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/*!
\page qt-embedded-opengl.html

\title Qt for Embedded Linux and OpenGL
\ingroup qt-embedded-linux

\section1 Introduction

\l {http://www.opengl.org}{OpenGL} is an industry standard API for
2D/3D graphics. It provides a powerful, low-level interface between
software and acceleration hardware, and it is operating system and
window system independent.

\l {http://www.khronos.org/opengles}{OpenGL ES} is a subset
of the \l {http://www.opengl.org}{OpenGL} standard.
Because it is meant for use in embedded systems, it has a smaller,
more constrained API. 

For reference, Nokia provides support for integrating \l
{http://www.khronos.org/opengles}{OpenGL ES} with Qt for Embedded Linux
for drawing into a QGLWidget.

The current implementation supports OpenGL and 2D painting within a
QGLWidget.  Using OpenGL to accelerate regular widgets and compositing
top-level windows with OpenGL are not currently supported.  These issues
will be addressed in future versions of Qt.

It is recommended that Qt for Embedded Linux is configured with the
\c{-DQT_QWS_CLIENTBLIT} and \c{-DQT_NO_QWS_CURSOR} options for optimum
performance.  OpenGL is rendered direct to the screen and these options
prevent Qt for Embedded Linux from trying to do its own non-OpenGL
compositing on the QGLWidget contents.

\section2 Using OpenGL 3D Graphics in Applications

The \l {QtOpenGL module} offers classes that make it easy to draw 3D
graphics in GUI applications. The module API is cross-platform, so it
is also available on Windows, X11, and Mac OS X.

To use OpenGL-enabled widgets in a Qt for Embedded Linux application,
all that is required is to subclass the QGLWidget and draw into instances of
the subclass with standard OpenGL functions. 

Note that on most embedded hardware, the OpenGL implementation is
actually \l{http://www.khronos.org/opengles/1_X/}{OpenGL/ES 1.1} or
\l{http://www.khronos.org/opengles/2_X/}{OpenGL/ES 2.0}.  When painting
within a QGLWidget::paintGL() override, it is necessary to limit the
application to only the features that are present in the OpenGL/ES
implementation.

\section2 Using OpenGL to Accelerate Normal 2D Painting

Qt provides a subclass of QPaintEngine that translates QPainter operations
into OpenGL calls (there are actually two subclasses, one for OpenGL/ES 1.1
and another for OpenGL/ES 2.0).  This specialized paint engine can be used
to improve 2D rendering performance on appropriate hardware.  It can also
overlay controls and decorations onto 3D scenes drawn using OpenGL.

As mentioned above, the OpenGL paint engine is not currently supported
in regular widgets.  However, any application that uses QGraphicsView
can set a QGLWidget as the viewport and obtain access to the
OpenGL paint engine that way:

\code
QGraphicsView view(&scene);
view.setViewport(new QGLWidget);
view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
view.showFullScreen();
\endcode

It is recommended that the QGraphicsView::FullViewportUpdate flag
be set because the default double-buffered behavior of QGLWidget
does not support partial updates.  It is also recommended that the
window be shown full-screen because that usually has the best
performance on current OpenGL/ES implementations.

Once a QGraphicsView has been initialized as above, regular widgets
can be added to the canvas using QGraphicsProxyWidget if the
application requires them.

\section2 Using OpenGL to Implement Window Compositing and Effects

Compositing effects can be simulated by adjusting the opacity and
other parameters of the items within a QGraphicsView canvas on a
QGLWidget viewport.

While Qt for Embedded Linux does include a complete windowing system,
using OpenGL to composite regular window surfaces can be quite difficult.
Most of Qt for Embedded Linux assumes that the window surface is a plain
raster memory buffer, with QGLWidget being the sole exception.
The need to constantly re-upload the raster memory buffers into OpenGL
textures for compositing can have a significant impact on performance,
which is why we do not recommend implementing that form of compositing.
We intend to address this problem in future versions of Qt.

\section1 Integrating OpenGL/ES into Qt for Embedded Linux

\section2 Reference Integration

The reference integration for OpenGL into Qt for Embedded Linux
is for the PowerVR chipset from \l{http://www.imgtec.com/}{Imagination
Technologies}.  It consists of two components: \c{pvreglscreen} which
provides the Qt for Embedded Linux screen driver, and \c{QWSWSEGL}
which implements a plug-in to the PowerVR EGL implementation to
implement low-level OpenGL drawing surfaces.

\section2 Integrating Other Chipsets

In this section we discuss the essential features of the reference
integration that need to be provided for any other chipset integration.

The QtOpenGL module assumes that a QGLWidget can be represented
by a \c EGLNativeWindowType value in some underlying window system
implementation, and that \c{eglSwapBuffers()} is sufficient to copy
the contents of the native window to the screen when requested.

However, many EGL implementations do not have a pre-existing window system.
Usually only a single full-screen window is provided, and everything else
must be simulated some other way.  This can be a problem because
of QtOpenGL's assumptions.  We intend to address these assumptions in a
future version of Qt, but for now it is the responsibility of the integrator
to provide a rudimentary window system within the EGL implementation.
This is the purpose of \c{QWSWSEGL} in the reference integration.

If it isn't possible for the EGL implementation to provide a rudimentary
window system, then full-screen windows using QGLWidget can be supported,
but very little else.

The screen driver needs to inherit from QGLScreen and perform the
following operations in its constructor:

\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 0

The \c{setSurfaceFunctions()} call supplies an object that takes care
of converting Qt paint devices such as widgets and pixmaps into
\c EGLNativeWindowType and \c EGLNativePixmapType values.  Here we
only support native windows.  Because OpenGL rendering is direct to
the screen, we also indicate that client blit is supported.

Next, we override the \c{createSurface()} functions in QGLScreen:

\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 1

Even if Qt for Embedded Linux is used in single-process mode, it is
necessary to create both client-side and server-side versions of the
window surface.  In our case, the server-side is just a stub because
the client side directly renders to the screen.

Note that we only create a \c{PvrEglWindowSurface} if the widget is a
QGLWidget.  All other widgets use the normal raster processing.
It can be tempting to make \c{createSurface()} create an OpenGL
window surface for other widget types as well.  This has not been
extensively tested and we do not recommend its use at this time.

The other main piece is the creation of the \c EGLNativeWindowType
value for the widget.  This is done in the \c{createNativeWindow()}
override:

\snippet src/plugins/gfxdrivers/powervr/pvreglscreen/pvreglscreen.cpp 2

The details of what needs to be placed in this function will vary
from chipset to chipset.  The simplest is to return the native window
handle corresponding to the "root" full-screen window:

\code
*native = rootWindowHandle;
return true;
\endcode

The most common value for \c rootWindowHandle is zero, but this may
not always be the case.  Consult the chipset documentation for the
actual value to use.  The important thing is that whatever value is
returned must be suitable for passing to the \c{eglCreateWindowSurface()}
function of the chipset's EGL implementation.

In the case of PowerVR, the rudimentary window system in \c{QWSWSEGL}
provides a \c PvrQwsDrawable object to represent the \c EGLNativeWindowType
value for the widget.

\section1 OpenVG Support

\l {http://www.khronos.org/openvg} {OpenVG} 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.  Acceleration of
regular widgets is supported with OpenVG, unlike with OpenGL/ES.
See \l{OpenVG Rendering in Qt} for more information on the
OpenVG support in Qt.

*/