opengles2.0 headfirst Simple_VertexShader

在这篇文章里记录笔者在阅读《OpenGL(R) ES 2.0 Programming Guide》1-8章里面觉得重要的知识点，在开始分析代码之前，先分析两个api调用：

void glDrawArrays(GLenum  mode , GLint  first , GLsizei  count )
mode specifies the primitive to render. Valid values are:
GL_POINTS
GL_LINES
GL_LINE_STRIP
GL_LINE_LOOP
GL_TRIANGLES
GL_TRIANGLE_STRIP
GL_TRIANGLE_FAN
first specifies the starting vertex index in the enabled vertex arrays
count specifies the number of indices to be drawn.

void glDrawElements(GLenum  mode , GLsizei  count ,
GLenum  type , const GLvoid * indices )
mode specifies the primitive to render. Valid values are:
GL_POINTS
GL_LINES
GL_LINE_STRIP
GL_LINE_LOOP
GL_TRIANGLES
GL_TRIANGLE_STRIP
GL_TRIANGLE_FAN
count specifies the number of indices
type specifies the type of element indices stored in indices.Valid
values are:
GL_UNSIGNED_BYTE
GL_UNSIGNED_SHORT
GL_UNSIGNED_INT —optional (can be used only if the
OES_element_index_uint extension is implemented)
indices specifies a pointer to location where element indices are stored.

大致概括下这块书上的分析，glDrawArrays在大量接续的，不共享的图元绘制情况下有很大优势；

但是在现实的绘制场景中(连续而大量共享顶点)，glDrawElements能表现出更优的性能，减少内存和子宽的占用；比如在下面的场景中：

使用glDrawArrays实现，

#define VERTEX_POS_INDX 0#define NUM_FACES 6GLfloat vertices[] = { … }; // (x, y, z) per vertexglEnableVertexAttribArray(VERTEX_POS_INDX);glVertexAttribPointer(VERTEX_POS_INDX, 3, GL_FLOAT, GL_FALSE,0, vertices);for (i=0; i<NUM_FACES; i++){glDrawArrays(GL_TRIANGLE_FAN, first, 4);first += 4;}

或者

glDrawArrays(GL_TRIANGLES, 0, 36);

为了绘制这个立方体，需要为每个面调用一次glDrawArrays，并且要为每个复用顶点进行复制，意味着本来的8个顶点，我们需要分配24个顶点的内存。或者36个顶点内存。

使用glDrawElements实现，

#define VERTEX_POS_INDX 0GLfloat vertices[] = { … };// (x, y, z) per vertexGLubyte indices[36] = { 0, 1, 2, 0, 2, 3,0, 3, 4, 0, 4, 5,0, 5, 6, 0, 6, 1,7, 6, 1, 7, 1, 2,7, 4, 5, 7, 5, 6,7, 2, 3, 7, 3, 4 };glEnableVertexAttribArray(VERTEX_POS_INDX);glVertexAttribPointer(VERTEX_POS_INDX, 3, GL_FLOAT, GL_FALSE,0, vertices);glDrawElements(GL_TRIANGLES, sizeof(indices)/sizeof(GLubyte),GL_UNSIGNED_BYTE, indices);

很多因素会导致GPU处理glDrawElements过程要比前者快很多，比如vertices被glDrawElements复用，所以在vertex里面需要的vertex attribute 会相应减少。并且可以减少glDrawElements节约了顶点的内存占用。

接下来我们分析下书本上的几个例子,这里贴出相关Opengles的部分，因为EGL部分都是通用的esutils来实现的，源码里面。书本上有几个知识点没有在例子代码上写出来，文章后面我单独列一下。

Simple_VertexShader：

//// Book:      OpenGL(R) ES 2.0 Programming Guide// Authors:   Aaftab Munshi, Dan Ginsburg, Dave Shreiner// ISBN-10:   0321502795// ISBN-13:   9780321502797// Publisher: Addison-Wesley Professional// URLs:      http://safari.informit.com/9780321563835//            http://www.opengles-book.com//// Simple_VertexShader.c////    This is a simple example that draws a rotating cube in perspective//    using a vertex shader to transform the object//#include <stdlib.h>#include "esUtil.h"typedef struct{   // Handle to a program object   GLuint programObject;   // Attribute locations   GLint  positionLoc;   // Uniform locations   GLint  mvpLoc;      // Vertex daata   GLfloat  *vertices;   GLuint   *indices;   int       numIndices;   // Rotation angle   GLfloat   angle;   // MVP matrix   ESMatrix  mvpMatrix;} UserData;///// Initialize the shader and program object//int Init ( ESContext *esContext ){   UserData *userData = esContext->userData;   GLbyte vShaderStr[] =        "uniform mat4 u_mvpMatrix;                   \n"      "attribute vec4 a_position;                  \n"      "void main()                                 \n"      "{                                           \n"      "   gl_Position = u_mvpMatrix * a_position;  \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";   // Load the shaders and get a linked program object   userData->programObject = esLoadProgram ( vShaderStr, fShaderStr );   // Get the attribute locations   userData->positionLoc = glGetAttribLocation ( userData->programObject, "a_position" );   // Get the uniform locations   userData->mvpLoc = glGetUniformLocation( userData->programObject, "u_mvpMatrix" );      // Generate the vertex data   userData->numIndices = esGenCube( 1.0, &userData->vertices,                                     NULL, NULL, &userData->indices );      // Starting rotation angle for the cube   userData->angle = 45.0f;   glClearColor ( 0.0f, 0.0f, 0.0f, 0.0f );   return TRUE;}///// Update MVP matrix based on time//void Update ( ESContext *esContext, float deltaTime ){   UserData *userData = (UserData*) esContext->userData;   ESMatrix perspective;   ESMatrix modelview;   float    aspect;      // Compute a rotation angle based on time to rotate the cube   userData->angle += ( deltaTime * 40.0f );   if( userData->angle >= 360.0f )      userData->angle -= 360.0f;   // Compute the window aspect ratio   aspect = (GLfloat) esContext->width / (GLfloat) esContext->height;      // Generate a perspective matrix with a 60 degree FOV   esMatrixLoadIdentity( &perspective );   esPerspective( &perspective, 60.0f, aspect, 1.0f, 20.0f );   // Generate a model view matrix to rotate/translate the cube   esMatrixLoadIdentity( &modelview );   // Translate away from the viewer   esTranslate( &modelview, 0.0, 0.0, -2.0 );   // Rotate the cube   esRotate( &modelview, userData->angle, 1.0, 0.0, 1.0 );      // Compute the final MVP by multiplying the    // modevleiw and perspective matrices together   esMatrixMultiply( &userData->mvpMatrix, &modelview, &perspective );}///// Draw a triangle using the shader pair created in Init()//void Draw ( ESContext *esContext ){   UserData *userData = esContext->userData;      // 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 position   glVertexAttribPointer ( userData->positionLoc, 3, GL_FLOAT,                            GL_FALSE, 3 * sizeof(GLfloat), userData->vertices );      glEnableVertexAttribArray ( userData->positionLoc );         // Load the MVP matrix   glUniformMatrix4fv( userData->mvpLoc, 1, GL_FALSE, (GLfloat*) &userData->mvpMatrix.m[0][0] );      // Draw the cube   glDrawElements ( GL_TRIANGLES, userData->numIndices, GL_UNSIGNED_INT, userData->indices );   eglSwapBuffers ( esContext->eglDisplay, esContext->eglSurface );}///// Cleanup//void ShutDown ( ESContext *esContext ){   UserData *userData = esContext->userData;   if ( userData->vertices != NULL )   {      free ( userData->vertices );   }   if ( userData->indices != NULL )   {      free ( userData->indices );   }   // Delete program object   glDeleteProgram ( userData->programObject );}int main ( int argc, char *argv[] ){   ESContext esContext;   UserData  userData;   esInitContext ( &esContext );   esContext.userData = &userData;   esCreateWindow ( &esContext, "Simple Texture 2D", 320, 240, ES_WINDOW_RGB );      if ( !Init ( &esContext ) )      return 0;   esRegisterDrawFunc ( &esContext, Draw );   esRegisterUpdateFunc ( &esContext, Update );      esMainLoop ( &esContext );   ShutDown ( &esContext );}