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How to mask out certain moving areas

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Jackson Parker
How to mask out certain moving areas
on Jan 10, 2011 at 4:32:34 pm

Hi there,


I could use your expertise on the following issue.

So I have this scene - the camera is showing the front of a car. Using motion tracking, I replaced the license plate with an image of a different license plate - so far so good.

Now after a few seconds, a person passes the car, its legs concealing parts of the license plate as it moves. Now obviously those parts of the "new" license plate (ergo the image placed on top of the real license plate) that are actually behind the person's legs should not be visible. Of course they are, since they are in a layer on top of the source video.

Now my question is - how can I use masks (i suppose this would work with masks) to mask out the legs of the person so that the "new" license plate is not shown in those certain areas of the frame?

I hope you get what I mean

Thanks for your help!
Jackson


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Stephen Smith
Re: How to mask out certain moving areas
on Jan 10, 2011 at 6:39:15 pm

You have it figured out. Duplicate the clip layer and drag the duplicate to the top. Add a mask to the legs. Key frame the control points and maybe jump every 5 frames or so and add another keyframe and move the control points around.

Motion can track the control points if you like but I've found it gets confused by the legs crossing back and forth and is easier to do the manual way. Check out my Kinetic Typography tutorial and it has a ad at the end about how my training dvd covers that subject. Or you can click here to learn more about Creative COWs Moving with Motion.

Hope this helps and best of luck.

Stephen Smith
Utah Video Productions

Check out my Motion Training DVD

Check out my Motion Tutorials


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Jeff Greenberg
Re: How to mask out certain moving areas
on Jan 11, 2011 at 8:14:21 pm

Yes, you have the right idea; what you want to do is called Rotoscoping (or RotoMatting)

You'll end up with a sandwich of video:

Original (masked)
replacement plate
Original (tracked)

You want to create a layer of her legs and animate a mask over it. Try to use a b-spline mask vs. bezier (as b-spline produces more 'natural' curves.) You want a few nodes to the mask and as few keyframes as possible. It's quite a bit of work to do this sorta roto work. I try to break down the movement into discrete specific motions and keyframe those and check inbetween.

Scott Squires is one of the experts in roto work - Commotion (a dead OSX program) used to do this exact sorta thing. He has info + a podcast that never really took off, but he talks about these ideas.

Last, I'd probably do each leg separately...so you might end up with two (or more) layers above your replacement plate.

Best,

Jeff G


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Jackson Parker
Re: How to mask out certain moving areas
on Jan 11, 2011 at 9:28:26 pm

Thanks a lot, I got it working this way. Quite a lot of work, since the legs are crossing each other over and over again ;-)

Thanks so much!


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