OpenGL中shader使用

       学了接近一个月的OpenGL，终于要排上用场了...好吧，就从学到的shader（着色器）开刀吧。

       先简单的介绍shader，shader其实是显卡的功能，就是利用显卡的GPU去做图像处理的工作，而不是CPU，这样可以在一些复杂的大程序中释放CPU空间而提高效率。这篇文章只是简单的介绍shader的使用，并没有介绍着色语言的语法结构等方面内容。后面等自己研究好了继续更新。

       使用shader，一般要经过一下几个步骤：

1、创建shader，这里会使用到glew的拓展库，应该包含glew.h和glew32.lib。利用一下函数创建：

GLhandleARB frag_shader;
//创建fragment Shader
//--------------------------------------------------------------------
// GLuint glCreateShader(GLenum shaderType);  
// 参数：  
// ·shaderType – GL_VERTEX_SHADER or GL_FRAGMENT_SHADER.  
//--------------------------------------------------------------------
frag_shader = glCreateShaderObjectARB(GL_FRAGMENT_SHADER_ARB);

2、向shader中导入着色程序，利用一下函数：

//---------------------------------------------------------------------------------------------------------------------
//void glShaderSource(GLuint shader, int numOfStrings, const char **strings, int *lenOfStrings);  
//参数：  
//·shader – the handler to the shader.  
//·numOfStrings – the number of strings in the array.  
//·strings – the array of strings.  
//·lenOfStrings – an array with the length of each string, or NULL, meaning that the strings are NULL terminated. 
//----------------------------------------------------------------------------------------------------------------------
glShaderSourceARB(frag_shader,1,(const char**)&inShader,NULL);

3、编译shader，就是将着色程序编译成shader可以识别的程序，以便使用：

//---------------------------------------------------------
//void glCompileShader(GLuint shader);  
//参数：  
//.shader – the handler to the shader.  
//----------------------------------------------------------
glCompileShaderARB(frag_shader);

4、查询shader的状态，shader并不能向C/C++编译器那样可以print信息出来以参考，但是可以通过logInfo来看：

glGetObjectParameterivARB(frag_shader,GL_OBJECT_COMPILE_STATUS_ARB,&result);

if(!result)
{
	char str[4096] = {0};
	//------------------------------------------------------------------------------------
	//将调试信息放入infoLog中
	//void glGetShaderInfoLog(GLuint object, int maxLen, int *len, char *log);
	//void glGetProgramInfoLog(GLuint object, int maxLen, int *len, char *log);
	//参数：
	//·object – the handler to the object. Either a shader or a program
	//·maxLen – The maximum number of chars to retrieve from the InfoLog.
	//·len – returns the actual length of the retrieved InfoLog.
	//·log – The log itself.
	//------------------------------------------------------------------------------
	glGetInfoLogARB(frag_shader,sizeof(str),NULL,str);
	glDeleteObjectARB(frag_shader);
	return ;
}

5、连接shader，前面只是把着色程序编译OK，现在要给创建好的shader使用：

shader = link(1,frag_shader);

6、link()函数是我自己写的，下面是link函数的代码：

GLhandleARB CShader::link(int shader_count,...)
{
	//创建一个程序
	GLhandleARB program = glCreateProgramObjectARB();
	GLhandleARB shaderT = NULL;

	va_list marker;

	//初始化
	va_start(marker,shader_count);
	for(int i = 0; i< shader_count; i++)
	{
		shaderT = va_arg(marker,GLhandleARB);
		//------------------------------------------------------------
		//void glAttachShader(GLuint program, GLuint shader);  
		//参数：  
		//	·program – the handler to the program.  
		//	·shader – the handler to the shader you want to attach.  
		//-------------------------------------------------------------
		glAttachObjectARB(program,shaderT);
	}
	va_end(marker);

	//连接程序
	glLinkProgramARB(program);

	int result = 0;
	glGetObjectParameterivARB(program,GL_OBJECT_LINK_STATUS_ARB,&result);
	if(!result)
	{
		char str[4096] = {0};
		glGetInfoLogARB(program,sizeof(str),NULL,str);
		glDeleteObjectARB(program);

		return NULL;
	}
	return program;
}

其实这个函数可以将多个着色程序给一个shader使用。

7、后面就是怎么使用Shader了，使用glUseProgram(program)和glUseProgram(NULL)要使用和卸载shader，

     还有可以从C++代码中传递给着色程序的参数。

8、还有就是怎么把着色程序导入到shader中，这里其实是一个纯读取文档的过程，并不难。

这里给出整个shader的程序：

CShader.h

#pragma once 

#ifndef GL_ARB_shader_objects
typedef GLhandleARB
typedef GLcharARB
#endif

class CShader
{
public:
	CShader(void);
public:
	~CShader(void);

public:
	//初始化shader
	void init(const char* inShader);

	//绑定纹理
	void bind();

	//撤销纹理
	void unBind();

	bool setParameter(char* keyword, GLfloat fv);
	bool setParameter(char* keyword, GLuint texID);
	char* loadShaderTex(const char* fileName);


private:
	GLhandleARB link(int shader_count,...);
public:
	GLhandleARB shader;

};

CShader.cpp

#include "stdafx.h"
#include <gl/glew.h>
#include "CShader.h"
#include <stdlib.h>
#include <stdarg.h>

#pragma comment(lib,"glew32.lib")

CShader::CShader(void)
{

}

CShader::~CShader(void)
{

}

void CShader::init(const char* inShader)
{
	GLhandleARB frag_shader;
	//创建fragment Shader
	//--------------------------------------------------------------------
	// GLuint glCreateShader(GLenum shaderType);  
	// 参数：  
	// ·shaderType – GL_VERTEX_SHADER or GL_FRAGMENT_SHADER.  
	//--------------------------------------------------------------------
	frag_shader = glCreateShaderObjectARB(GL_FRAGMENT_SHADER_ARB);

	//---------------------------------------------------------------------------------------------------------------------
	//void glShaderSource(GLuint shader, int numOfStrings, const char **strings, int *lenOfStrings);  
	//参数：  
	//	·shader – the handler to the shader.  
	//	·numOfStrings – the number of strings in the array.  
	//	·strings – the array of strings.  
	//	·lenOfStrings – an array with the length of each string, or NULL, meaning that the strings are NULL terminated. 
	//----------------------------------------------------------------------------------------------------------------------
	glShaderSourceARB(frag_shader,1,(const char**)&inShader,NULL);
	
	//---------------------------------------------------------
	//void glCompileShader(GLuint shader);  
	//参数：  
	//	.shader – the handler to the shader.  
	//----------------------------------------------------------
	glCompileShaderARB(frag_shader);

	int result = 0;
	//查询状态
	glGetObjectParameterivARB(frag_shader,GL_OBJECT_COMPILE_STATUS_ARB,&result);

	if(!result)
	{
		char str[4096] = {0};
		//------------------------------------------------------------------------------------
		//将调试信息放入infoLog中
		//void glGetShaderInfoLog(GLuint object, int maxLen, int *len, char *log);
		//void glGetProgramInfoLog(GLuint object, int maxLen, int *len, char *log);
		//参数：
		//	·object – the handler to the object. Either a shader or a program
		//	·maxLen – The maximum number of chars to retrieve from the InfoLog.
		//	·len – returns the actual length of the retrieved InfoLog.
		//	·log – The log itself.
		//------------------------------------------------------------------------------
		glGetInfoLogARB(frag_shader,sizeof(str),NULL,str);
		glDeleteObjectARB(frag_shader);
		return ;
	}
	//Compile shader
	shader = link(1,frag_shader);
	glDeleteObjectARB(frag_shader);
}

GLhandleARB CShader::link(int shader_count,...)
{
	//创建一个程序
	GLhandleARB program = glCreateProgramObjectARB();
	GLhandleARB shaderT = NULL;

	va_list marker;

	//初始化
	va_start(marker,shader_count);
	for(int i = 0; i< shader_count; i++)
	{
		shaderT = va_arg(marker,GLhandleARB);
		//------------------------------------------------------------
		//void glAttachShader(GLuint program, GLuint shader);  
		//参数：  
		//	·program – the handler to the program.  
		//	·shader – the handler to the shader you want to attach.  
		//-------------------------------------------------------------
		glAttachObjectARB(program,shaderT);
	}
	va_end(marker);

	//连接程序
	glLinkProgramARB(program);

	int result = 0;
	glGetObjectParameterivARB(program,GL_OBJECT_LINK_STATUS_ARB,&result);
	if(!result)
	{
		char str[4096] = {0};
		glGetInfoLogARB(program,sizeof(str),NULL,str);
		glDeleteObjectARB(program);

		return NULL;
	}
	return program;
}

void CShader::bind()
{
	//if shader link successfully,then use it
	glUseProgram( shader);	
}
bool CShader::setParameter(char* keyword,GLfloat fv)
{
	//find out where the flicker constant lives
	GLint ret = glGetUniformLocation(shader, keyword);
	if(ret != -1)
	{
		glUniform1f(ret,fv);
		return true;
	}
	return false;
}

bool CShader::setParameter(char* keyword,GLuint texID)
{
	//Find out where the flicker constant lives
	GLint ret = glGetUniformLocation(shader,keyword);

	if(ret != -1)
	{
		glUniform1iARB(ret, texID);
		return true;
	}
	return false;
}

void CShader::unBind()
{
	glUseProgramObjectARB(NULL);
}

char* CShader::loadShaderTex(const char* fileName)
{
	char* shaderText = NULL;
	GLint shaderLength = 0;
	FILE *fp;

	fp = fopen(fileName,"r");
	if(fp != NULL)
	{
		//get the char length 
		while(fgetc(fp) != EOF)
		{
			shaderLength++;
		}
		rewind(fp);

		shaderText = (GLchar*)malloc(shaderLength);
		if(shaderText != NULL)
		{
			fread(shaderText,1,shaderLength,fp);
		}
		shaderText[shaderLength] = '\0';
		fclose(fp);
	}
	return shaderText;
}

上面有把shader只用写成接口，并没有给出着色程序的代码，你需要将着色程序保存为文本文档，然后用loadShaderText()读取程序，然后就是init(),利用setParameter()设置好参数就可以了。