OpenGL ES2.0編程三部曲（轉載自MyArrow）

1. 保存全局變量的數據結構

以下例子程序均基於Linux平臺。

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渲染環境和相關元素（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主要實現以下功能：

       1) 創建Shader對象

       2) 裝載Shader源碼

       3) 編譯Shader

      其實現參考代碼如下：

 

/* 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 
       它創建一個空的shader對象，它用於維護用來定義shader的源碼字符串。支持以下兩種shader:
      （1） GL_VERTEX_SHADER: 它運行在可編程的“頂點處理器”上，用於代替固定功能的頂點處理；
      （ 2） GL_FRAGMENT_SHADER: 它運行在可編程的“片斷處理器”上，用於代替固定功能的片段處理；

2）glShaderSource
        shader對象中原來的源碼全部被新的源碼所代替。

3）glCompileShader
       編譯存儲在shader對象中的源碼字符串，編譯結果被當作shader對象狀態的一部分被保存起來，可通過glGetShaderiv函數獲取編譯狀態。

4）glGetShaderiv
       獲取shader對象參數，參數包括：GL_SHADER_TYPE, GL_DELETE_STATUS, GL_COMPILE_STATUS, GL_INFO_LOG_LENGTH, GL_SHADER_SOURCE_LENGTH.

3.2 LoadProgram

其參考代碼如下：

 

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
      建立一個空的program對象，shader對象可以被連接到program對像
2）glAttachShader
      program對象提供了把需要做的事連接在一起的機制。在一個program中，在shader對象被連接在一起之前，必須先把shader連接到program上。
3）glBindAttribLocation 
       把program的頂點屬性索引與頂點shader中的變量名進行綁定。
4）glLinkProgram
       連接程序對象。如果任何類型爲GL_VERTEX_SHADER的shader對象連接到program,它將產生在“可編程頂點處理器”上可執行的程序；如果任何類型爲GL_FRAGMENT_SHADER的shader對象連接到program,它將產生在“可編程片斷處理器”上可執行的程序。
5）glGetProgramiv
       獲取program對象的參數值，參數有：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中只實現了Shader的編譯，在3.2中只實現了Program的鏈接，現在還缺少真正供進行編譯和鏈接的源碼，其參考代碼如下：

 

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 = 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. 安裝並執行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到預設值。可清除以下四類buffer:

      1）GL_COLOR_BUFFER_BIT

      2）GL_DEPTH_BUFFER_BIT

      3）GL_ACCUM_BUFFER_BIT

      4）GL_STENCIL_BUFFER_BIT

      預設值通過glClearColor, glClearIndex, glClearDepth, glClearStencil, 和glClearAccum來設置。

1）gClearColor

       指定color buffer的清除值，當調用glClear(GL_COLOR_BUFFER_BIT)時才真正用設定的顏色值清除color buffer。參數值的範圍爲: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，取值範圍爲:0~1，默認值爲1。

       void glClearDepth( GLclampd   depth);

4）glClearStencil

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

5）glClearAccum

       指定accumulation buffer的清除值，初始值爲0，取值範圍爲：-1~1

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

4.2 glUseProgram

       安裝一個program object，並把它作爲當前rendering state的一部分。

       1) 當一個可執行程序被安裝到vertex processor，下列OpenGL固定功能將被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) 當一個可執行程序被安裝到fragment processor，下列OpenGL固定功能將被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

       定義一個通用頂點屬性數組。當渲染時，它指定了通用頂點屬性數組從索引index處開始的位置和數據格式。其定義如下：

 

void glVertexAttribPointer(
      GLuint   index,           // 指示將被修改的通用頂點屬性的索引
       GLint   size,             // 指點每個頂點元素個數(1~4)
      GLenum   type,            // 數組中每個元素的數據類型
       GLboolean   normalized,   //指示定點數據值是否被歸一化(歸一化<[-1,1]或[0,1]>：GL_TRUE,直接使用:GL_FALSE)
      GLsizei   stride,         // 連續頂點屬性間的偏移量，如果爲0，相鄰頂點屬性間緊緊相鄰
       const GLvoid *   pointer);//頂點數組
:其index應該小於#define GL_MAX_VERTEX_ATTRIBS               0x8869

4.4 glEnableVertexAttribArray

      Enable由索引index指定的通用頂點屬性數組。

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

      默認狀態下，所有客戶端的能力被disabled，包括所有通用頂點屬性數組。如果被Enable，通用頂點屬性數組中的值將被訪問並被用於rendering，通過調用頂點數組命令：glDrawArrays, glDrawElements, glDrawRangeElements, glArrayElement, glMultiDrawElements, or glMultiDrawArrays.

4.5 glDrawArrays

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

    1) mode:指明render原語，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數組中起始索引。

    3) count: 指明被render的原語個數。

    可以預先使用單獨的數據定義vertex、normal和color，然後通過一個簡單的glDrawArrays構造一系列原語。當調用glDrawArrays時，它使用每個enable的數組中的count個連續的元素，來構造一系列幾何原語，從第first個元素開始。

4.6 eglSwapBuffers

      把EGL surface中的color buffer提交到native window進行顯示。 

      EGLBoolean eglSwapBuffers(EGLDisplay display,EGLSurface surface)

5. 協調組織

    在前面的描述中，三步曲已經完成了：

    1）初始化EGL環境，爲繪圖做好準備

    2）生成Program

    3）安裝並執行Program

    只有這三個關鍵人物，還不能運行，還需要一個協調組織者。其參考代碼如下：   

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\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;
}