I got it to work - that's the good news. Thanks again Phil for the pointers.

The bad news is that the pre-computed deltas are only effective over a small range of object shapes (which I'd have realised if I'd thought about it a bit more thoroughly, duh). I really need to calculate the deltas according to the shape of the object.

Unfortunately that makes it more trouble than it's worth, given my level of trigonometrical expertise. The project is highly specific and unlikely to be of general interest, I think, so I won't beg for assistance in this forum. :)