# How to tell which faces of a convex solid are visible.

I have a series of faces that describe a 3-d solid. If I draw these faces, I've drawn the solid (my light source is at infinity). Except, for most of the solids I'm drawing (platonic solids and tetartoid so far), only half the total faces are visible from any angle. Now, I'm capturing the solid from the "front" and rotating it. The question is, how do I decide which faces are visible (and should be rendered) and which ones are behind the solid and should not be rendered. I must say, this problem has proven to be considerably harder than I imagined. I thought I'd take the cross product of two edges of the solid and see if it's positive. But, if I just reverse the order of the vectors in the cross product, the sign flips. I also tried taking the center of each face and seeing if it's z-coordinate (my camera is at infinite z) is positive or negative. This worked quite well for platonic solids where all the faces are regular. However, it didn't work for a tetartoid where the faces are irregular (see below).