Finding a Subset of Points by Relative Distances

I'm writing a game in which the player may manipulate a great many objects at one time. I would like the player to be able to select objects according to the distances between them.

Given the locations of all objects, a starting object, and a distance threshold, what is the fastest way to find the subset containing the starting object for which the di开发者_开发百科stance between any two objects does not exceed the threshold? Heuristic solutions are perfectly acceptable.

This library seems to do the trick:

"ANN is a library written in the C++ programming language to support both exact and approximate nearest neighbor searching in spaces of various dimensions.

[...]

In the nearest neighbor problem a set P of data points in d-dimensional space is given. These points are preprocessed into a data structure, so that given any query point q, the nearest (or generally k nearest) points of P to q can be reported efficiently."

Depends on your data structure. Primarily, are your objects already sorted / partitioned by distance? I can't think of any distance heuristic... but you could certainly do this in parallel which should help.