Hi Burr, I'm not quite following why you are talking about the Y direction there - the closest and furthest points between 2 curves can be in any direction, not just in the Y direction.

Once you have found a maxima or minima, you can't just take one base point of it and then go in some completely other direction to still maintain the max/min property of it...

Are you trying to find some more constrained type distance than you mentioned originally? If so then you may be looking for a kind of "collision detection" mechanism and not actually a "closest point between curves" type operation.

- Michael