source: trunk/doc/src/platforms/emb-openvg.qdocinc@ 568

\section1 Introduction

\l {http://www.khronos.org/openvg}{OpenVG} is a standard API from the
\l{http://www.khronos.org/openvg}{Khronos Group} for accelerated 2D vector
graphics and raster graphics. It is a dedicated API for 2D graphics on
mobile devices. It is therefore more likely to be a better alternative for
2D acceleration than OpenGL/ES. 

\tableofcontents

\section1 Using OpenVG with Qt
The QtOpenVG plugin provides support for OpenVG painting. OpenVG is
optimized for 2D vector operations, and closely matches the functionality
in QPainter. To translate QPainter operations into OpenVG calls, Qt uses a
subclass of QPaintEngine. Unlike with OpenGL ES, OpenVG can be used for
acceleration of regular widgets. It can therefore be an excellent substitute
for the default raster-based QPaintEngine on hardware that supports the
OpenVG API.

\section2 Configure
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.

\section2 Supported OpenVG Engines

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.

\section2 Supported features

\table
    \header
        \o Feature
        \o Description

    \row
        \o Context modes
        \o 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.

    \row
        \o Transformation matrices
        \o 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.

    \row
        \o Composition modes
        \o 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.

    \row
        \o Pens and brushes
        \o 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.


    \row
        \o Rectangles, lines, and points
        \o 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.

    \row
        \o Polygons and Ellipses
        \o 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.

    \row
        \o Other Objects
        \o 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.

    \row
        \o Clipping
        \o 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().

    \row
        \o Opacity
        \o 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.

    \row
        \o Text Drawing
        \o 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.

    \row
        \o Pixmaps
        \o 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.

    \row
        \o Pixmap filters
        \o Convolution, colorize, drop shadow, and blur filters are
accelerated using OpenVG operations.

\endtable

\section2 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.