gluLookAt的实现的疑问[已解:2f]

CR苏杭

1. void GLAPIENTRY
   
2. gluLookAt(GLdouble eyex, GLdouble eyey, GLdouble eyez, GLdouble centerx,
   
3.           GLdouble centery, GLdouble centerz, GLdouble upx, GLdouble upy,
   
4.           GLdouble upz)
   
5. {
   
6.     float forward[3], side[3], up[3];
   
7.     GLfloat m[4][4];
   
8. 
   
9.     forward[0] = centerx - eyex;
   
10.     forward[1] = centery - eyey;
    
11.     forward[2] = centerz - eyez;
    
12. 
    
13.     up[0] = upx;
    
14.     up[1] = upy;
    
15.     up[2] = upz;
    
16. 
    
17.     normalize(forward);
    
18. 
    
19.     /* Side = forward x up */
    
20.     cross(forward, up, side);
    
21.     normalize(side);
    
22. 
    
23.     /* Recompute up as: up = side x forward */        // ？？？？
    
24.     cross(side, forward, up);
    
25. 
    
26.     __gluMakeIdentityf(&m[0][0]);
    
27.     m[0][0] = side[0];
    
28.     m[1][0] = side[1];
    
29.     m[2][0] = side[2];
    
30. 
    
31.     m[0][1] = up[0];
    
32.     m[1][1] = up[1];
    
33.     m[2][1] = up[2];
    
34. 
    
35.     m[0][2] = -forward[0];
    
36.     m[1][2] = -forward[1];
    
37.     m[2][2] = -forward[2];
    
38. 
    
39.     glMultMatrixf(&m[0][0]);
    
40.     glTranslated(-eyex, -eyey, -eyez);
    
41. }

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为什么这里的up要重新side叉乘forward一次？ 而不直接normalize(up)？

array

因为用户传进来的up可能是一个并非垂直于forward和side的值，需要重新换算成标准。据我所知至少在国内有不少人都愿意始终指定up为(0,0,1)……