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What Does Nearby Mean?
The Distance, Sharpness, and Consistency controls all factor into the decision whether two trackers are close enough to coalesce. It is a fairly complex decision, taking into account both 2-D and 3-D locations, and is not particularly amenable to human second-guessing. The controls are pretty straightforward, though.
As an aside, it might seem that all that is needed is to measure the 3-D distance between the computed tracker points, and coalesce them if the points are within a certain distance measured in 3-D ( not in pixels). However, this simplistic approach would perform remarkable poorly, because the depth uncertainty of a tracker is often much larger than the uncertainty in its horizontal image-plane position. If the distance was large enough to coalesce the desired trackers, it would be large enough to incorrectly coalesce other trackers.
Instead, SynthEyes uses a more sophisticated and compute-intensive approach which is evaluated over all the active frames of the trackers.
The first and most important parameter is the Distance , measured in horizontal pixels. It is the maximum distance between two trackers that can be considered for coalescing. If they are further apart than this in all frames, they will definitely not be coalesced. If they are closer, some of the time, they may be coalesced, increasingly likely the closer they are.
The second most important parameter, the Consistency, controls how much of the time the trackers must be sufficiently close, compared to their overall lifetime. So very roughly, at 0.7 the trackers must be within the given distance on 70% of the frames. If a track is already geometrically accurate, the consistency can be made higher, but if the solution is marginal, the consistency can be reduced to permit matches even if the two trackers slide past one another.
The third parameter, Sharpness, controls the extent to which the exact distance between trackers affects the result, versus the fact that they are within the required Distance at all. If Sharpness is zero, the exact distance will not matter at all, while at a sharpness of one (the maximum), if the trackers are at almost the maximum distance, they might as well be past it.
Sharpness can be used to trade off some computer time versus quality of result: a small distance and low sharpness will give a faster but less precise result. Settings with a larger distance and larger sharpness will take longer to run but produce a more carefully-thought-out result—though the two sets of results may be very similar most of the time, because the larger sharpness will make the larger distance nearly equivalent to the smaller distance and low sharpness.
If you are handling a shot with a lot of jitter in the trackers, due to large film grain or severe compression artifacts, you should decrease the sharpness, because those small differences in distance are in fact meaningless.
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