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Motion tracking of large one-color circle

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vid klopcic
Motion tracking of large one-color circle
on Mar 22, 2015 at 12:46:23 pm


I recently recorded solar eclipse through telescope. It's postion was adjusted every few minutes. Now I want to speed up video to create timelapse of Monn passing. Before that, I must set position of the Sun to a fixed spot, so it won't shake on high speed.

Usually I do this using "stabilize motion" on tracker pane, but this time, Sun doesn't have any visible spot throuh entire footage (Moon covers it while passing the Sun). I tried to position tracker around entire Sun, but it would took forever to analyze. I am sure, that tracking high contrast circle shouldn't be hard.

I am wondering, which tools can I use to track movement of the Sun? I attached an image of what video looks like.

Thank you in advance for any help :).

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Graham Macfarlane
Re: Motion tracking of large one-color circle
on Mar 23, 2015 at 9:09:24 am

Hi Vid,

There is one small dark spot on the sun disk (top left) which would help early on in the track. Have you tried increasing the contrast to see if there are some other revealed details you could track?

If that doesn’t work it might be worth trying to use the dark spot top left as much as possible, and once it is occluded, start using the sharp intersection point that is created at the top left of the sun disk where the moon overlaps.

This intersection point does not represent a true tracking feature and normally you would never use it as its result will be a movement around the edge of the sun disk. Also note that when the moon gets close to covering the whole sun you may lose the track and have to go full manual until you can pick up the next sharp corner as the moon leaves the sun disk.

If you have no other option though, it will at least give you a smooth motion which you could then counter with manual keyframes (hopefully less than one per frame!!) to keep the sun position locked.

Tip: if you do go this route, when countering the movement, use a null aligned to that sharp corner you tracked. Animate the start and finish positions and then arrange your Bezier handles in the viewport to align the path visually along the edge of the sun disk. This should help create a nice circular path more quickly.
This will give you a null that follows your track which you can then negate with a second null. Just pick-whip the positions and negate with a *-1 at the end of the expression generated on the second null.

Hope that helps!

Graham Macfarlane
3D | VFX

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Roland R. Kahlenberg
Re: Motion tracking of large one-color circle
on Mar 23, 2015 at 12:44:04 pm

You can try AE's tracker to track a small area on the edge of the Sun. If that fails, you can scale down the image drastically to reduce the tracking area of the entire Sun. It will be a good idea to set the Anchor Point to a point close to the Sun's center prior to scaling. When tracking position, all you need is a single point - once tracking is done and data pasted onto a Null, you can Parent the original layer (un-scaled) to the Null.

In future, for such shots, you will want to put a small gaffer tape onto the camera lens and use that as a tracking marker. This is ideal if your camera is locked and the taped area can be easily removed or garbage matted out because it's not required for the final composite.

- Roland

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