OpenGL ES2.0编程实例

1. 保存全局变量的数据结构 以下例子程序均基于Linux平台。 [cpp] view plaincopy typedef struct _escontext { void *userData; //Putyouruserdatahere... GLintwidth; //Windowwidth GLintheight; //Windowheight EGLNativeWindowTypehWnd; //Windowhandle

1. 保存全局变量的数据结构

以下例子程序均基于Linux平台。

[cpp] view plaincopy

1. typedef struct _escontext
2. {
3. void* userData; // Put your user data here...
4. GLint width; // Window width
5. GLint height; // Window height
6. EGLNativeWindowType hWnd; // Window handle
7. EGLDisplay eglDisplay; // EGL display
8. EGLContext eglContext; // EGL context
9. EGLSurface eglSurface; // EGL surface
11. // Callbacks
12. void (ESCALLBACK *drawFunc) ( struct _escontext * );
13. void (ESCALLBACK *keyFunc) ( struct _escontext *, unsigned char, int, int );
14. void (ESCALLBACK *updateFunc) ( struct _escontext *, float deltaTime );
15. }ESContext;

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1. typedef struct
2. {
3. // Handle to a program object
4. GLuint programObject;
6. // Atrribute Location
7. GLint positionLoc;
8. GLint textureLoc;
10. // Uniform location
11. GLint matrixModeLoc;
12. GLint matrixViewLoc;
13. GLint matrixPerspectiveLoc;
15. // Sampler location
16. GLint samplerLoc;
18. // texture
19. GLuint texture;
20. } UserData;

2. 初始化EGL渲染环境和相关元素（第一步曲）

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1. int InitEGL(ESContext * esContext)
2. {
3. NativeWindowType eglWindow = NULL;
5. EGLDisplay display;
6. EGLContext context;
7. EGLSurface surface;
9. EGLConfig configs[2];
10. EGLBoolean eRetStatus;
11. EGLint majorVer, minorVer;
12. EGLint context_attribs[] = {EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE};
14. EGLint numConfigs;
15. EGLint cfg_attribs[] = {EGL_BUFFER_SIZE, EGL_DONT_CARE,
16. EGL_DEPTH_SIZE, 16,
17. EGL_RED_SIZE, 5,
18. EGL_GREEN_SIZE, 6,
19. EGL_BLUE_SIZE, 5,
20. EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT,
21. EGL_NONE};
23. // Get default display connection
24. display = eglGetDisplay((EGLNativeDisplayType)EGL_DEFAULT_DISPLAY);
25. if ( display == EGL_NO_DISPLAY )
26. {
27. return EGL_FALSE;
28. }
30. // Initialize EGL display connection
31. eRetStatus = eglInitialize(display, &majorVer, &minorVer);
32. if( eRetStatus != EGL_TRUE )
33. {
34. return EGL_FALSE;
35. }
37. //Get a list of all EGL frame buffer configurations for a display
38. eRetStatus = eglGetConfigs (display, configs, 2, &numConfigs);
39. if( eRetStatus != EGL_TRUE )
40. {
41. return EGL_FALSE;
42. }
44. // Get a list of EGL frame buffer configurations that match specified attributes
45. eRetStatus = eglChooseConfig (display, cfg_attribs, configs, 2, &numConfigs);
46. if( eRetStatus != EGL_TRUE || !numConfigs)
47. {
48. return EGL_FALSE;
49. }
51. // Create a new EGL window surface
52. surface = eglCreateWindowSurface(display, configs[0], eglWindow, NULL);
53. if (surface == EGL_NO_SURFACE)
54. {
55. return EGL_FALSE;
56. }
58. // Set the current rendering API (EGL_OPENGL_API, EGL_OPENGL_ES_API,EGL_OPENVG_API)
59. eRetStatus = eglBindAPI(EGL_OPENGL_ES_API);
60. if (eRetStatus != EGL_TRUE)
61. {
62. return EGL_FALSE;
63. }
65. // Create a new EGL rendering context
66. context = eglCreateContext (display, configs[0], EGL_NO_CONTEXT, context_attribs);
67. if (context == EGL_NO_CONTEXT)
68. {
69. return EGL_FALSE;
70. }
72. // Attach an EGL rendering context to EGL surfaces
73. eRetStatus = eglMakeCurrent (display, surface, surface, context);
74. if( eRetStatus != EGL_TRUE )
75. {
76. return EGL_FALSE;
77. }
78. //If interval is set to a value of 0, buffer swaps are not synchronized to a video frame, and the swap happens as soon as the render is complete.
79. eglSwapInterval(display,0);
81. // Return the context elements
82. esContext->eglDisplay = display;
83. esContext->eglSurface = surface;
84. esContext->eglContext = context;
86. return EGL_TRUE;
87. }

3. 生成Program （第二步曲）

3.1 LoadShader

LoadShader主要实现以下功能：

1) 创建Shader对象

2) 装载Shader源码

3) 编译Shader

其实现参考代码如下：

[cpp] view plaincopy

1. /* type specifies the Shader type: GL_VERTEX_SHADER or GL_FRAGMENT_SHADER */
2. GLuint LoadShader ( GLenum type, const char *shaderSrc )
3. {
4. GLuint shader;
5. GLint compiled;
7. // Create an empty shader object, which maintain the source code strings that define a shader
8. shader = glCreateShader ( type );
10. if ( shader == 0 )
11. return 0;
13. // Replaces the source code in a shader object
14. glShaderSource ( shader, 1, &shaderSrc, NULL );
16. // Compile the shader object
17. glCompileShader ( shader );
19. // Check the shader object compile status
20. glGetShaderiv ( shader, GL_COMPILE_STATUS, &compiled );
22. if ( !compiled )
23. {
24. GLint infoLen = 0;
26. glGetShaderiv ( shader, GL_INFO_LOG_LENGTH, &infoLen );
28. if ( infoLen > 1 )
29. {
30. char* infoLog = malloc (sizeof(char) * infoLen );
32. glGetShaderInfoLog ( shader, infoLen, NULL, infoLog );
33. esLogMessage ( "Error compiling shader:\n%s\n", infoLog );
35. free ( infoLog );
36. }
38. glDeleteShader ( shader );
39. return 0;
40. }
42. return shader;
43. }

1）glCreateShader
它创建一个空的shader对象，它用于维护用来定义shader的源码字符串。支持以下两种shader:
（1） GL_VERTEX_SHADER: 它运行在可编程的“顶点处理器”上，用于代替固定功能的顶点处理；
（ 2） GL_FRAGMENT_SHADER: 它运行在可编程的“片断处理器”上，用于代替固定功能的片段处理；

2）glShaderSource
shader对象中原来的源码全部被新的源码所代替。

3）glCompileShader
编译存储在shader对象中的源码字符串，编译结果被当作shader对象状态的一部分被保存起来，可通过glGetShaderiv函数获取编译状态。

4）glGetShaderiv
获取shader对象参数，参数包括：GL_SHADER_TYPE, GL_DELETE_STATUS, GL_COMPILE_STATUS, GL_INFO_LOG_LENGTH, GL_SHADER_SOURCE_LENGTH.

3.2 LoadProgram

其参考代码如下：

[cpp] view plaincopy

1. GLuint LoadProgram ( const char *vShaderStr, const char *fShaderStr )
2. {
3. GLuint vertexShader;
4. GLuint fragmentShader;
5. GLuint programObject;
6. GLint linked;
8. // Load the vertex/fragment shaders
9. vertexShader = LoadShader ( GL_VERTEX_SHADER, vShaderStr );
10. fragmentShader = LoadShader ( GL_FRAGMENT_SHADER, fShaderStr );
12. // Create the program object
13. programObject = glCreateProgram ( );
14. if ( programObject == 0 )
15. return 0;
17. // Attaches a shader object to a program object
18. glAttachShader ( programObject, vertexShader );
19. glAttachShader ( programObject, fragmentShader );
20. // Bind vPosition to attribute 0
21. glBindAttribLocation ( programObject, 0, "vPosition" );
22. // Link the program object
23. glLinkProgram ( programObject );
25. // Check the link status
26. glGetProgramiv ( programObject, GL_LINK_STATUS, &linked );
28. if ( !linked )
29. {
30. GLint infoLen = 0;
32. glGetProgramiv ( programObject, GL_INFO_LOG_LENGTH, &infoLen );
34. if ( infoLen > 1 )
35. {
36. char* infoLog = malloc (sizeof(char) * infoLen );
38. glGetProgramInfoLog ( programObject, infoLen, NULL, infoLog );
39. esLogMessage ( "Error linking program:\n%s\n", infoLog );
41. free ( infoLog );
42. }
44. glDeleteProgram ( programObject );
45. return GL_FALSE;
46. }
48. // Free no longer needed shader resources
49. glDeleteShader ( vertexShader );
50. glDeleteShader ( fragmentShader );
52. return programObject;
53. }

1）glCreateProgram
建立一个空的program对象，shader对象可以被连接到program对像
2）glAttachShader
program对象提供了把需要做的事连接在一起的机制。在一个program中，在shader对象被连接在一起之前，必须先把shader连接到program上。
3）glBindAttribLocation
把program的顶点属性索引与顶点shader中的变量名进行绑定。
4）glLinkProgram
连接程序对象。如果任何类型为GL_VERTEX_SHADER的shader对象连接到program,它将产生在“可编程顶点处理器”上可执行的程序；如果任何类型为GL_FRAGMENT_SHADER的shader对象连接到program,它将产生在“可编程片断处理器”上可执行的程序。
5）glGetProgramiv
获取program对象的参数值，参数有：GL_DELETE_STATUS, GL_LINK_STATUS, GL_VALIDATE_STATUS, GL_INFO_LOG_LENGTH, GL_ATTACHED_SHADERS, GL_ACTIVE_ATTRIBUTES, GL_ACTIVE_ATTRIBUTE_MAX_LENGTH, GL_ACTIVE_UNIFORMS, GL_ACTIVE_UNIFORM_MAX_LENGTH.

3.3 CreateProgram

在3.1中只实现了Shader的编译，在3.2中只实现了Program的链接，现在还缺少真正供进行编译和链接的源码，其参考代码如下：

[cpp] view plaincopy

1. int CreateProgram(ESContext * esContext)
2. {
3. GLuint programObject;
5. GLbyte vShaderStr[] =
6. "attribute vec4 vPosition; \n"
7. "void main() \n"
8. "{ \n"
9. " gl_Position = vPosition; \n"
10. "} \n";
12. GLbyte fShaderStr[] =
13. "precision mediump float;\n"\
14. "void main() \n"
15. "{ \n"
16. " gl_FragColor = vec4 ( 1.0, 0.0, 0.0, 1.0 );\n"
17. "} \n";
19. // Create user data
20. esContext->userData = malloc(sizeof(UserData));
21. UserData *userData = esContext->userData;
23. // Load the shaders and get a linked program object
24. programObject = LoadProgram ( (const char*)vShaderStr, (const char*)fShaderStr );
25. if(programObject == 0)
26. {
27. return GL_FALSE;
28. }
30. // Store the program object
31. userData->programObject = programObject;
33. // Get the attribute locations
34. userData->positionLoc = glGetAttribLocation ( g_programObject, "v_position" );
35. glClearColor ( 0.0f, 0.0f, 0.0f, 1.0f );
36. return 0;
37. }

4. 安装并执行Program（第三步）

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1. void Render ( ESContext *esContext )
2. {
3. UserData *userData = esContext->userData;
4. GLfloat vVertices[] = { 0.0f, 0.5f, 0.0f,
5. -0.5f, -0.5f, 0.0f,
6. 0.5f, -0.5f, 0.0f };
8. // Set the viewport
9. glViewport ( 0, 0, esContext->width, esContext->height );
11. // Clear the color buffer
12. glClear ( GL_COLOR_BUFFER_BIT );
14. // Use the program object
15. glUseProgram ( userData->programObject );
17. // Load the vertex data
18. glVertexAttribPointer ( 0, 3, GL_FLOAT, GL_FALSE, 0, vVertices );
19. glEnableVertexAttribArray ( 0 );
20. glDrawArrays ( GL_TRIANGLES, 0, 3 );
21. eglSwapBuffers(esContext->eglDisplay, esContext->eglSurface);

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1. }

4.1 glClear

清除指定的buffer到预设值。可清除以下四类buffer:

1）GL_COLOR_BUFFER_BIT

2）GL_DEPTH_BUFFER_BIT

3）GL_ACCUM_BUFFER_BIT

4）GL_STENCIL_BUFFER_BIT

预设值通过glClearColor, glClearIndex, glClearDepth, glClearStencil, 和glClearAccum来设置。

1）gClearColor

指定color buffer的清除值，当调用glClear(GL_COLOR_BUFFER_BIT)时才真正用设定的颜色值清除color buffer。参数值的范围为:0~1。

void glClearColor( GLclampf red, GLclampf green, GLclampf blue, GLclampf alpha);

2）glClearIndex

指定color index buffer清除值。void glClearIndex( GLfloat c);

3）glClearDepth

指定depth buffer的清除值，取值范围为:0~1，默认值为1。

void glClearDepth( GLclampd depth);

4）glClearStencil

指定stencil buffer清除值的索引，初始值为0。void glClearStencil( GLint s);

5）glClearAccum

指定accumulation buffer的清除值，初始值为0，取值范围为：-1~1

void glClearAccum( GLfloat red,GLfloat green,GLfloat blue,GLfloat alpha);

4.2 glUseProgram

安装一个program object，并把它作为当前rendering state的一部分。

1) 当一个可执行程序被安装到vertex processor，下列OpenGL固定功能将被disable:

• The modelview matrix is not applied to vertex coordinates.
• The projection matrix is not applied to vertex coordinates.
• The texture matrices are not applied to texture coordinates.
• Normals are not transformed to eye coordinates.
• Normals are not rescaled or normalized.
• Normalization of GL_AUTO_NORMAL evaluated normals is not performed.
• Texture coordinates are not generated automatically.
• Per-vertex lighting is not performed.
• Color material computations are not performed.
• Color index lighting is not performed.
• This list also applies when setting the current raster position.

2) 当一个可执行程序被安装到fragment processor，下列OpenGL固定功能将被disable:

• Texture environment and texture functions are not applied.
• Texture application is not applied.
• Color sum is not applied.
• Fog is not applied.

4.3 glVertexAttribPointer

定义一个通用顶点属性数组。当渲染时，它指定了通用顶点属性数组从索引index处开始的位置和数据格式。其定义如下：

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1. void glVertexAttribPointer(
2. GLuint index, // 指示将被修改的通用顶点属性的索引
3. GLint size, // 指点每个顶点元素个数(1~4)
4. GLenum type, // 数组中每个元素的数据类型
5. GLboolean normalized, //指示定点数据值是否被归一化(归一化：GL_TRUE,直接使用:GL_FALSE)
6. GLsizei stride, // 连续顶点属性间的偏移量，如果为0，相邻顶点属性间紧紧相邻
7. const GLvoid * pointer);//顶点数组
8. :其index应该小于#define GL_MAX_VERTEX_ATTRIBS 0x8869

4.4 glEnableVertexAttribArray

Enable由索引index指定的通用顶点属性数组。

void glEnableVertexAttribArray( GLuint index);
void glDisableVertexAttribArray( GLuint index);

默认状态下，所有客户端的能力被disabled，包括所有通用顶点属性数组。如果被Enable，通用顶点属性数组中的值将被访问并被用于rendering，通过调用顶点数组命令：glDrawArrays, glDrawElements, glDrawRangeElements, glArrayElement, glMultiDrawElements, or glMultiDrawArrays.

4.5 glDrawArrays

void glDrawArrays( GLenum mode,
GLint first,
GLsizei count);

1) mode:指明render原语，如：GL_POINTS, GL_LINE_STRIP, GL_LINE_LOOP, GL_LINES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_TRIANGLES, GL_QUAD_STRIP, GL_QUADS, 和 GL_POLYGON。

2) first: 指明Enable数组中起始索引。

3) count: 指明被render的原语个数。

可以预先使用单独的数据定义vertex、normal和color，然后通过一个简单的glDrawArrays构造一系列原语。当调用glDrawArrays时，它使用每个enable的数组中的count个连续的元素，来构造一系列几何原语，从第first个元素开始。

4.6 eglSwapBuffers

把EGL surface中的color buffer提交到native window进行显示。

EGLBoolean eglSwapBuffers(EGLDisplay display,EGLSurface surface)

5. 协调组织

在前面的描述中，三步曲已经完成了：

1）初始化EGL环境，为绘图做好准备

2）生成Program

3）安装并执行Program

只有这三个关键人物，还不能运行，还需要一个协调组织者。其参考代码如下：

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1. int main(int argc, char** argv)
2. {
3. ESContext esContext;
4. UserData userData;
5. int iFrames;
6. unsigned long iStartTime,iEndTime;
7. int iDeltaTime;
9. memset( &esContext, 0, sizeof( ESContext) );
10. esContext.userData = &userData;
12. esContext.width = 1280;
13. esContext.height = 720;
14. // Init EGL display, surface and context
15. if(!InitEGL(&esContext))
16. {
17. printf("Init EGL fail\n");
18. return GL_FALSE;
19. }
20. // compile shader, link program
21. if(!CreateProgram(&esContext))
22. {
23. printf("Create Program fail\n");
24. return GL_FALSE;
25. }
28. iStartTime = GetCurTime();
29. iFrames = 0;
31. while(1)
32. { // render a frame
33. Render();
34. iFrames++;
36. iEndTime = GetCurTime();
37. iDeltaTime = iEndTime - iStartTime;
38. if(iDeltaTime >= 5000)
39. {
40. iStartTime = iEndTime;
41. float fFrame = iFrames * 1000.0 / iDeltaTime;
42. iFrames = 0;
44. printf("Frame: %f\n", fFrame);
45. }
46. }
47. glDeleteProgram (esContext.userData->programObject);
48. return GL_TRUE;
49. }