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\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 designed for use with embedded systems, it has a smaller, more
constrained API.
\l {http://www.khronos.org/opengles/1_X}{OpenGL ES version 1.x} is designed for
fixed function hardware, while its successor \l
{http://www.khronos.org/opengles/2_X}{OpenGL ES version 2.x} is designed for
programmable hardware. It is worth noting that there is a significant
difference between the two, and that they are not compatible with each
other. OpenGL ES 1.x limits processing to a pre-defined set of fixed
options for drawing and lighting objects. OpenGL 2.x has a significantly
shorter graphics pipeline than 1.x. Instead of using function
transformation and a fragment pipeline, 2.x uses the \l
{http://www.khronos.org/registry/gles/specs/2.0/GLSL_ES_Specification_1.0.17.pdf}{OpenGL
ES Shading Language (GLSL ES)}. Instead of using the pre-defined functions,
the programmer writes small shader programs telling the hardware in detail
how to render each object.
The \l {QtOpenGL module} offers classes that make it easy to draw 3D
graphics in GUI applications using OpenGL ES. Qt provides a plugin that
integrates both OpenGL ES versions \l
{http://www.khronos.org/opengles/1_X}{1.x} and \l
{http://www.khronos.org/opengles/2_X}{2.x} with Qt for Embedded. However,
Qt for Embedded can be adapted to a wide range of OpenGL versions.
To translate QPainter operations into OpenGL ES calls (there are actually
two subclasses, one for OpenGL/ES 1.1 and another for OpenGL/ES 2.0), Qt
uses a subclass of QPaintEngine. 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.
\tableofcontents
\section1 Using OpenGL ES with Qt
To use OpenGL-enabled widgets in a Qt for Embedded application, all that is
required is to subclass QGLWidget and draw into instances of the subclass
with standard OpenGL functions. The current implementation only
supports OpenGL ES and 2D painting within a QGLWidget. Using OpenGL ES to
accelerate regular widgets as well as compositing top-level windows with
OpenGL ES are not currently supported. These issues will be addressed in
future versions of Qt.
\note 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.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
view.setViewportUpdateMode(QGraphicsView::FullViewportUpdate);
view.setFrameStyle(0);
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.
\note OpenGL ES 2.X does not support PBuffers, so QGLPixelBuffer will not
work. In this case, QGLFramebufferObject should be used instead. However,
OpenGL ES 1.X does not support Framebuffer objects, with the exception of
some OpenGL ES 1.X extensions. In this case, please use QGLPixelBuffer.
\note 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.
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