iOS开发——图形编程OC篇&OpenGL ES2.0编程步骤

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。

 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[];
      EGLBoolean eRetStatus;
      EGLint majorVer, minorVer;
      EGLint context_attribs[] = {EGL_CONTEXT_CLIENT_VERSION, , EGL_NONE};

      EGLint numConfigs;
      EGLint cfg_attribs[] = {EGL_BUFFER_SIZE,    EGL_DONT_CARE,
                              EGL_DEPTH_SIZE,     ,
                              EGL_RED_SIZE,       ,
                              EGL_GREEN_SIZE,     ,
                              EGL_BLUE_SIZE,      ,
                              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, , &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, , &numConfigs);
      if( eRetStatus != EGL_TRUE  || !numConfigs)
      {
           return EGL_FALSE;
      }

      // Create a new EGL window surface
      surface = eglCreateWindowSurface(display, configs[], 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[], 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,);

      // 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 );

     )
        ;

    // Replaces the source code in a shader object
    glShaderSource ( shader, , &shaderSrc, NULL );

    // Compile the shader object
    glCompileShader ( shader );

    // Check the shader object compile status
    glGetShaderiv ( shader, GL_COMPILE_STATUS, &compiled );

    if ( !compiled )
    {
       GLint infoLen = ;

       glGetShaderiv ( shader, GL_INFO_LOG_LENGTH, &infoLen );

        )
       {
          char* infoLog = malloc (sizeof(char) * infoLen );

          glGetShaderInfoLog ( shader, infoLen, NULL, infoLog );
          esLogMessage ( "Error compiling shader:\n%s\n", infoLog );            

          free ( infoLog );
       }

       glDeleteShader ( shader );
       ;
    }

    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 ( );
     )
       ;

    // Attaches a shader object to a program object
    glAttachShader ( programObject, vertexShader );
    glAttachShader ( programObject, fragmentShader );
    // Bind vPosition to attribute 0  
    glBindAttribLocation ( programObject, , "vPosition" );
    // Link the program object
    glLinkProgram ( programObject );

    // Check the link status
    glGetProgramiv ( programObject, GL_LINK_STATUS, &linked );

    if ( !linked )
    {
       GLint infoLen = ;

       glGetProgramiv ( programObject, GL_INFO_LOG_LENGTH, &infoLen );

        )
       {
          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 = malloc(sizeof(UserData));
     UserData *userData = esContext->userData;

     // Load the shaders and get a linked program object
     programObject = LoadProgram ( (const char*)vShaderStr, (const char*)fShaderStr );
     )
     {
     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 );
     ;
 }

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 ( , , 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 ( , , GL_FLOAT, GL_FALSE, , vVertices );
    glEnableVertexAttribArray (  );
    glDrawArrays ( GL_TRIANGLES, ,  );
    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
 //注:其index应该小于#define GL_MAX_VERTEX_ATTRIBS&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 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, , sizeof( ESContext) );
     esContext.userData = &userData;

     esContext.width = ;
     esContext.height = ;
     // 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 = ;

     )
     {    // render a frame
          Render(&esContext);
          iFrames++;

          iEndTime = GetCurTime();
     iDeltaTime  = iEndTime - iStartTime;
     )
     {
               iStartTime = iEndTime;
         float fFrame = iFrames * 1000.0 / iDeltaTime;
         iFrames = ;

         printf("Frame: %f\n", fFrame);
     }
     }
     glDeleteProgram (esContext.userData->programObject);
     return GL_TRUE;
 }
 
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