPixmap, and will not be accelerated. This issue may be addressed in
+ a future version of the engine.
+
+ ShivaVG support is highly experimental and limited to Qt/X11. It is
+ provided as an example of how to integrate a non-EGL engine.
+*/