Fig. 6. Perspective
projection of a circle to an ellipse. A: the circle inscribed
on the face of a cube. B: the simple ellipse, with its centre
marked. C: the projection when the cube is on the visual
axis forms an ellipse whose centre (and vertical axis) is
displaced from that of the projected circle. D: for a circle
above the visual axis, the major axis of the ellipse is
rotated from the vertical, making its construction a challenging
geometric problem with no known geometric solution.
