OpenGL|OpenGL ES2.0编程三步曲 OpenGL|ES

1. 保存全局变量的数据结构
以下例子程序均基于Linux平台。

typedef struct _escontext { void*userData; // Put your user data here... GLintwidth; // Window width GLintheight; // Window height EGLNativeWindowTypehWnd; // Window handle EGLDisplayeglDisplay; // EGL display EGLContexteglContext; // EGL context EGLSurfaceeglSurface; // EGL surface// Callbacks void (ESCALLBACK *drawFunc) ( struct _escontext * ); void (ESCALLBACK *keyFunc) ( struct _escontext *, unsigned char, int, int ); void (ESCALLBACK *updateFunc) ( struct _escontext *, float deltaTime ); }ESContext;

typedef struct { // Handle to a program object GLuint programObject; // Atrribute Location GLint positionLoc; GLint textureLoc; // Uniform location GLint matrixModeLoc; GLint matrixViewLoc; GLint matrixPerspectiveLoc; // Sampler location GLint samplerLoc; // texture GLuint texture; } UserData;

2. 初始化EGL渲染环境和相关元素（第一步曲）

int InitEGL(ESContext * esContext) { NativeWindowType eglWindow = NULL; EGLDisplay display; EGLContext context; EGLSurface surface; EGLConfig configs[2]; EGLBoolean eRetStatus; EGLint majorVer, minorVer; EGLint context_attribs[] = {EGL_CONTEXT_CLIENT_VERSION, 2, EGL_NONE}; EGLint numConfigs; EGLint cfg_attribs[] = {EGL_BUFFER_SIZE,EGL_DONT_CARE, EGL_DEPTH_SIZE,16, EGL_RED_SIZE,5, EGL_GREEN_SIZE,6, EGL_BLUE_SIZE,5, EGL_RENDERABLE_TYPE, EGL_OPENGL_ES2_BIT, EGL_NONE}; // Get default display connection display = eglGetDisplay((EGLNativeDisplayType)EGL_DEFAULT_DISPLAY); if ( display == EGL_NO_DISPLAY ) { return EGL_FALSE; }// Initialize EGL display connection eRetStatus = eglInitialize(display, &majorVer, &minorVer); if( eRetStatus != EGL_TRUE ) { return EGL_FALSE; }//Get a list of all EGL frame buffer configurations for a display eRetStatus = eglGetConfigs (display, configs, 2, &numConfigs); if( eRetStatus != EGL_TRUE ) { return EGL_FALSE; }// Get a list of EGL frame buffer configurations that match specified attributes eRetStatus = eglChooseConfig (display, cfg_attribs, configs, 2, &numConfigs); if( eRetStatus != EGL_TRUE|| !numConfigs) { return EGL_FALSE; }// Create a new EGL window surface surface = eglCreateWindowSurface(display, configs[0], eglWindow, NULL); if (surface == EGL_NO_SURFACE) { return EGL_FALSE; }// Set the current rendering API (EGL_OPENGL_API, EGL_OPENGL_ES_API,EGL_OPENVG_API) eRetStatus = eglBindAPI(EGL_OPENGL_ES_API); if (eRetStatus != EGL_TRUE) { return EGL_FALSE; }// Create a new EGL rendering context context = eglCreateContext (display, configs[0], EGL_NO_CONTEXT, context_attribs); if (context == EGL_NO_CONTEXT) { return EGL_FALSE; }// Attach an EGL rendering context to EGL surfaces eRetStatus = eglMakeCurrent (display, surface, surface, context); if( eRetStatus != EGL_TRUE ) { return EGL_FALSE; } //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. eglSwapInterval(display,0); // Return the context elements esContext->eglDisplay = display; esContext->eglSurface = surface; esContext->eglContext = context; return EGL_TRUE; }

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
【OpenGL|OpenGL ES2.0编程三步曲】3) 编译Shader
其实现参考代码如下：

/* type specifies the Shader type: GL_VERTEX_SHADER or GL_FRAGMENT_SHADER */ GLuint LoadShader ( GLenum type, const char *shaderSrc ) { GLuint shader; GLint compiled; // Create an empty shader object, which maintain the source code strings that define a shader shader = glCreateShader ( type ); if ( shader == 0 ) return 0; // Replaces the source code in a shader object glShaderSource ( shader, 1, &shaderSrc, NULL ); // Compile the shader object glCompileShader ( shader ); // Check the shader object compile status glGetShaderiv ( shader, GL_COMPILE_STATUS, &compiled ); if ( !compiled ) { GLint infoLen = 0; glGetShaderiv ( shader, GL_INFO_LOG_LENGTH, &infoLen ); if ( infoLen > 1 ) { char* infoLog = malloc (sizeof(char) * infoLen ); glGetShaderInfoLog ( shader, infoLen, NULL, infoLog ); esLogMessage ( "Error compiling shader:\n%s\n", infoLog ); free ( infoLog ); }glDeleteShader ( shader ); return 0; }return shader; }

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：

GLuint LoadProgram ( const char *vShaderStr, const char *fShaderStr ) { GLuint vertexShader; GLuint fragmentShader; GLuint programObject; GLint linked; // Load the vertex/fragment shaders vertexShader = LoadShader ( GL_VERTEX_SHADER, vShaderStr ); fragmentShader = LoadShader ( GL_FRAGMENT_SHADER, fShaderStr ); // Create the program object programObject = glCreateProgram ( ); if ( programObject == 0 ) return 0; // Attaches a shader object to a program object glAttachShader ( programObject, vertexShader ); glAttachShader ( programObject, fragmentShader ); // Bind vPosition to attribute 0 glBindAttribLocation ( programObject, 0, "vPosition" ); // Link the program object glLinkProgram ( programObject ); // Check the link status glGetProgramiv ( programObject, GL_LINK_STATUS, &linked ); if ( !linked ) { GLint infoLen = 0; glGetProgramiv ( programObject, GL_INFO_LOG_LENGTH, &infoLen ); if ( infoLen > 1 ) { char* infoLog = malloc (sizeof(char) * infoLen ); glGetProgramInfoLog ( programObject, infoLen, NULL, infoLog ); esLogMessage ( "Error linking program:\n%s\n", infoLog ); free ( infoLog ); }glDeleteProgram ( programObject ); return GL_FALSE; } // Free no longer needed shader resources glDeleteShader ( vertexShader ); glDeleteShader ( fragmentShader ); return programObject; }

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：

int CreateProgram(ESContext * esContext) { GLuint programObject; GLbyte vShaderStr[] = "attribute vec4 vPosition; \n" "void main()\n" "{\n" "gl_Position = vPosition; \n" "}\n"; GLbyte fShaderStr[] = "precision mediump float; \n"\ "void main()\n" "{\n" "gl_FragColor = vec4 ( 1.0, 0.0, 0.0, 1.0 ); \n" "}\n"; // Create user data esContext->userData = https://www.it610.com/article/malloc(sizeof(UserData)); UserData *userData = https://www.it610.com/article/esContext->userData; // Load the shaders and get a linked program object programObject = LoadProgram ( (const char*)vShaderStr, (const char*)fShaderStr ); if(programObject == 0) { return GL_FALSE; }// Store the program object userData->programObject = programObject; // Get the attribute locations userData->positionLoc = glGetAttribLocation ( g_programObject, "v_position" ); glClearColor ( 0.0f, 0.0f, 0.0f, 1.0f ); return 0; }

4. 安装并执行Program（第三步）

void Render ( ESContext *esContext ) { UserData *userData = https://www.it610.com/article/esContext->userData; GLfloat vVertices[] = {0.0f,0.5f, 0.0f, -0.5f, -0.5f, 0.0f, 0.5f, -0.5f, 0.0f }; // Set the viewport glViewport ( 0, 0, esContext->width, esContext->height ); // Clear the color buffer glClear ( GL_COLOR_BUFFER_BIT ); // Use the program object glUseProgram ( userData->programObject ); // Load the vertex data glVertexAttribPointer ( 0, 3, GL_FLOAT, GL_FALSE, 0, vVertices ); glEnableVertexAttribArray ( 0 ); glDrawArrays ( GL_TRIANGLES, 0, 3 ); 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( GLclampfred, GLclampfgreen,GLclampfblue,GLclampfalpha);
2）glClearIndex
指定color index buffer清除值。void glClearIndex( GLfloatc);
3）glClearDepth
指定depth buffer的清除值，取值范围为:0~1，默认值为1。
void glClearDepth( GLclampddepth);
4）glClearStencil
指定stencil buffer清除值的索引，初始值为0。void glClearStencil( GLints);
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：

void glVertexAttribPointer( GLuintindex,// 指示将被修改的通用顶点属性的索引 GLintsize,// 指点每个顶点元素个数(1~4) GLenumtype,// 数组中每个元素的数据类型 GLbooleannormalized,//指示定点数据值是否被归一化(归一化：GL_TRUE,直接使用:GL_FALSE) GLsizeistride,// 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 const GLvoid *pointer); //顶点数组 //注:其index应该小于#define GL_MAX_VERTEX_ATTRIBS0x8869

4.4 glEnableVertexAttribArray
Enable由索引index指定的通用顶点属性数组。
void glEnableVertexAttribArray( GLuintindex);
void glDisableVertexAttribArray( GLuintindex);
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( GLenummode,
GLintfirst,
GLsizeicount);
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：

int main(int argc, char** argv) { ESContext esContext; UserDatauserData; int iFrames; unsigned long iStartTime,iEndTime; int iDeltaTime; memset( &esContext, 0, sizeof( ESContext) ); esContext.userData = https://www.it610.com/article/&userData; esContext.width = 1280; esContext.height = 720; // Init EGL display, surface and context if(!InitEGL(&esContext)) { printf("Init EGL fail\n"); return GL_FALSE; } // compile shader, link program if(!CreateProgram(&esContext)) { printf("Create Program fail\n"); return GL_FALSE; }iStartTime = GetCurTime(); iFrames = 0; while(1) {// render a frame Render(&esContext); iFrames++; iEndTime = GetCurTime(); iDeltaTime= iEndTime - iStartTime; if(iDeltaTime >= 5000) { iStartTime = iEndTime; float fFrame = iFrames * 1000.0 / iDeltaTime; iFrames = 0; printf("Frame: %f\n", fFrame); } } glDeleteProgram (esContext.userData->programObject); return GL_TRUE; }