How do I read the an environment map CPU side? (Similar to texCube function on GPU)

Question

I need to extract the one pixel from a environment mapping (I already have the 6 textures) using the normal vector. How can I achieve this ?

I want something very similar to what texCUBE(link) does in GPU but in CPU.

I am using XNA and C#, but an example in any language may help.

Answer 1

Someone answered in another forum - the original post is here:

The largest component of the normal vector tells you which face it intersects. Next divide the normal vector by the absolute value of the largest component. Then scale and shit the other two components into the usual range for uvs ([0, 1] on each axis).

For example if you had the vector <-3, -1, 2>.

First it intersects the negative x face. Then

<-3, -1, 2> / |-3| = <-1, -1/3, 2/3>.

Finally just scale the remaining two components from [-1, 1] to [0, 1], <-1/3, 2/3> * 0.5 + 0.5 = <2/3, 5/6>. And similarly for each other face.