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3 ** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies). |
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38 ** $QT_END_LICENSE$ |
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40 ****************************************************************************/ |
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41 |
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42 /*! |
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43 \example opengl/hellogl_es |
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44 \title Hello GL ES Example |
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45 |
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46 The Hello GL ES example is the \l{Hello GL Example} ported to OpenGL ES. |
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47 It also included some effects from the OpenGL \l{Overpainting Example}. |
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48 |
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49 \image hellogl-es-example.png |
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50 |
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51 A complete introduction to OpenGL ES and a description of all differences |
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52 between OpenGL and OpenGL ES is out of the scope of this document; but |
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53 we will describe some of the major issues and differences. |
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54 |
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55 Since Hello GL ES is a direct port of standard OpenGL code, it is a fairly |
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56 good example for porting OpenGL code to OpenGL ES. |
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57 |
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58 \tableofcontents |
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59 |
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60 \section1 Using QGLWidget |
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61 |
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62 QGLWidget can be used for OpenGL ES similar to the way it is used with |
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63 standard OpenGL; but there are some differences. We use EGL 1.0 to embedd |
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64 the OpenGL ES window within the native window manager. In |
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65 QGLWidget::initializeGL() we initialize OpenGL ES. |
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66 |
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67 \section1 Using OpenGL ES rendering commands |
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68 |
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69 To update the scene, we reimplment QGLWidget::paintGL(). We use OpenGL ES |
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70 rendering commands just like we do with standard OpenGL. Since the OpenGL |
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71 ES common light profile only supports fixed point functions, we need to |
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72 abstract it somehow. Hence, we define an abstraction layer in |
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73 \c{cl_helper.h}. |
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74 |
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75 \snippet examples/opengl/hellogl_es/cl_helper.h 0 |
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76 |
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77 Instead of \c glFogxv() or \c glFogfv() we use \c q_glFogv() and to |
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78 convert the coordinates of a vertice we use the macro \c f2vt(). That way, |
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79 if QT_OPENGL_ES_CL is defined we use the fixed point functions and every |
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80 float is converted to fixed point. |
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81 |
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82 If QT_OPENGL_ES_CL is not defined we use the floating point functions. |
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83 |
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84 \snippet examples/opengl/hellogl_es/cl_helper.h 1 |
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85 |
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86 This way we support OpenGL ES Common and Common Light with the same code |
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87 and abstract the fact that we use either the floating point functions or |
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88 otherwise the fixed point functions. |
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89 |
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90 \section1 Porting OpenGL to OpenGL ES |
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91 |
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92 Since OpenGL ES is missing the immediate mode and does not support quads, |
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93 we have to create triangle arrays. |
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94 |
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95 We create a quad by adding vertices to a QList of vertices. We create both |
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96 sides of the quad and hardcode a distance of 0.05f. We also compute the |
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97 correct normal for each face and store them in another QList. |
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98 |
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99 \snippet examples/opengl/hellogl_es/glwidget.cpp 0 |
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100 |
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101 And then we convert the complete list of vertexes and the list of normals |
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102 into the native OpenGL ES format that we can use with the OpenGL ES API. |
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103 |
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104 \snippet examples/opengl/hellogl_es/glwidget.cpp 1 |
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105 |
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106 In \c paintQtLogo() we draw the triangle array using OpenGL ES. We use |
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107 q_vertexTypeEnum to abstract the fact that our vertex and normal arrays |
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108 are either in float or in fixed point format. |
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109 |
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110 \snippet examples/opengl/hellogl_es/glwidget.cpp 2 |
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111 |
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112 \section1 Using QGLPainter |
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113 |
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114 Since the \c QGLPainter is slower for OpenGL ES we paint the bubbles with |
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115 the rasterizer and cache them in a QImage. This happends only once during |
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116 the initialiazation. |
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117 |
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118 \snippet examples/opengl/hellogl_es/bubble.cpp 0 |
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119 |
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120 For each bubble this QImage is then drawn to the QGLWidget by using the |
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121 according QPainter with transparency enabled. |
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122 |
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123 \snippet examples/opengl/hellogl_es/bubble.cpp 1 |
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124 |
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125 Another difference beetwen OpenGL and OpenGL ES is that OpenGL ES does not |
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126 support glPushAttrib(GL_ALL_ATTRIB_BITS). So we have to restore all the |
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127 OpenGL states ourselves, after we created the QPainter in |
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128 GLWidget::paintGL(). |
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129 |
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130 \snippet examples/opengl/hellogl_es/glwidget.cpp 3 |
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131 |
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132 Setting up up the model view matrix and setting the right OpenGL states is |
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133 done in the same way as for standard OpenGL. |
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134 |
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135 \snippet examples/opengl/hellogl_es/glwidget.cpp 4 |
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136 |
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137 Now we have to restore the OpenGL state for the QPainter. This is not done |
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138 automatically for OpenGL ES. |
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139 |
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140 \snippet examples/opengl/hellogl_es/glwidget.cpp 5 |
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141 |
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142 Now we use the QPainter to draw the transparent bubbles. |
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143 |
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144 \snippet examples/opengl/hellogl_es/glwidget.cpp 6 |
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145 |
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146 In the end, we calculate the framerate and display it using the QPainter |
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147 again. |
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148 |
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149 \snippet examples/opengl/hellogl_es/glwidget.cpp 7 |
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150 |
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151 After we finished all the drawing operations we swap the screen buffer. |
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152 |
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153 \snippet examples/opengl/hellogl_es/glwidget.cpp 8 |
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154 |
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155 \section1 Summary |
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156 |
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157 Similar to the \l{Hello GL Example}, we subclass QGLWidget to render |
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158 a 3D scene using OpenGL ES calls. QGLWidget is a subclass of QWidget. |
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159 Hence, its \l{QGLWidget}'s subclasses can be placed in layouts and |
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160 provided with interactive features just like normal custom widgets. |
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161 |
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162 QGLWidget allows pure OpenGL ES rendering to be mixed with QPainter calls, |
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163 but care must be taken to maintain the state of the OpenGL ES |
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164 implementation. |
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165 */ |