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Attach point drifts from keyframe marker while tracking...?

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Seth MarshallAttach point drifts from keyframe marker while tracking...?
by on Mar 24, 2010 at 5:09:13 pm

I'm stabilizing a time lapse where the camera was fixed on a tripod. There is no real motion that needs tracking, just the occasional bumps on the tripod that need fixing. Unfortunately over time as lighting changes the trackers in AE don't do a very good job regardless of the setting I use in options. Unless I'm doing something wrong it needs supervising.

I've found a good area to track and made my search area big enough. Frame by frame tracking is of course the best but extremely time consuming. I would like to use the track forward function and watch.
The strangest thing is that over time the attach point drifts inside the feature region, but the keyframe marker stays centered. Therefore over time you see the keyframe marker wander as the attach point stays fixed on the feature although not centered in the feature region.

Why would this happen? It is rather hard to tell if the attach point has moved, only to discover it once you realize the keyframe marker is wandering. With a long time lapse there are so many keyframe markers it is hard to keep track of the current one until it is too late.

Does anyone with experience have any helpful guidance on how to do this more efficiently? Thank you!


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Roland R. KahlenbergRe: Attach point drifts from keyframe marker while tracking...?
by on Mar 24, 2010 at 6:10:46 pm

[Seth Marshall] "The strangest thing is that over time the attach point drifts inside the feature region, but the keyframe marker stays centered."

Strange indeed! The Attach Point is supposed to be within the Feature Region at all times.

[Seth Marshall] "Therefore over time you see the keyframe marker wander as the attach point stays fixed on the feature although not centered in the feature region."

[Seth Marshall] "Why would this happen? It is rather hard to tell if the attach point has moved, only to discover it once you realize the keyframe marker is wandering. With a long time lapse there are so many keyframe markers it is hard to keep track of the current one until it is too late."

AE tracks on a per frame per second basis. Un:ess you're traking with fields and your footage is interlace and comp is set to double frame rate, you can't have more keyframes than you need. All trackers, regardless of software track on a per frame basis unless told to do so with interlaced footage.

We obviously have issues with a lot of what you've posted. Best is for you to post your clip for ALL to take a look and then you may have a better opportunity for the community here to collectively look at an hopefully solve your issue(s).

Cheers
RoRK

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Darby EdelenRe: Attach point drifts from keyframe marker while tracking...?
by on Mar 25, 2010 at 3:35:31 am

[Roland R. Kahlenberg] "Strange indeed! The Attach Point is supposed to be within the Feature Region at all times."

The attach point can be moved anywhere, but it should always move exactly the same way that its associated feature region does. If the feature region moves 10 pixels up, the attach point should move 10 pixels up. If you're supervising the track you may or may not want to move the attach point of the tracker.

If you want to track the same feature, but AE got the track wrong, then you should move the feature/search regions AND the attach point back to the correct location on the feature.

If you want to switch to a new feature because your previous feature has left the frame or is no longer trackable, then you want to leave the attach point where it is and move the feature/search regions to another feature on the same plane of depth (this matters more with tracking shots, if you're on a tripod then matching the depth/parallax is less important).

I've never seen an attach point wander separate from its feature region. If you could post some screenshots of your settings or even a screen capture of it happening, that may help.

I'm only guessing, but one suggestion would be to select the track layer and hit 'u' and see which values are animated. Make sure the Attach Point Offset isn't animated.

Darby Edelen


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