\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.