本例子选自OpenGL Shading Language中的第一个Shader例子，使用颜色平滑地表示一个表面的温度。温度及其颜色的范围在应用程序中进行设置。
//先看顶点着色器temp.vert：
uniform float CoolestTemp;
uniform float TempRange;
attribute float VertexTemp;//每个定点都对应一个温度值
varying float Temperature;//传递到片段处理器进行后续处理
void main(void)
{
//进行插值
Temperature = (VertexTemp - CoolestTemp) / TempRange;
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
}
//片段着色器temp.frag：
uniform vec3 CoolestColor;
uniform vec3 HottestColor;
varying float Temperature;
void main(void)
{
//通过温度值寻找一个相应的颜色，位于最冷和最热之间
vec3 color = mix(CoolestColor, HottestColor,Temperature);
gl_FragColor = vec4(color,1.0);
}
下面看看如何将着色器和应用程序进行链接（第一步是创建着色器对象，指定着色器源代码，进行编译，并程序对象和着色器进行绑定和链接），同时指定了一些一致变量的值
/*public*/
int installShaders(const GLchar *Vertex, const GLchar *Fragment)
{
GLint vertCompiled, fragCompiled;
// status values
GLint linked;
// Create a vertex shader object and a fragment shader object
VertexShaderObject = glCreateShader(GL_VERTEX_SHADER);
FragmentShaderObject = glCreateShader(GL_FRAGMENT_SHADER); /
/ Load source code strings into shaders
glShaderSource(VertexShaderObject, 1, &Vertex, NULL);
glShaderSource(FragmentShaderObject, 1, &Fragment, NULL);
// Compile the brick vertex shader, and print out
// the compiler log file.
glCompileShader(VertexShaderObject);
glGetShaderiv(VertexShaderObject, GL_COMPILE_STATUS, &vertCompiled);
// Compile the brick vertex shader, and print out
// the compiler log file.
glCompileShader(FragmentShaderObject);
glGetShaderiv(FragmentShaderObject, GL_COMPILE_STATUS, &fragCompiled);
if (!vertCompiled || !fragCompiled)
return 0;
// Create a program object and attach the two compiled shaders
ProgramObject = glCreateProgram();
glAttachShader(ProgramObject, VertexShaderObject);
glAttachShader(ProgramObject, FragmentShaderObject);
// Link the program object and print out the info log
glLinkProgram(ProgramObject);
glGetProgramiv(ProgramObject, GL_LINK_STATUS, &linked);
if (!linked)
return 0;
// Install program object as part of current state
glUseProgram(ProgramObject);
// Set up initial uniform values
glUniform1f(glGetUniformLocation(ProgramObject, "CoolestTemp"), 0.0f);
glUniform1f(glGetUniformLocation(ProgramObject, "TempRange"), 1.0f);
glUniform3f(glGetUniformLocation(ProgramObject, "CoolestColor"), 0.0, 0.0, 1.0);
glUniform3f(glGetUniformLocation(ProgramObject, "HottestColor"), 1.0, 0.0, 0.0);
return 1;
}
这其中用到了一些读取外部着色器代码的函数，顶点着色器后缀自定义为.vert，片段着色器自定义为.frag，我们也可以将着色器放在程序中存储在一个字符串中。
//Shader related functions static int shaderSize(char *fileName, EShaderType shaderType)
{
//返回顶点着色器或者片段着色器的大小
char name[100];
strcpy(name, fileName);
switch (shaderType)
{
case EVertexShader:
strcat(name, ".vert");
break;
case EFragmentShader:
strcat(name, ".frag");
break;
default:
printf("ERROR: unknown shader file type
");
exit(1);
break;
}
int count = -1;
// Open the file, seek to the end to find its length
int fd = _open(name, _O_RDONLY);
if (fd != -1)
{
count = _lseek(fd, 0, SEEK_END) + 1;
_close(fd);
}
return count;
}
static int readShader(char *fileName, EShaderType shaderType, char *shaderText, int size)
{
// Reads a shader from the supplied file and returns the shader in the
// arrays passed in.
Returns 1 if successful, 0 if an error occurred.
// The parameter size is an upper limit of the amount of bytes to read.
// It is ok for it to be too big.
FILE *fh;
char name[100];
int count;
strcpy(name, fileName);
switch (shaderType)
{
case EVertexShader:
strcat(name, ".vert");
break;
case EFragmentShader:
strcat(name, ".frag");
break;
default:
printf("ERROR: unknown shader file type
");
exit(1);
break;
}
// Open the file
fh = fopen(name, "r");
if (!fh)
return -1;
// Get the shader from a file.
fseek(fh, 0, SEEK_SET);
count = (int) fread(shaderText, 1, size, fh);
shaderText[count] = ' ';
if (ferror(fh))
count = 0;
fclose(fh);
return count;
}
/*public*/
int readShaderSource(char *fileName, GLchar **vertexShader, GLchar **fragmentShader)
{
int vSize, fSize;
// // Allocate memory to hold the source of our shaders.
vSize = shaderSize(fileName, EVertexShader);
fSize = shaderSize(fileName, EFragmentShader);
if ((vSize == -1) || (fSize == -1))
{
printf("Cannot determine size of the shader %s
", fileName); return 0;
}
*vertexShader = (GLchar *) malloc(vSize);
*fragmentShader = (GLchar *) malloc(fSize);
// // Read the source code //
if (!readShader(fileName, EVertexShader, *vertexShader, vSize))
{
printf("Cannot read the file %s.vert
", fileName);
return 0;
}
if (!readShader(fileName, EFragmentShader, *fragmentShader, fSize))
{
printf("Cannot read the file %s.frag
", fileName);
return 0;
}
return 1;
}
这里我们只绘制一个简单的三角形，需要为每个顶点指定一个属性值，代表其温度值，着色器中我们对其进行了读取并插值。对于复杂的图形，应当使用顶点数组进行指定。
static void display(void)
{
glLoadIdentity();
glTranslatef(0.0, 0.0, -5.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//查询定点着色器属性变量VertexTemp，并对它进行设置，每个定点需要指定一个属性
GLint tempLoc = glGetAttribLocationARB(ProgramObject, "VertexTemp");
glBegin(GL_TRIANGLES);
glVertexAttrib1f(tempLoc, 0.0f);
glVertex3f(1.0f,0.0f,0.0f);
glVertexAttrib1f(tempLoc, 0.5f);
glVertex3f(-1.0f,0.0f,0.0f);
glVertexAttrib1f(tempLoc, 1.0f);
glVertex3f(0.0f,1.0f,0.0f);
glEnd();
glFlush();
glutSwapBuffers();
}
运行效果如下：
整个程序如下：
#include
#include
#include
#include
#include
#define GLEW_STATIC 1
#include
#include
#include
//窗口句柄
static GLint window;
GLuint ProgramObject = 0; //程序对象
GLuint VertexShaderObject = 0; //顶点着色器对象
GLuint FragmentShaderObject = 0; //片段着色器对象
//Shader类型
typedef enum {
EVertexShader,
EFragmentShader,
} EShaderType;
//用来检查OpenGL版本，需要GLSL 2.0支持
void getGlVersion( int *major, int *minor )
{
constchar* verstr = (constchar*)glGetString( GL_VERSION );
if( (verstr == NULL) || (sscanf( verstr, "%d.%d", major, minor ) != 2) )
{
*major = *minor = 0;
fprintf( stderr, "Invalid GL_VERSION format!!!
" );
}
}
staticvoid display(void)
{
glLoadIdentity();
glTranslatef(0.0, 0.0, -5.0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//查询定点着色器属性变量VertexTemp，并对它进行设置，每个定点需要指定一个属性
GLint tempLoc = glGetAttribLocationARB(ProgramObject, "VertexTemp");
glBegin(GL_TRIANGLES);
glVertexAttrib1f(tempLoc, 0.0f);
glVertex3f(1.0f,0.0f,0.0f);
glVertexAttrib1f(tempLoc, 0.5f);
glVertex3f(-1.0f,0.0f,0.0f);
glVertexAttrib1f(tempLoc, 1.0f);
glVertex3f(0.0f,1.0f,0.0f);
glEnd();
glFlush();
glutSwapBuffers();
}
staticvoid reshape(int wid, int ht)
{
float vp = 0.8f;
float aspect = (float) wid / (float) ht;
glViewport(0, 0, wid, ht);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//glOrtho(-1.0, 1.0, -1.0, 1.0, -10.0, 10.0);
glFrustum(-vp, vp, -vp / aspect, vp / aspect, 3, 10.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
//Shader related functions
staticint shaderSize(char *fileName, EShaderType shaderType)
{
//返回顶点着色器或者片段着色器的大小
char name[100];
strcpy(name, fileName);
switch (shaderType)
{
case EVertexShader:
strcat(name, ".vert");
break;
case EFragmentShader:
strcat(name, ".frag");
break;
default:
printf("ERROR: unknown shader file type
");
exit(1);
break;
}
int count = -1;
// Open the file, seek to the end to find its length
int fd = _open(name, _O_RDONLY);
if (fd != -1)
{
count = _lseek(fd, 0, SEEK_END) + 1;
_close(fd);
}
return count;
}
staticint readShader(char *fileName, EShaderType shaderType, char *shaderText, int size)
{
//
// Reads a shader from the supplied file and returns the shader in the
// arrays passed in. Returns 1 if successful, 0 if an error occurred.
// The parameter size is an upper limit of the amount of bytes to read.
// It is ok for it to be too big.
//
FILE *fh;
char name[100];
int count;
strcpy(name, fileName);
switch (shaderType)
{
case EVertexShader:
strcat(name, ".vert");
break;
case EFragmentShader:
strcat(name, ".frag");
break;
default:
printf("ERROR: unknown shader file type
");
exit(1);
break;
}
// Open the file
fh = fopen(name, "r");
if (!fh)
return -1;
// Get the shader from a file.
fseek(fh, 0, SEEK_SET);
count = (int) fread(shaderText, 1, size, fh);
shaderText[count] = ' ';
if (ferror(fh))
count = 0;
fclose(fh);
return count;
}
/*public*/
int readShaderSource(char *fileName, GLchar **vertexShader, GLchar **fragmentShader)
{
int vSize, fSize;
//
// Allocate memory to hold the source of our shaders.
//
vSize = shaderSize(fileName, EVertexShader);
fSize = shaderSize(fileName, EFragmentShader);
if ((vSize == -1) || (fSize == -1))
{
printf("Cannot determine size of the shader %s
", fileName);
return 0;
}
*vertexShader = (GLchar *) malloc(vSize);
*fragmentShader = (GLchar *) malloc(fSize);
//
// Read the source code
//
if (!readShader(fileName, EVertexShader, *vertexShader, vSize))
{
printf("Cannot read the file %s.vert
", fileName);
return 0;
}
if (!readShader(fileName, EFragmentShader, *fragmentShader, fSize))
{
printf("Cannot read the file %s.frag
", fileName);
return 0;
}
return 1;
}
/*public*/
int installShaders(const GLchar *Vertex,
const GLchar *Fragment)
{
GLint vertCompiled, fragCompiled; // status values
GLint linked;
// Create a vertex shader object and a fragment shader object
VertexShaderObject = glCreateShader(GL_VERTEX_SHADER);
FragmentShaderObject = glCreateShader(GL_FRAGMENT_SHADER);
// Load source code strings into shaders
glShaderSource(VertexShaderObject, 1, &Vertex, NULL);
glShaderSource(FragmentShaderObject, 1, &Fragment, NULL);
// Compile the brick vertex shader, and print out
// the compiler log file.
glCompileShader(VertexShaderObject);
glGetShaderiv(VertexShaderObject, GL_COMPILE_STATUS, &vertCompiled);
// Compile the brick vertex shader, and print out
// the compiler log file.
glCompileShader(FragmentShaderObject);
glGetShaderiv(FragmentShaderObject, GL_COMPILE_STATUS, &fragCompiled);
if (!vertCompiled || !fragCompiled)
return 0;
// Create a program object and attach the two compiled shaders
ProgramObject = glCreateProgram();
glAttachShader(ProgramObject, VertexShaderObject);
glAttachShader(ProgramObject, FragmentShaderObject);
// Link the program object and print out the info log
glLinkProgram(ProgramObject);
glGetProgramiv(ProgramObject, GL_LINK_STATUS, &linked);
if (!linked)
return 0;
// Install program object as part of current state
glUseProgram(ProgramObject);
// Set up initial uniform values
glUniform1f(glGetUniformLocation(ProgramObject, "CoolestTemp"), 0.0f);
glUniform1f(glGetUniformLocation(ProgramObject, "TempRange"), 1.0f);
glUniform3f(glGetUniformLocation(ProgramObject, "CoolestColor"), 0.0, 0.0, 1.0);
glUniform3f(glGetUniformLocation(ProgramObject, "HottestColor"), 1.0, 0.0, 0.0);
return 1;
}
/******************************************************************************/
/*
/* Main
/*
/******************************************************************************/
int main( int argc, char **argv )
{
int success = 0;
int gl_major, gl_minor;
GLchar *VertexShaderSource, *FragmentShaderSource;
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGB | GLUT_DEPTH | GLUT_DOUBLE);
glutInitWindowSize(500, 500);
window = glutCreateWindow( "Temperature Shader");
glutDisplayFunc(display);
glutReshapeFunc(reshape);
// Initialize the "OpenGL Extension Wrangler" library
glewInit();
// Make sure that OpenGL 2.0 is supported by the driver
getGlVersion(&gl_major, &gl_minor);
printf("GL_VERSION major=%d minor=%d
", gl_major, gl_minor);
if (gl_major < 2)
{
printf("GL_VERSION major=%d minor=%d
", gl_major, gl_minor);
printf("Support for OpenGL 2.0 is required for this demo...exiting
");
exit(1);
}
readShaderSource("temp", &VertexShaderSource, &FragmentShaderSource);
success = installShaders(VertexShaderSource, FragmentShaderSource);
if (success)
glutMainLoop();
return 0;
}