+Just a side note about the possibility of ending up with a divide by 0 case when manipulating
+the endpoints. Sometimes this results in a single frame anomaly but you can workaround this
+by setting the X value for the middle keyframe of a series to just a little more than 0,
+such as .01 so that dividing by 0 is avoided. The actual cause of the problem is that the
+interpolated X1..X4 auto coordinates used by perspective eventually have x1==x2, x3==x4, so
+that all points x are scaled by zero. Another solution is to tweak the frame count to an even
+number, so that the center is not over the point where x1==x2, x3==x4, or perturb the midpoint
+position a little so that the answers are not exactly zero.
+