Getting the frustum planes

Whats the math to find the frustum planes? I already read the frustum culling tutorial at <http://www.markmorley.com/opengl/frustumculling.html>, but dont know why do I have to multiply the projection matrix by the modelview matrix to do that…

Thank you

Well, your trying to find numbers. Lets make an extremely simply example for comparison:

2 * 3 = 6

if you dont multiply the modelview (3) by the projection (2) you dont get the correct answer (6). It really does break down to simple math, only using lots of numbers rather than a few. If you want to exactly how all the math behind projecting a 3d point onto a 2d surface works I’d suggested Foley, van Dam, Feighner, & Hughes : “Computer Graphics Principles and Practice” in particular chapters 5 and 6. I’m sure others can point out online resources as well (it is quite an expensive book).

cheers