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/*!
    \example opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/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 examples/opengl/hellogl_es/glwidget.cpp 5

    Now we use the QPainter to draw the transparent bubbles.

    \snippet examples/opengl/hellogl_es/glwidget.cpp 6

    In the end, we calculate the framerate and display it using the QPainter
    again.

    \snippet examples/opengl/hellogl_es/glwidget.cpp 7

    After we finished all the drawing operations we swap the screen buffer.

    \snippet examples/opengl/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.
*/