OpenGL的GLSL編程初探

先發個截圖:當然看到這個界面大家肯定覺得這不就是個三角形，這不是我想說的，重點在後邊。

先附上代碼,首先是main.cpp文件:

1. #include "GL\glew.h"
2. #include "GL\glut.h"
3. #include <gl\GL.h>
4. #include <gl\GLU.h>
6. #include <Windows.h>
7. #include <stdio.h>
9. GLuint program;
11. GLint attribute_coord2d;
13. char* file_read(const char* filename)
14. {
15. FILE* input = fopen(filename, "rb");
16. if(input == NULL) return NULL;
18. if(fseek(input, 0, SEEK_END) == -1) return NULL;
19. long size = ftell(input);
20. if(size == -1) return NULL;
21. if(fseek(input, 0, SEEK_SET) == -1) return NULL;
23. /*if using c-compiler: dont cast malloc's return value*/
24. char *content = (char*) malloc( (size_t) size +1 );
25. if(content == NULL) return NULL;
27. fread(content, 1, (size_t)size, input);
28. if(ferror(input)) {
29. free(content);
30. return NULL;
31. }
33. fclose(input);
34. content[size] = '\0';
35. return content;
36. }
38. /**
39. * Display compilation errors from the OpenGL shader compiler
40. */
41. void print_log(GLuint object)
42. {
43. GLint log_length = 0;
44. if (glIsShader(object))
45. glGetShaderiv(object, GL_INFO_LOG_LENGTH, &log_length);
46. else if (glIsProgram(object))
47. glGetProgramiv(object, GL_INFO_LOG_LENGTH, &log_length);
48. else {
49. fprintf(stderr, "printlog: Not a shader or a program\n");
50. return;
51. }
53. char* log = (char*)malloc(log_length);
55. if (glIsShader(object))
56. glGetShaderInfoLog(object, log_length, NULL, log);
57. else if (glIsProgram(object))
58. glGetProgramInfoLog(object, log_length, NULL, log);
60. fprintf(stderr, "%s", log);
61. free(log);
62. }
64. /**
65. * Compile the shader from file 'filename', with error handling
66. */
67. GLuint create_shader(const char* filename, GLenum type)
68. {
69. const GLchar* source = file_read(filename);
70. if (source == NULL) {
71. fprintf(stderr, "Error opening %s: ", filename); perror("");
72. return 0;
73. }
74. GLuint res = glCreateShader(type);
75. const GLchar* sources[2] = {
76. #ifdef GL_ES_VERSION_2_0
77. "#version 100\n"
78. "#define GLES2\n",
79. #else
80. "#version 120\n",
81. #endif
82. source };
83. glShaderSource(res, 2, sources, NULL);
84. free((void*)source);
86. glCompileShader(res);
87. GLint compile_ok = GL_FALSE;
88. glGetShaderiv(res, GL_COMPILE_STATUS, &compile_ok);
89. if (compile_ok == GL_FALSE) {
90. fprintf(stderr, "%s:", filename);
91. print_log(res);
92. glDeleteShader(res);
93. return 0;
94. }
96. return res;
97. }
100. int init_resources(void)
101. {
102. /* FILLED IN LATER */
103. GLint compile_ok = GL_FALSE, link_ok = GL_FALSE;
105. GLuint vs, fs;
106. if ((vs = create_shader("triangle.v.glsl", GL_VERTEX_SHADER)) == 0) return 0;
107. if ((fs = create_shader("triangle.f.glsl", GL_FRAGMENT_SHADER)) == 0) return 0;
108. program = glCreateProgram();
109. glAttachShader(program, vs);
110. glAttachShader(program, fs);
111. glLinkProgram(program);
112. glGetProgramiv(program, GL_LINK_STATUS, &link_ok);
113. if (!link_ok) {
114. fprintf(stderr, "glLinkProgram:");
115. print_log(program);
116. }
117. return 0;
118. }
120. void onDisplay()
121. {
122. /* Clear the background as white */
123. glClearColor(1.0, 1.0, 1.0, 1.0);
124. glClear(GL_COLOR_BUFFER_BIT);
126. glUseProgram(program);
127. glEnableVertexAttribArray(attribute_coord2d);
128. GLfloat triangle_vertices[] = {
129. 0.0, 0.8,
130. -0.8, -0.8,
131. 0.8, -0.8,
132. };
133. /* Describe our vertices array to OpenGL (it can't guess its format automatically) */
134. glVertexAttribPointer(
135. attribute_coord2d, // attribute
136. 2, // number of elements per vertex, here (x,y)
137. GL_FLOAT, // the type of each element
138. GL_FALSE, // take our values as-is
139. 0, // no extra data between each position
140. triangle_vertices // pointer to the C array
141. );
143. /* Push each element in buffer_vertices to the vertex shader */
144. glDrawArrays(GL_TRIANGLES, 0, 3);
145. glDisableVertexAttribArray(attribute_coord2d);
147. /* Display the result */
148. glutSwapBuffers();
149. }
151. void free_resources()
152. {
153. glDeleteProgram(program);
154. }
156. int main(int argc, char* argv[])
157. {
158. /* Glut-related initialising functions */
159. glutInit(&argc, argv);
160. glutInitDisplayMode(GLUT_RGBA|GLUT_DOUBLE|GLUT_DEPTH);
161. glEnable(GL_BLEND);
162. glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
163. glutInitWindowSize(640, 480);
164. glutCreateWindow("My First Triangle");
168. /* Extension wrangler initialising */
169. GLenum glew_status = glewInit();
170. if (glew_status != GLEW_OK)
171. {
172. fprintf(stderr, "Error: %s\n", glewGetErrorString(glew_status));
173. return EXIT_FAILURE;
174. }
176. /* When all init functions runs without errors,
177. the program can initialise the resources */
178. if (0 == init_resources())
179. {
180. /* We can display it if everything goes OK */
181. glutDisplayFunc(onDisplay);
182. glutMainLoop();
183. }
185. /* If the program exits in the usual way,
186. free resources and exit with a success */
187. free_resources();
188. return EXIT_SUCCESS;
189. }

我的操作系統是win7 x64位，采用了glew 和freeglut庫，這些庫可以從相應的網站上下載，這裡就不說了，主要看一個函數create_shader這個函數，裡面負責從文件中讀取GLSL的代碼，然後將其編譯，後面將其連接然後進行執行。將三個文件放在同一目錄下即可編譯通過，那些繁瑣的理論知識這裡就不廢話了。

接下來來看這兩個文件吧（triangle.v.glsl和triangle.f.glsl）：

//triangle.v.glsl

1. attribute vec2 coord2d;
2. void main(void)
3. {
4. gl_Position = vec4(coord2d, 0.0, 1.0);
5. }

//triangle.f.glsl

1. void main(void)
2. {
3. gl_FragColor[0] = gl_FragCoord.x/640.0;
4. gl_FragColor[1] = gl_FragCoord.y/480.0;
5. gl_FragColor[2] = 0.5;
6. gl_FragColor[3] = floor(mod(gl_FragCoord.y, 2.0));
7. };