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How do I pin a bezier line to two points in footage?

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Simon Tyler
How do I pin a bezier line to two points in footage?
on Mar 18, 2013 at 3:58:07 pm

I have footage of a rotating earth and I'd like a curved bezier line to grow from one point to another (using Write On) and have the start and end of the line locked onto those two points as the earth rotates. I'd also like to preserve the curved line or at least have it change slowly. With the rotation, the distance between the two points does change.

I can track the start and end locations on the earth footage with Analyze Motion or Match Move, but I don't know how to apply the resulting tracks to the start and end control points of the bezier line.

Can anyone help?


Motion 4, Mac Pro Snow Leopard 10.6.8


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Simon Ubsdell
Re: How do I pin a bezier line to two points in footage?
on Mar 18, 2013 at 4:40:26 pm

Use the Shape Track Points Behavior for the end point of the line that you want to track..

You can add the Write On behavior over the top of this to get the effect you want.

Simon Ubsdell
http://www.tokyo-uk.com


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Simon Ubsdell
Re: How do I pin a bezier line to two points in footage?
on Mar 18, 2013 at 6:38:09 pm

Here's a demo video tracking both ends of the line to different targets plus write-on behavior:







Simon Ubsdell
http://www.tokyo-uk.com


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Simon Tyler
Re: How do I pin a bezier line to two points in footage?
on Mar 18, 2013 at 7:03:05 pm

Thanks Simon. Track Points doesn't work for the rotating earth which is a graphic and has no control points. It doesn't work on the bezier line because that's not yet moving in the right way.

It almost works when I take the two tracks I made using Analyze Motion on the earth graphic and make those the source for the Track Points behaviour on the bezier line. But the bezier curve flips to a completely different shape after the first couple of frames and can't be edited.


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Simon Ubsdell
Re: How do I pin a bezier line to two points in footage?
on Mar 18, 2013 at 7:08:13 pm

You should be applying the track points behavior to the line itself not the thing you're trying to track.

Add two track point behaviors and use one for each end of the line. make sure you use Mimic Source and you're only attaching one point to each track behavior, i.e. track point 1 for one end of the line and track point 2 for the other end.

If you can track the movement of your graphic in this way (seems unlikely that you'd have problems with this), then you can just keyframe the position of the start and end point of the line.

Simon Ubsdell
http://www.tokyo-uk.com


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Simon Tyler
Re: How do I pin a bezier line to two points in footage?
on Mar 19, 2013 at 8:55:02 am

Thanks Simon for your advice. I now have a track and line that works! I deleted the old Bezier line and started again. I didn't need to use keyframes, but used the Analyze Motion track points that I'd created for the rotating earth. So this was what I did:

1. Applied Analyse Motion to the rotating earth footage, placing the first tracker on the point where I wanted my bezier line to start, and a second tracker where I wanted it to finish. Clicked Analyze and got some good tracks.
2. At the right point on the timeline drew my bezier line with the first control point on the start location and the second on the end location. I used handles to give it a curve.
3. As you suggested, applied the Track Points behaviour to the bezier line. I made the Analyze Motion behaviour the Source and selected Mimic Source.

When I ran it this time, the curve on the bezier line didn't flip around at all, but retained its shape, just what I needed!


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Simon Ubsdell
Re: How do I pin a bezier line to two points in footage?
on Mar 19, 2013 at 10:13:09 am

Glad you got it to work but I do think you went the long way around it.

If you'd used the Track Points behavior correctly you wouldn't have needed to use the extra Analyse Motion Behaviors because the Track Points behavior would have tracked your graphic start and end points and applied the result to the ends of your line all in one go.

Still at least you got there which is the main thing ;-)

Simon Ubsdell
http://www.tokyo-uk.com


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Simon Tyler
Re: How do I pin a bezier line to two points in footage?
on Mar 19, 2013 at 11:23:22 am

Thanks Simon, of course you're right.

I've tried it again using just Track Points and it worked first time. I wasn't initially sure that Motion would offer me two trackers to position, but it did, I applied the Write On behaviour afterwards and it's fine. I have a few more of these to do and it will be a lot quicker than the first one! Thanks for helping out.


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Simon Ubsdell
Re: How do I pin a bezier line to two points in footage?
on Mar 19, 2013 at 11:51:49 am

Good news.

To be honest, I was unsure until I tried it that you could have a write-on together with a track points behavior, let alone two - so I've learnt something as well.

Sometimes Motion is much easier than one thinks ...

... and of course sometimes not ;-)

Simon Ubsdell
http://www.tokyo-uk.com


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