[XXShadowXX]

Originally Posted by GaryVoshol

The vision target on the trailer is at a fixed height. Your camera will be mounted on your robot at a fixed height; make it lower than the vision target. If you measure the angle (above horizontal) that your camera is at to point at the target, you can use trig to figure out how far away you are from the target.

sounds by far the simplest solution, of course the object will still get smaller the further you are from the object, so it wall have some margin of error, it will still need some optic curve...