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View Full Version : S,T, R & Q planes. How do they work?

Anders
11-07-2002, 01:13 AM
Hi

I'm currently looking at the automatic texture generation function glGenTex in openGL. I am a little puzzled by it. Could someone please explain what the S, T, R, Q planes are, what they do, and how to set them up? I've seen some examples using this function, but they all just enter the indentity matrix as the s,t,r,q matrix. I realise, that they have a similar function to the homogenous coordinates X,Y,Z and W, but I haven't understood the function completely. So if someone could explain it to me or point me to a site that does, I would appreciate it. Thank you.
Also, when is it appropriate to use the GenTex function? What do professionals do?

Anders

Tom Nuydens
11-07-2002, 03:55 AM
The TexGen planes specify the origin and scale of your generated texture coordinates.

A plane has the equation Ax + By + Cz + D = 0. Substituting a point for (x, y, z) gives you the distance of that point to the plane. When you specify the TexGen planes, what you give to the GL are A, B, C and D. OpenGL then uses your vertex coordinates to fill in x, y and z, and then outputs Ax + By + Cz + D as your final texture coordinate. It does this for S, T, R and Q.

You could also think of this as a dot product (although it doesn't really help), or you could consider the combination of the four TexGen planes to be a matrix that is applied to your vertex coordinates to produce texcoords.

Important to remember here is that if you use EYE_LINEAR TexGen, the planes are multiplied by the inverse modelview matrix right after you specify them.

-- Tom

dorbie
11-07-2002, 03:59 AM
I like to think of the plane equation as a normal of the plane with the 4th parameter being the displacement from the origin along the normal. It's quite intuitive then to work with the numbers.