Here's a tool some might find very useful.
From www.scorchworks.com who also more than dabbles in home Injection Plastic Molding (way cool !)
Here's what he did with G-code Ripper "auto probe" to map a lithophane unto the odd shape of a pumpkin (of all things) :
The workflow used can be found here : CNC Lithophane Jack-O-Lantern | Scorch Works Blog
Have fun ...