Custom distance functor for flann KDTree index.

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I am using the flann module to index a set of image descriptors for quick image matching. Everything is working great so far, but I would like to use my own distance metrics instead of the metrics currently available.

Is there any easy way to use custom made distance functions with the opencv flann implementation? Is the only option to add a distance functors to the source and rebuild opencv? If that is the only way, does someone know what files I need to edit and look at, in order to successfully add other distance metrics into the flann module?