Page 7 - We now have an an algebraic relation between world co-ordinates and the new paper co-ordinates based on the midpoint.

Although it appears that I have assumed the curve C is an ellipse, this is not the case. Even with b as constant the ellipse equations below can describe any curve if a is not assumed to be a constant. The only restriction on the curve is that it goes no higher that +b nor lower than -b. This is not an unreasonable restriction. In other words the perspective view of the cirle (whatever curve it is) does not fall outside the the perspective view of the far and near edges of the square.

What the rest of the mathematics does is to show, using only already known facts about about the circle and the perspective set up, that a is indeed a constant.