OpenGL进阶(八) - GLSL入门

简介

        随着图形硬件的发展，渲染管线由固定不可更改想着可编程和更平滑的方向不断发展。越来越多的基于GPU的编程语言开始出现,cg,cuda,各种着色语言等等。

        今天要介绍的就是和OpenGL结合非常紧密的GLSL（OpenGL Shading Language）. 通过OpenGL的API我们可以绘制图元，变换图形等等，当并不能改变基础的渲染管线。在OpenGL中使用GLSL，就能将渲染管线中固定的功能阶段转变成可编程的。

        编程环境：Ubuntu12.04 32bit  GTX480

GLSL简介

         OpenGL着色语言（GLSL――OpenGL Shading Language）是用来在OpenGL中着色编程的语言，也即开发人员写的短小的自定义程序，他们是在图形卡的GPU （Graphic Processor Unit图形处理单元）上执行的，代替了固定的渲染管线的一部分。比如：视图转换、投影转换等。GLSL（GL Shading Language）的着色器代码分成2个部分：Vertex Shader（顶点着色器）和Fragment（片断着色器），有时还会有Geometry Shader（几何着色器）。负责运行顶点着色的是顶点着色器。它可以得到当前OpenGL 中的状态，GLSL内置变量进行传递。它拥有一下的一些特点：

1.是一种高级的过程式语言；

2.作为OpenGL标准的一个部分，也就意味着开源，跨平台；

3.基于C和C++的语法和流程控制；

4.天然支持向量和矩阵的运算；

5.比C和C++更加严格的变量控制；

6.使用变量来处理输入和输出而不是读写文档；

7.Shader的长度并没有限制，也没有必要去查询。

     为什要使用OpenGL shader？

1.增加材料的真材实感 - 石头，草地，木头等等；

2.增加光照效果的真材实感 - 面光源，软阴影等等；

3.高级的渲染效果 - 全局照明，光线追踪等等；

4.非真实的材质 - 模拟画笔效果，钢笔绘制效果等等；

5.阶段贴图 - 动态生成2D和3D的纹理，而不是静态的图像；

6.图像处理 - 卷积，遮罩，复杂混合等；

7.动态效果 - 关键帧插值，粒子系统，动画；

8.可编程反走样方法；

9.通用计算 - 排序，数学建模，流体计算；

这些特性在使用opengl的时候可能可以去实现，但是都会有些局限，而现在，通过shader,我们可以通过显卡的硬件加速来显著增加渲染的速度，同时可以解放CPU。

写一个简单的Shader

首先来看一下电脑的OpenGL环境，终端运行:

glxinfo | grep OpenGL

基于SDL的OpenGL已经安装好(参考这里：SDL入门学习)，接下来需要安装一下OpenGL的扩展库。

sudo apt-get install glew-utils libglew1.6

这次先绘制一个简单的矩形。

在工程文件夹下创建一个basic.vert,作为vertex shader.

#version 400in vec3 VertexPosition;in vec3 VertexColor;out vec3 Color;void main(){Color = VertexColor;gl_Position = vec4( VertexPosition, 1.0);}

·in – for input parameters
·out – for outputs of the function. The returnstatement is also an option for sending the result of a function.
·inout – for parameters that are both input andoutput of a function (新版本的GLSL似乎已经废除)

再创建一个basic.frag，作为fragment shader.

#version 400void main(void){gl_FragColor = vec4(0.0, 1.0, 0.0, 1.0);}

创建main.c,代码如下：

/*****************************************************************************Copyright: 2013, ustc All rights reserved.contact:[e-mail] name: main.cDescription:Using opengl shading language in SDL.Author:Silang QuanVersion: 1.0Date: 2013.7.30*****************************************************************************/#include <SDL/SDL.h>#include <GL/glew.h>#include <GL/gl.h>#include <GL/glu.h>#include <stdio.h>#include <stdlib.h>const int SCREEN_WIDTH = 800;const int SCREEN_HEIGHT =800;const int SCREEN_BPP = 32;SDL_Surface *screen;//Whether the window is windowed or notbool windowed;//Whether the window is finebool windowOK;//Handler for GLSL programGLuint programHandle;GLuint vShader;GLuint fShader;void quit( int code ){    SDL_Quit( );    /* Exit program. */    exit( code );}char *textFileRead(char *fn) {     FILE *fp;    char *content = NULL;     int count=0;     if (fn != NULL) {        fp = fopen(fn,"rt");         if (fp != NULL) {       fseek(fp, 0, SEEK_END);      count = ftell(fp);      rewind(fp);             if (count > 0) {                content = (char *)malloc(sizeof(char) * (count+1));                count = fread(content,sizeof(char),count,fp);                content[count] = '\0';            }            fclose(fp);        }    }    return content;}void toggle_fullscreen(){//If the screen is windowedif( windowed == true ){//Set the screen to fullscreenscreen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_OPENGL|SDL_RESIZABLE| SDL_FULLSCREEN );//If there's an errorif( screen == NULL ){windowOK = false;return;}//Set the window state flagwindowed = false;}//If the screen is fullscreenelse if( windowed == false ){//Window the screenscreen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_OPENGL|SDL_RESIZABLE );//If there's an errorif( screen == NULL ){windowOK = false;return;}//Set the window state flagwindowed = true;}}void handleKeyEvent( SDL_keysym* keysym ){    switch( keysym->sym ){    case SDLK_ESCAPE:        quit( 0 );        break;    case SDLK_SPACE:        break;    case SDLK_F1:toggle_fullscreen();break;    default:        break;    }}void resizeGL(int width,int height){    if ( height == 0 )    {        height = 1;    }    //Reset View    glViewport( 0, 0, (GLint)width, (GLint)height );    //Choose the Matrix mode    glMatrixMode( GL_PROJECTION );    //reset projection    glLoadIdentity();    //set perspection    gluPerspective( 45.0, (GLfloat)width/(GLfloat)height, 0.1, 100.0 );    //choose Matrix mode    glMatrixMode( GL_MODELVIEW );    glLoadIdentity();}void handleEvents(){    // Our SDL event placeholder.    SDL_Event event;    //Grab all the events off the queue.    while( SDL_PollEvent( &event ) ) {        switch( event.type ) {        case SDL_KEYDOWN:            // Handle key Event            handleKeyEvent( &event.key.keysym );            break;        case SDL_QUIT:            // Handle quit requests (like Ctrl-c).            quit( 0 );            break;        case SDL_VIDEORESIZE://Handle resize event            screen = SDL_SetVideoMode(event.resize.w, event.resize.h, 16,                                      SDL_OPENGL|SDL_RESIZABLE);            if ( screen )            {                resizeGL(screen->w, screen->h);            }            break;        }    }}void initSDL(int width,int height,int bpp,int flags){    // First, initialize SDL's video subsystem.    if( SDL_Init( SDL_INIT_VIDEO ) < 0 )    {        fprintf( stderr, "Video initialization failed: %s\n",                 SDL_GetError( ) );        quit( 1 );    }    atexit(SDL_Quit);//Set some Attribute of OpenGL in SDL    SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 5 );    SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 5 );    SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 5 );    SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 16 );    SDL_GL_SetAttribute( SDL_GL_DOUBLEBUFFER, 1 );    //Set the video mode    screen= SDL_SetVideoMode( width, height, bpp,flags);    if(!screen )    {        fprintf( stderr, "Video mode set failed: %s\n",SDL_GetError( ) );        quit( 1 );    windowed=false;}else windowed=true;    resizeGL(screen->w, screen->h);    //Set caption    SDL_WM_SetCaption( "OpenGL Shading Language Test", NULL );    }void initShader(){vShader = glCreateShader( GL_VERTEX_SHADER );fShader = glCreateShader( GL_FRAGMENT_SHADER );printf("Here\n");if(0 == vShader || 0 == fShader){fprintf(stderr, "Error creating vertex shader.\n");quit(1);}GLchar* vShaderCode = textFileRead("basic.vert");GLchar* fShaderCode = textFileRead("basic.frag");const GLchar* vCodeArray[1] = {vShaderCode};const GLchar* fCodeArray[1] = {fShaderCode};glShaderSource(vShader, 1, vCodeArray, NULL);glShaderSource(fShader, 1, fCodeArray, NULL);glCompileShader(vShader);glCompileShader(fShader);free(vShaderCode);free(fShaderCode);//const GLchar* codeArray[] = {shaderCode};//Check the compile resultGLint logLen;glGetShaderiv(vShader, GL_INFO_LOG_LENGTH, &logLen);if(logLen > 0){char *log = (char *)malloc(logLen);GLsizei written;glGetShaderInfoLog(vShader, logLen, &written, log);printf("Shader compile error log: %s\n",log);free(log);}programHandle = glCreateProgram();if(0 == programHandle){fprintf(stderr, "Error creating programHandle.\n");quit(1);}glAttachShader(programHandle, vShader);glAttachShader(programHandle, fShader);glLinkProgram(programHandle);//glUseProgram(programHandle);}void freeShader(){glDetachShader(programHandle, fShader);glDetachShader(programHandle, vShader);glDeleteShader(fShader);glDeleteShader(vShader);//glDetachShader(fShader);//glDetachShader(vShader);//glDetachShader(programHandle);}void renderGL(){/* These are to calculate our fps */    static GLint T0     = 0;static GLint Frames = 0;    // Clear the color and depth buffers.    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );    // We don't want to modify the projection matrix. */    glMatrixMode( GL_MODELVIEW );    glLoadIdentity( );    // Move down the z-axis.    glTranslatef( 0.0, 0.0, -5.0 );//Draw a square    glUseProgram(programHandle);    glBegin(GL_QUADS);            glVertex2f(-0.5f, -0.5f);            glVertex2f( 0.5f, -0.5f);            glVertex2f( 0.5f,  0.5f);            glVertex2f(-0.5f,  0.5f);    glEnd();        // Unbind shader    glUseProgram(0);    SDL_GL_SwapBuffers( );        /* Gather our frames per second */    Frames++;    {GLint t = SDL_GetTicks();if (t - T0 >= 5000) {    GLfloat seconds = (t - T0) / 1000.0;    GLfloat fps = Frames / seconds;    printf("%d frames in %g seconds = %g FPS\n", Frames, seconds, fps);    T0 = t;    Frames = 0;}    }}void initGL( int width, int height ){    float ratio = (float) width / (float) height;    // Our shading model--Gouraud (smooth).    glShadeModel( GL_SMOOTH );    // Set the clear color.    glClearColor( 0, 0, 0, 0 );    // Setup our viewport.    glViewport( 0, 0, width, height );    //Change to the projection matrix and set our viewing volume.    glMatrixMode( GL_PROJECTION );    glLoadIdentity();    gluPerspective( 60.0, ratio, 1.0, 100.0 );}int main( int argc, char* argv[] ){// Color depth in bits of our window.int flags= SDL_OPENGL|SDL_RESIZABLE;//Set the SDLinitSDL(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP,flags);if(glewInit() != GLEW_OK) exit(EXIT_FAILURE);//Init vertext shaderinitShader();//Set the OpenGLinitGL(SCREEN_WIDTH, SCREEN_HEIGHT );    //main loop    while(true){        /* Process incoming events. */        handleEvents( );        /* Draw the screen. */        renderGL( );    }    // Free Shader    freeShader();    return 0;}

主要是增加了几个关于Shader的函数，initShader用于shader的初始化，freeShader用于删除shader，释放内存。使用shader之前还需要调用glewInit来初始化glew。

终端编译命令：

g++ main.c -o main -lSDL -lGL -lGLU -lGLEW

解释一下几个相关的API。

GLuint glCreateShader(GLenum shaderType);Parameter:shaderType – GL_VERTEX_SHADER, GL_GEOMETRY_SHADER, GL_TESS_CONTROL_SHADER, GL_TESS_EVALUATION_SHADER, or GL_FRAGMENT_SHADER.Return Value:the shader handler

void glShaderSource(GLuint shader, int numOfStrings, const char **strings, int *lengthOfStrings);Parameters:shader – the handler to the shader.numOfStrings – the number of strings in the array.strings – the array of strings.lengthOfStrings – an array with the length of each string, or NULL, meaning that the strings are NULL terminated.

void glCompileShader(GLuint shader);Parameters:shader – the handler to the shader.

void glUseProgram(GLuint program); Installs a program object as part of current rendering stateParameters：programSpecifies the handle of the program object whose executables are to be used as part of current rendering state.

...

整个opengl程序执行的流程如下：

更多函数参考OpenGL reference - http://www.opengl.org/sdk/docs/man/

参考

OpenGL 4.0 Shading Language Cookbook

GLSL Core Tutorial – Creating a Shader - http://www.lighthouse3d.com/tutorials/glsl-core-tutorial/creating-a-shader/

Hello GLSL - http://sindney.com/blog/posts/hello-glsl/

OpenGL reference - http://www.opengl.org/sdk/docs/man/