01 三角形的绘制

#include "esUtil.h"
 
typedef struct
{
   // Handle to a program object
   GLuint programObject;
 
} UserData;
 
///
// 加载着色器源代码，返回一个着色器的对象 GLuint
//
GLuint LoadShader ( GLenum type, const char *shaderSrc )
{
   GLuint shader;
   GLint compiled;
 
   // 创建一个着色器对象
   shader = glCreateShader ( type );
 
   if ( shader == 0 )
   {
      return 0;
   }
 
   // 加载着色器源代码，并输出一个着色器对象：shader
   glShaderSource ( shader, 1, &shaderSrc, NULL );
 
   // 编译着色器
   glCompileShader ( shader );
 
   // 检查编译的状态，将结果输出给：compiled
   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;
 
}
 
///
// Initialize the shader and program object
//
int Init ( ESContext *esContext )
{
   UserData *userData = esContext->userData;
    //顶点着色器
//    vec4 vPosition ： 声明一个输入属性数组
//    layout(location = 0) 限定符表示这个变量的位置是顶点属性0
   char vShaderStr[] =
      "#version 300 es                          \n"
      "layout(location = 0) in vec4 vPosition;  \n"
      "void main()                              \n"
      "{                                        \n"
      "   gl_Position = vPosition;              \n"
      "}                                        \n";
 
    //片元着色器
    //
   char fShaderStr[] =
      "#version 300 es                              \n"
      "precision mediump float;                     \n"
      "out vec4 fragColor;                          \n"
      "void main()                                  \n"
      "{                                            \n"
      "   fragColor = vec4 ( 0.2, 0.3, 0.7, 1.0 );  \n"
      "}                                            \n";
 
   GLuint vertexShader;
   GLuint fragmentShader;
   GLuint programObject;
   GLint linked;
 
   // 创建顶点着色器对象
   vertexShader = LoadShader ( GL_VERTEX_SHADER, vShaderStr );
    
    // 创建片段着色器对象
   fragmentShader = LoadShader ( GL_FRAGMENT_SHADER, fShaderStr );
 
   // 创建一个空的程序对象，链接顶点着色器，片段着色器，不同的着色器源码编译成着色器对象后，他们必须连接到一个程序对象并一起链接，才能绘制图形
   programObject = glCreateProgram ( );
 
   if ( programObject == 0 )
   {
      return 0;
   }
 
    // 将着色器shader添加到program
   glAttachShader ( programObject, vertexShader );
   glAttachShader ( programObject, fragmentShader );
 
   // 把片元着色器和顶点着色器连接到程序对象， 创建可执行的 OpenGL ES program
   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 FALSE;
   }
 
   // Store the program object
   userData->programObject = programObject;
 
   glClearColor ( 1.0f, 1.0f, 1.0f, 0.0f );
   return TRUE;
}
 
///
// Draw a triangle using the shader pair created in Init()
//
void Draw ( ESContext *esContext )
{
   UserData *userData = esContext->userData;
   GLfloat vVertices[] = {  -0.5f,  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 );
 
   // 每次绘制之前需要清除颜色缓冲区
   glClear ( GL_COLOR_BUFFER_BIT );
 
   // 绑定程序对象，进行渲染
   glUseProgram ( userData->programObject );
 
//第一个参数指定我们要配置的顶点属性。和顶点着色器对应： layout(location = 0) in vec4 vPosition值(Location)吗？它可以把顶点属性的位置值设置为0。因为我们希望把数据传递到这一个顶点属性中，所以这里我们传入0。
//第二个参数指定顶点属性的大小。顶点属性是一个vec3，它由3个值组成，所以大小是3。
//第三个参数指定数据的类型，这里是GL_FLOAT(GLSL中vec*都是由浮点数值组成的)。
   glVertexAttribPointer ( 0, 3, GL_FLOAT, GL_FALSE, 0, vVertices );
   glEnableVertexAttribArray ( 0 );
 
    // 绘制图元
   glDrawArrays ( GL_TRIANGLES, 0, 3 );
}
 
void Shutdown ( ESContext *esContext )
{
   UserData *userData = esContext->userData;
 
   glDeleteProgram ( userData->programObject );
}
 
int esMain ( ESContext *esContext )
{
   esContext->userData = malloc ( sizeof ( UserData ) );
 
   esCreateWindow ( esContext, "Hello Triangle", 220, 240, ES_WINDOW_RGB );
 
   if ( !Init ( esContext ) )
   {
      return GL_FALSE;
   }
 
   esRegisterShutdownFunc ( esContext, Shutdown );
   esRegisterDrawFunc ( esContext, Draw );
 
   return GL_TRUE;
}