/sci/ - Science & Math

Help me out. I am trying to teach myself to draw and I want to study proper perspective in order to do it.

I'm having trouble trying to rotate a cube by its axis. By drawing the elipse at the base I have an Idea where should the 45° angle of the corner lie (blue line), but I don't know how to figure out where to cut it so the figure keeps the same size but in perspective. Anyone has an idea how to do it? I tried reading some book called "Viewpoints" but it was garbage that didn't explain shit, to be solved in a classroom as part of a course.