Mechanical tool and freehand layout methods
b. The back line of the new square is determined by the method
presented in Figure 3-5. Now diagonal lines are drawn from corner to corner
in the perspective square. Within the original square, short vertical lines
are drawn downward to the picture plane from the points where the circle
line and the diagonal lines intersect.
From these two points, draw lines
back to the vanishing point.
The points at which these lines cross the
diagonal lines in the perspective square are the points through which the
curve is drawn.