求解投影视锥体水平夹角Foh

已知

• Fov
• Aspect = Width / Height
• 构造如下三角形，分别为视锥体的垂直切面，水平切面

triangle.jpg

tan(Fov/2) = (H/2) / Near
=> H = 2 * Near * tan(Fov/2)

Aspect = W / H
=> W = H * Aspect
=> W = 2 * Near * tan(Fov/2) * Aspect

tan(Foh/2) = (W/2) / Near 
= 2 * Near * tan(Fov/2) * Aspect / Near
= 2 * Aspect * tan(Fov/2)

最后根据反三角函数，就可以计算出Foh的角度了