I am not a graphic artist or 3D animator. What I am doing right now is just learning Opengl. While on the topic of cube map, I came across this R texture coordinate. How can you visualize it? Say even if you visualize it as a cube. It is nothing more than a cube covered in 2D texture. Or for example you are sampling a texture on a triangle(2D or 3D) it will always just use a 2D texture. So I am just wondering, why is there an R coordinate? I think S and T would be enough in many cases( I am not sure on this since I am just beginning to learn OpenGL).
Searching in google I found this
Which is kind of confusing. If you map it on cube and cube is basically composed on 2D square, you are just going to use 2D texture as well(S,T) right?