OpenGL ES2.0编程步骤
OpenGL ES (OpenGL for Embedded Systems) 是 OpenGL 三维图形 API 的子集，针对手机、PDA和游戏主机等嵌入式设备而设计。该API由Khronos集团定义推广，Khronos是一个图形软硬件行业协会，该协会主要关注图形和多媒体方面的开放标准。
1. 保存全局变量的数据结构
以下例子程序均基于Linux平台。
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
10
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;
1 typedef struct
2 {
3 // Handle to a program object
4 GLuint programObject;
5
6 // Atrribute Location
7 GLint positionLoc;
8 GLint textureLoc;
9
10 // Uniform location
11 GLint matrixModeLoc;
12 GLint matrixViewLoc;
13 GLint matrixPerspectiveLoc;
14
15 // Sampler location
16 GLint samplerLoc;
17
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）
1 int InitEGL(ESContext * esContext)
2 {
3 NativeWindowType eglWindow = NULL;
4
5 EGLDisplay display;
6 EGLContext context;
7 EGLSurface surface;
8
9 EGLConfig configs[2];
10 EGLBoolean eRetStatus;
11 EGLint majorVer, minorVer;
12 EGLint context_attribs[] = {EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE};
13
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};
22
23 // Get default display connection
24 display = eglGetDisplay((EGLNativeDisplayType)EGL_DEFAULT_DISPLAY);
25 if ( display == EGL_NO_DISPLAY )
26 {
27 return EGL_FALSE;
28 }
29
30 // Initialize EGL display connection
31 eRetStatus = eglInitialize(display, &majorVer, &minorVer);
32 if( eRetStatus != EGL_TRUE )
33 {
34 return EGL_FALSE;
35 }
36
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 }
43
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 }
50
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 }
57
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 }
64
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 }
71
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);
80
81 // Return the context elements
82 esContext->eglDisplay = display;
83 esContext->eglSurface = surface;
84 esContext->eglContext = context;
85
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：
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;
6
7 // Create an empty shader object, which maintain the source code strings that define a shader
8 shader = glCreateShader ( type );
9
10 if ( shader == 0 )
11 return 0;
12
13 // Replaces the source code in a shader object
14 glShaderSource ( shader, 1, &shaderSrc, NULL );
15
16 // Compile the shader object
17 glCompileShader ( shader );
18
19 // Check the shader object compile status
20 glGetShaderiv ( shader, GL_COMPILE_STATUS, &compiled );
21
22 if ( !compiled )
23 {
24 GLint infoLen = 0;
25
26 glGetShaderiv ( shader, GL_INFO_LOG_LENGTH, &infoLen );
27
28 if ( infoLen > 1 )
29 {
30 char* infoLog = malloc (sizeof(char) * infoLen );
31
32 glGetShaderInfoLog ( shader, infoLen, NULL, infoLog );
33 esLogMessage ( "Error compiling shader:\n%s\n", infoLog );
34
35 free ( infoLog );
36 }
37
38 glDeleteShader ( shader );
39 return 0;
40 }
41
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：
1 GLuint LoadProgram ( const char *vShaderStr, const char *fShaderStr )
2 {
3 GLuint vertexShader;
4 GLuint fragmentShader;
5 GLuint programObject;
6 GLint linked;
7
8 // Load the vertex/fragment shaders
9 vertexShader = LoadShader ( GL_VERTEX_SHADER, vShaderStr );
10 fragmentShader = LoadShader ( GL_FRAGMENT_SHADER, fShaderStr );
11
12 // Create the program object
13 programObject = glCreateProgram ( );
14 if ( programObject == 0 )
15 return 0;
16
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 );
24
25 // Check the link status
26 glGetProgramiv ( programObject, GL_LINK_STATUS, &linked );
27
28 if ( !linked )
29 {
30 GLint infoLen = 0;
31
32 glGetProgramiv ( programObject, GL_INFO_LOG_LENGTH, &infoLen );
33
34 if ( infoLen > 1 )
35 {
36 char* infoLog = malloc (sizeof(char) * infoLen );
37
38 glGetProgramInfoLog ( programObject, infoLen, NULL, infoLog );
39 esLogMessage ( "Error linking program:\n%s\n", infoLog );
40
41 free ( infoLog );
42 }
43
44 glDeleteProgram ( programObject );
45 return GL_FALSE;
46 }
47
48 // Free no longer needed shader resources
49 glDeleteShader ( vertexShader );
50 glDeleteShader ( fragmentShader );
51
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：
1 int CreateProgram(ESContext * esContext)
2 {
3 GLuint programObject;
4
5 GLbyte vShaderStr[] =
6 "attribute vec4 vPosition; \n"
7 "void main() \n"
8 "{ \n"
9 " gl_Position = vPosition; \n"
10 "} \n";
11
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";
18
19 // Create user data
20 esContext->userData = malloc(sizeof(UserData));
21 UserData *userData = esContext->userData;
22
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 }
29
30 // Store the program object
31 userData->programObject = programObject;
32
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）
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 };
7
8 // Set the viewport
9 glViewport ( 0, 0, esContext->width, esContext->height );
10
11 // Clear the color buffer
12 glClear ( GL_COLOR_BUFFER_BIT );
13
14 // Use the program object
15 glUseProgram ( userData->programObject );
16
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);
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：
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<[-1,1]或[0,1]>：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 //注: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：
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;
8
9 memset( &esContext, 0, sizeof( ESContext) );
10 esContext.userData = &userData;
11
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 }
26
27
28 iStartTime = GetCurTime();
29 iFrames = 0;
30
31 while(1)
32 { // render a frame
33 Render(&esContext);
34 iFrames++;
35
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;
43
44 printf("Frame: %f\n", fFrame);
45 }
46 }
47 glDeleteProgram (esContext.userData->programObject);
48 return GL_TRUE;
49 }