OpenGL ES2.0编程三步曲 -转

原地址：http://blog.csdn.net/myarrow/article/details/7707943

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...
   GLint       width;                          // Window width
   GLint       height;                         // Window height
   EGLNativeWindowType  hWnd;  // Window handle
   EGLDisplay  eglDisplay;             // EGL display
   EGLContext  eglContext;            // EGL context
   EGLSurface  eglSurface;            // 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

       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:
%s
", 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:
%s
", 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;    
"
      "void main()                  
"
      "{                            
"
      "   gl_Position = vPosition;  
"
      "}                            
";

     GLbyte fShaderStr[] =
      "precision mediump float;
"
      "void main()                                  
"
      "{                                            
"
      "  gl_FragColor = vec4 ( 1.0, 0.0, 0.0, 1.0 );
"
      "}                                                    
";

    // Create user data
    esContext->userData = malloc(sizeof(UserData));
    UserData *userData = 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 = 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( 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：

void glVertexAttribPointer(
      GLuint   index,           // 指示将被修改的通用顶点属性的索引
       GLint   size,             // 指点每个顶点元素个数(1~4)
      GLenum   type,            // 数组中每个元素的数据类型
       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)
      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
       const GLvoid *   pointer);//顶点数组
: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：   

int main(int argc, char** argv)
{
    ESContext esContext;
    UserData  userData;
    int iFrames;
    unsigned long iStartTime,iEndTime;
    int iDeltaTime;

    memset( &esContext, 0, sizeof( ESContext) );
    esContext.userData = &userData;

    esContext.width = 1280;
    esContext.height = 720;
    // Init EGL display, surface and context
    if(!InitEGL(&esContext))
    {
        printf("Init EGL fail
");
        return GL_FALSE;
    }
    // compile shader, link program
    if(!CreateProgram(&esContext))
    {
        printf("Create Program fail
");
        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
", fFrame);
    }
    }
    glDeleteProgram (esContext.userData->programObject);
    return GL_TRUE;
}