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 400
in 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 400
void 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:k283228391@126.com
File name: main.c
Description:Using opengl shading language in SDL.
Author:Silang Quan
Version: 1.0
Date: 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 not
bool windowed;
//Whether the window is fine
bool windowOK;
//Handler for GLSL program
GLuint 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 windowed
	if( windowed == true )
	{
		//Set the screen to fullscreen
		screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_OPENGL|SDL_RESIZABLE| SDL_FULLSCREEN );

		//If there's an error
		if( screen == NULL )
		{
			windowOK = false;
			return;
		}

		//Set the window state flag
		windowed = false;
	}
	//If the screen is fullscreen
	else if( windowed == false )
	{
		//Window the screen
		screen = SDL_SetVideoMode( SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP, SDL_OPENGL|SDL_RESIZABLE );

		//If there's an error
		if( screen == NULL )
		{
			windowOK = false;
			return;
		}

		//Set the window state flag
		windowed = 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 result
	GLint 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 SDL
	initSDL(SCREEN_WIDTH, SCREEN_HEIGHT, SCREEN_BPP,flags);
	if(glewInit() != GLEW_OK) exit(EXIT_FAILURE);
	//Init vertext shader
	initShader();
	//Set the OpenGL
	initGL(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 state
Parameters：
program
Specifies 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/