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As you track through a shot, you can change the tracker size and search size to adapt to changes in the shot. SynthEyes will record these changes as keys onto the corresponding animated tracks, which you can see and modify in the Graph Editor.
Values between keys are linearly interpolated, by default. There are some fine points to consider: tracking is a bit different than animating!
Tip : Normal tracker position keys appear as black triangles on the time bar. When there is a key on a secondary channel, such as tracker size or search size, and not on the tracker position, a gray tracker key is shown instead.
First, suppose you are tracking with a vertical search size of 0.03. At frame 30, the shot is bouncier, and you increase the vertical search size to 0.04 so that the pattern continues to be found.
Later, you go back and have SynthEyes play through the shot again, re-tracking the tracker. Frame 29 was originally tracked with a search size of 0.03, but now it will be tracked with a search size close to 0.04, say 0.395 substantially larger. Depending on the situation, there is a chance that the tracker will no longer be placed at the same, presumably correct, location as it was originally. The same is true of the earlier frames, to correspondingly smaller chances.
For a 100% reproducible effect, the search size keys could be interpolated using staircase interpolation. But that doesn't really correspond to the underlying situation, and requires some bizarre and incomplete changes if you later change the tracking
direction. If you'd tracked backwards, you'd probably have set the sizes the other way, resulting in utterly different sizes in the middle frames. With linear interpolation, the results are the same in both directions.
The same effect occurs when you change the tracker size and aspect ratio: due to the linear interpolation, changes are effectively retroactive and may cause problems to surface in previously-tracked frames.
But especially in the case of tracker size and aspect, the linear interpolation may result in more accurate tracking data to result if the earlier frames are re-tracked.
Depending on the tracker settings, in particular if you use Smooth after Keying, the affected earlier frames may automatically be re-tracked.
Linear interpolation may sometimes cause a change upon re-tracking, but it makes more sense in general!
©2024 Boris FX, Inc. — UNOFFICIAL — Converted from original PDF.