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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.

*/