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Motion tracking with mask

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Stephen VilasMotion tracking with mask
by on Jan 8, 2010 at 9:50:17 pm

I just encountered a problem with a composite i am trying to do in After Effects. at first i thought it would be fairly straighforward cause i have done this with a series of shots already, but this is the first time i have to use mask.

the setup is as follows:

I have a shot of an old retro TV. there's a slow track out from CloseUp of the TV screen to Total of the TV. the track out is very clean, no bumps, no rotation, just simple straight line out.

Then I have some archive footage which i want to composite in the TV.

my approach was the following:

1) I motion tracked the screen of the TV with a corner pin track (4 points) and applied it to the archive footage layer. this was succesfull, i just had to upscale the footage a little bit.

2) because its a retro TV with those round corners (its not a simple square as with a computer screen, etc) i decided to mask the footage of the TV and place the archive footage behind it.

and then realised the mask didnt follow the motion tracking data.

3) tried to apply the mask to the archive footage layer instead of the TV layer, but no luck there either. the same problem occurs of course.

am i taking the wrong approach here?
the shot itself is about 12 seconds long, so i really hope i dont have to rotoscope this frame by frame, even if the camera movement is smooth.

thanks in advance for any advice!



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Bruce WainerRe: Motion tracking with mask
by on Jan 8, 2010 at 10:04:37 pm

what if you make another layer (out of a solid) and apply the mask to it, pre-compose the layer (to make it like a still image with no masks), apply the motion track, and use that as a track matte? You could use an alpha matte on the footage or an inverted alpha matte on the TV layer.


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Stephen VilasRe: Motion tracking with mask
by on Jan 9, 2010 at 8:16:42 am

[Bruce Wainer] "what if you make another layer (out of a solid) and apply the mask to it, pre-compose the layer (to make it like a still image with no masks), apply the motion track, and use that as a track matte? You could use an alpha matte on the footage or an inverted alpha matte on the TV layer."

When I do this and apply the tracker on the pre-comp layer it becomes scaled down to the size of the 4 corner pin points. so i end up with a layer that is the size of the TV screen. since the layer is scaled down, so is the mask making it useless. scaling up the layer again afterwards doesnt do the trick.


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Bruce WainerRe: Motion tracking with mask
by on Jan 9, 2010 at 10:04:42 pm

I don't know how to explain what I mean, but I'll make a quick example for you in a bit.


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Bruce WainerRe: Motion tracking with mask
by on Jan 9, 2010 at 10:34:35 pm

here's my example: http://brwainer.110mb.com/screenshot_1-9-2010.htm

what I did was create a white solid in a new composition (size doesn't matter, but should be at least your footage or output size). then corner pinned the composition to the TV, so that the entire solid (with corners still attached - I'll get to that.) follows the TV. next, I masked the solid to fit the round corners of the TV (go 'view>new viewer' to get two windows so you can edit and see the result at the same time). finally, in your TV composition, use the white solid composition as an alpha track matte for your footage. That's it!

Just remember to apply the corner pin BEFORE you mask.


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Stephen VilasRe: Motion tracking with mask
by on Jan 11, 2010 at 12:35:17 am

Thanks a lot for the advice, Bruce!
your approach did the trick. i still had to mess around a bit with the mask, but i applied the tracking data to the corner pin of the solid and now everything seems to work.
a little bit of cleaning up and i've got the shot that i want.

thanks for even bothering making a mock up!

all the best


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Bruce WainerRe: Motion tracking with mask
by on Jan 11, 2010 at 2:37:50 am

no problem. I'm a visual learner, so I can't even explain things any other way sometimes. Besides, other than being a still image, that was no mock-up.


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