source: trunk/doc/src/howtos/openvg.qdoc@ 846

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/*!
    \page openvg.html
    \title OpenVG Rendering in Qt
    \since 4.6
    \ingroup best-practices
    \ingroup technology-apis


    \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
    unless the \c{VG_KHR_advanced_blending} extension is present,
    in which case the following additional modes are supported:

    \list
    \o QPainter::CompositionMode_Overlay
    \o QPainter::CompositionMode_ColorDodge
    \o QPainter::CompositionMode_ColorBurn
    \o QPainter::CompositionMode_HardLight
    \o QPainter::CompositionMode_SoftLight
    \o QPainter::CompositionMode_Difference
    \o QPainter::CompositionMode_Exclusion
    \o QPainter::CompositionMode_SourceOut
    \o QPainter::CompositionMode_DestinationOut
    \o QPainter::CompositionMode_SourceAtop
    \o QPainter::CompositionMode_DestinationAtop
    \o QPainter::CompositionMode_Xor
    \endlist

    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 need to convert
    the path into a series of rectangles.  If the number of rectangles
    exceeds VG_MAX_SCISSOR_RECTS, then the results will not be exact.

    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.

    \section2 Scrolling

    By default, accelerated scrolling is not enabled because the impact on
    performance is very much tied to the hardware platform. To enable
    accelerated scrolling, you should ensure that QVG_BUFFER_SCROLLING is
    defined when compiling the QtOpenVG module.

    You should only enable this feature if vgCopyPixels() is known to be
    efficient on your hardware platform.

    \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.
*/