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/****************************************************************************
**
** Copyright (C) 2012 Digia Plc and/or its subsidiary(-ies).
** Contact: http://www.qt-project.org/legal
**
** This file is part of the documentation of the Qt Toolkit.
**
** $QT_BEGIN_LICENSE:FDL$
** Commercial License Usage
** Licensees holding valid commercial Qt licenses may use this file in
** accordance with the commercial license agreement provided with the
** Software or, alternatively, in accordance with the terms contained in
** a written agreement between you and Digia. For licensing terms and
** conditions see http://qt.digia.com/licensing. For further information
** use the contact form at http://qt.digia.com/contact-us.
**
** GNU Free Documentation License Usage
** Alternatively, this file may be used under the terms of the GNU Free
** Documentation License version 1.3 as published by the Free Software
** Foundation and appearing in the file included in the packaging of
** this file. Please review the following information to ensure
** the GNU Free Documentation License version 1.3 requirements
** will be met: http://www.gnu.org/copyleft/fdl.html.
** $QT_END_LICENSE$
**
****************************************************************************/
/*!
\example hellogl_es
\title Hello GL ES Example
\brief The Hello GL ES example is the \l{Hello GL Example} ported to OpenGL ES.
It also included some effects from the OpenGL \l{Overpainting Example}.
\image hellogl-es-example.png
A complete introduction to OpenGL ES and a description of all differences
between OpenGL and OpenGL ES is out of the scope of this document; but
we will describe some of the major issues and differences.
Since Hello GL ES is a direct port of standard OpenGL code, it is a fairly
good example for porting OpenGL code to OpenGL ES.
\tableofcontents
\section1 Using QGLWidget
QGLWidget can be used for OpenGL ES similar to the way it is used with
standard OpenGL; but there are some differences. We use EGL 1.0 to embedd
the OpenGL ES window within the native window manager. In
QGLWidget::initializeGL() we initialize OpenGL ES.
\section1 Porting OpenGL to OpenGL ES
Since OpenGL ES is missing the immediate mode and does not support quads,
we have to create triangle arrays.
We create a quad by adding vertices to a QList of vertices. We create both
sides of the quad and hardcode a distance of 0.05f. We also compute the
correct normal for each face and store them in another QList.
\snippet hellogl_es/glwidget.cpp 0
And then we convert the complete list of vertexes and the list of normals
into the native OpenGL ES format that we can use with the OpenGL ES API.
\snippet hellogl_es/glwidget.cpp 1
In \c paintQtLogo() we draw the triangle array using OpenGL ES. We use
q_vertexTypeEnum to abstract the fact that our vertex and normal arrays
are either in float or in fixed point format.
\snippet hellogl_es/glwidget.cpp 2
\section1 Using QGLPainter
Since the \c QGLPainter is slower for OpenGL ES we paint the bubbles with
the rasterizer and cache them in a QImage. This happends only once during
the initialiazation.
\snippet hellogl_es/bubble.cpp 0
For each bubble this QImage is then drawn to the QGLWidget by using the
according QPainter with transparency enabled.
\snippet hellogl_es/bubble.cpp 1
Another difference beetwen OpenGL and OpenGL ES is that OpenGL ES does not
support glPushAttrib(GL_ALL_ATTRIB_BITS). So we have to restore all the
OpenGL states ourselves, after we created the QPainter in
GLWidget::paintGL().
\snippet hellogl_es/glwidget.cpp 3
Setting up up the model view matrix and setting the right OpenGL states is
done in the same way as for standard OpenGL.
\snippet hellogl_es/glwidget.cpp 4
Now we have to restore the OpenGL state for the QPainter. This is not done
automatically for OpenGL ES.
\snippet hellogl_es/glwidget.cpp 5
Now we use the QPainter to draw the transparent bubbles.
\snippet hellogl_es/glwidget.cpp 6
In the end, we calculate the framerate and display it using the QPainter
again.
\snippet hellogl_es/glwidget.cpp 7
After we finished all the drawing operations we swap the screen buffer.
\snippet hellogl_es/glwidget.cpp 8
\section1 Summary
Similar to the \l{Hello GL Example}, we subclass QGLWidget to render
a 3D scene using OpenGL ES calls. QGLWidget is a subclass of QWidget.
Hence, its \l{QGLWidget}'s subclasses can be placed in layouts and
provided with interactive features just like normal custom widgets.
QGLWidget allows pure OpenGL ES rendering to be mixed with QPainter calls,
but care must be taken to maintain the state of the OpenGL ES
implementation.
*/
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