Where should I project a polygon corner when it is behind me?
Trying to explain a problem seems to help the thought process. This is what eventually worked for me:

I realized that I can easily find the point (in 3D space) where a line between two of the corners brakes through one of the walls in my pyramid field of view, given the coordinates of the out-of-view corner and one of the visible ones. It is only a matter of finding the intersection between two lines. The position of the intersection point gives away the height on the frame wall.



Self answer, yes, but nothing came to mind before posting at SE.