OpenGL的GLSL编程初探

先发个截图:当然看到这个界面大家肯定觉得这不就是个三角形，这不是我想说的，重点在后边。

先附上代码,首先是main.cpp文件:

#include "GL\glew.h"
#include "GL\glut.h"
#include <gl\GL.h>
#include <gl\GLU.h>

#include <Windows.h>
#include <stdio.h>

GLuint program;

GLint attribute_coord2d;

char* file_read(const char* filename)
{
  FILE* input = fopen(filename, "rb");
  if(input == NULL) return NULL;
 
  if(fseek(input, 0, SEEK_END) == -1) return NULL;
  long size = ftell(input);
  if(size == -1) return NULL;
  if(fseek(input, 0, SEEK_SET) == -1) return NULL;
 
  /*if using c-compiler: dont cast malloc's return value*/
  char *content = (char*) malloc( (size_t) size +1  ); 
  if(content == NULL) return NULL;
 
  fread(content, 1, (size_t)size, input);
  if(ferror(input)) {
    free(content);
    return NULL;
  }
 
  fclose(input);
  content[size] = '\0';
  return content;
}

/**
 * Display compilation errors from the OpenGL shader compiler
 */
void print_log(GLuint object)
{
  GLint log_length = 0;
  if (glIsShader(object))
    glGetShaderiv(object, GL_INFO_LOG_LENGTH, &log_length);
  else if (glIsProgram(object))
    glGetProgramiv(object, GL_INFO_LOG_LENGTH, &log_length);
  else {
    fprintf(stderr, "printlog: Not a shader or a program\n");
    return;
  }
 
  char* log = (char*)malloc(log_length);
 
  if (glIsShader(object))
    glGetShaderInfoLog(object, log_length, NULL, log);
  else if (glIsProgram(object))
    glGetProgramInfoLog(object, log_length, NULL, log);
 
  fprintf(stderr, "%s", log);
  free(log);
}

/**
 * Compile the shader from file 'filename', with error handling
 */
GLuint create_shader(const char* filename, GLenum type)
{
  const GLchar* source = file_read(filename);
  if (source == NULL) {
    fprintf(stderr, "Error opening %s: ", filename); perror("");
    return 0;
  }
  GLuint res = glCreateShader(type);
  const GLchar* sources[2] = {
#ifdef GL_ES_VERSION_2_0
    "#version 100\n"
    "#define GLES2\n",
#else
    "#version 120\n",
#endif
    source };
  glShaderSource(res, 2, sources, NULL);
  free((void*)source);
 
  glCompileShader(res);
  GLint compile_ok = GL_FALSE;
  glGetShaderiv(res, GL_COMPILE_STATUS, &compile_ok);
  if (compile_ok == GL_FALSE) {
    fprintf(stderr, "%s:", filename);
    print_log(res);
    glDeleteShader(res);
    return 0;
  }
 
  return res;
}


int init_resources(void)
{
  /* FILLED IN LATER */
	GLint compile_ok = GL_FALSE, link_ok = GL_FALSE;
 
	GLuint vs, fs;
  if ((vs = create_shader("triangle.v.glsl", GL_VERTEX_SHADER))   == 0) return 0;
  if ((fs = create_shader("triangle.f.glsl", GL_FRAGMENT_SHADER)) == 0) return 0;
   program = glCreateProgram();
  glAttachShader(program, vs);
  glAttachShader(program, fs);
  glLinkProgram(program);
  glGetProgramiv(program, GL_LINK_STATUS, &link_ok);
   if (!link_ok) {
    fprintf(stderr, "glLinkProgram:");
    print_log(program);
   }
  return 0;
}
 
void onDisplay()
{
  /* Clear the background as white */
  glClearColor(1.0, 1.0, 1.0, 1.0);
  glClear(GL_COLOR_BUFFER_BIT);
 
  glUseProgram(program);
  glEnableVertexAttribArray(attribute_coord2d);
  GLfloat triangle_vertices[] = {
     0.0,  0.8,
    -0.8, -0.8,
     0.8, -0.8,
  };
  /* Describe our vertices array to OpenGL (it can't guess its format automatically) */
  glVertexAttribPointer(
    attribute_coord2d, // attribute
    2,                 // number of elements per vertex, here (x,y)
    GL_FLOAT,          // the type of each element
    GL_FALSE,          // take our values as-is
    0,                 // no extra data between each position
    triangle_vertices  // pointer to the C array
  );
 
  /* Push each element in buffer_vertices to the vertex shader */
  glDrawArrays(GL_TRIANGLES, 0, 3);
  glDisableVertexAttribArray(attribute_coord2d);
 
  /* Display the result */
  glutSwapBuffers();
}

void free_resources()
{
  glDeleteProgram(program);
}
 
int main(int argc, char* argv[])
{
	/* Glut-related initialising functions */
	glutInit(&argc, argv);
	glutInitDisplayMode(GLUT_RGBA|GLUT_DOUBLE|GLUT_DEPTH);
	glEnable(GL_BLEND);
	glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
	glutInitWindowSize(640, 480);
	glutCreateWindow("My First Triangle");
 


	/* Extension wrangler initialising */
	GLenum glew_status = glewInit();
	if (glew_status != GLEW_OK)
	{
		fprintf(stderr, "Error: %s\n", glewGetErrorString(glew_status));
		return EXIT_FAILURE;
	}
 
	/* When all init functions runs without errors,
	the program can initialise the resources */
	if (0 == init_resources())
	{
		/* We can display it if everything goes OK */
		glutDisplayFunc(onDisplay);
		glutMainLoop();
	}
 
	/* If the program exits in the usual way,
	free resources and exit with a success */
	free_resources();
	return EXIT_SUCCESS;
}
           

我的操作系统是win7 x64位，采用了glew 和freeglut库，这些库可以从相应的网站上下载，这里就不说了，主要看一个函数create_shader这个函数，里面负责从文件中读取GLSL的代码，然后将其编译，后面将其连接然后进行执行。将三个文件放在同一目录下即可编译通过，那些繁琐的理论知识这里就不废话了。

接下来来看这两个文件吧（triangle.v.glsl和triangle.f.glsl）：

//triangle.v.glsl

attribute vec2 coord2d;                  
void main(void) 
{                        
	gl_Position = vec4(coord2d, 0.0, 1.0); 
}
           

//triangle.f.glsl

void main(void) 
{        
	gl_FragColor[0] = gl_FragCoord.x/640.0;
	gl_FragColor[1] = gl_FragCoord.y/480.0;
	gl_FragColor[2] = 0.5;
	gl_FragColor[3] = floor(mod(gl_FragCoord.y, 2.0));
};