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27
28/*!
29 \example opengl/hellogl_es
30 \title Hello GL ES Example
31
32 The Hello GL ES example is the \l{Hello GL Example} ported to OpenGL ES.
33 It also included some effects from the OpenGL \l{Overpainting Example}.
34
35 \image hellogl-es-example.png
36
37 A complete introduction to OpenGL ES and a description of all differences
38 between OpenGL and OpenGL ES is out of the scope of this document; but
39 we will describe some of the major issues and differences.
40
41 Since Hello GL ES is a direct port of standard OpenGL code, it is a fairly
42 good example for porting OpenGL code to OpenGL ES.
43
44 \tableofcontents
45
46 \section1 Using QGLWidget
47
48 QGLWidget can be used for OpenGL ES similar to the way it is used with
49 standard OpenGL; but there are some differences. We use EGL 1.0 to embedd
50 the OpenGL ES window within the native window manager. In
51 QGLWidget::initializeGL() we initialize OpenGL ES.
52
53 \section1 Porting OpenGL to OpenGL ES
54
55 Since OpenGL ES is missing the immediate mode and does not support quads,
56 we have to create triangle arrays.
57
58 We create a quad by adding vertices to a QList of vertices. We create both
59 sides of the quad and hardcode a distance of 0.05f. We also compute the
60 correct normal for each face and store them in another QList.
61
62 \snippet examples/opengl/hellogl_es/glwidget.cpp 0
63
64 And then we convert the complete list of vertexes and the list of normals
65 into the native OpenGL ES format that we can use with the OpenGL ES API.
66
67 \snippet examples/opengl/hellogl_es/glwidget.cpp 1
68
69 In \c paintQtLogo() we draw the triangle array using OpenGL ES. We use
70 q_vertexTypeEnum to abstract the fact that our vertex and normal arrays
71 are either in float or in fixed point format.
72
73 \snippet examples/opengl/hellogl_es/glwidget.cpp 2
74
75 \section1 Using QGLPainter
76
77 Since the \c QGLPainter is slower for OpenGL ES we paint the bubbles with
78 the rasterizer and cache them in a QImage. This happends only once during
79 the initialiazation.
80
81 \snippet examples/opengl/hellogl_es/bubble.cpp 0
82
83 For each bubble this QImage is then drawn to the QGLWidget by using the
84 according QPainter with transparency enabled.
85
86 \snippet examples/opengl/hellogl_es/bubble.cpp 1
87
88 Another difference beetwen OpenGL and OpenGL ES is that OpenGL ES does not
89 support glPushAttrib(GL_ALL_ATTRIB_BITS). So we have to restore all the
90 OpenGL states ourselves, after we created the QPainter in
91 GLWidget::paintGL().
92
93 \snippet examples/opengl/hellogl_es/glwidget.cpp 3
94
95 Setting up up the model view matrix and setting the right OpenGL states is
96 done in the same way as for standard OpenGL.
97
98 \snippet examples/opengl/hellogl_es/glwidget.cpp 4
99
100 Now we have to restore the OpenGL state for the QPainter. This is not done
101 automatically for OpenGL ES.
102
103 \snippet examples/opengl/hellogl_es/glwidget.cpp 5
104
105 Now we use the QPainter to draw the transparent bubbles.
106
107 \snippet examples/opengl/hellogl_es/glwidget.cpp 6
108
109 In the end, we calculate the framerate and display it using the QPainter
110 again.
111
112 \snippet examples/opengl/hellogl_es/glwidget.cpp 7
113
114 After we finished all the drawing operations we swap the screen buffer.
115
116 \snippet examples/opengl/hellogl_es/glwidget.cpp 8
117
118 \section1 Summary
119
120 Similar to the \l{Hello GL Example}, we subclass QGLWidget to render
121 a 3D scene using OpenGL ES calls. QGLWidget is a subclass of QWidget.
122 Hence, its \l{QGLWidget}'s subclasses can be placed in layouts and
123 provided with interactive features just like normal custom widgets.
124
125 QGLWidget allows pure OpenGL ES rendering to be mixed with QPainter calls,
126 but care must be taken to maintain the state of the OpenGL ES
127 implementation.
128*/
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