I apologize that I didn’t explain the perspective (gui-points.scm) homework clearly! My intention was for you to create two 3-D objects that look the same from one point of view but whose differences are revealed when the viewpoint changes. The cube and truncated pyramid are such a pair of 3-D objects: they look the same from the front but different from other places. You should be able to make another pair of 3-D objects whose 2-D projections behave the same way.

Please feel free to send me—or not send me—more changes to gui-points.scm as you play with it.