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Mask drifting from its points

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Adrian Graham-SmithMask drifting from its points
by on May 29, 2015 at 11:09:55 am

I'm doing a screen replacement on a television, which is a tracking shot. And something odd's happening...

As the television moves, the mask is drifting slightly. The track is rock solid and all the control points are where they should be, but the mask in practice isn't matching up exactly to the points. If I take one of the points and drag it way across the screen, the mask starts to change shape slightly but not in proportion to how far I'm moving the point.

What am I missing here?


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Dave LaRondeRe: Mask drifting from its points
by on May 29, 2015 at 4:22:09 pm

Are you using Mocha AE for this? You should.

Dave LaRonde
Promotion Producer
KGAN (CBS) & KFXA (Fox) Cedar Rapids, IA


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Adrian Graham-SmithRe: Mask drifting from its points
by on May 30, 2015 at 11:26:17 am

I did have a look in Mocha, which I come back to once a year or so. I just find it infuriatingly counter intuitive.

The After Effects Mast Tracker seems to have everything I wanted from Mocha but with a more familiar interface. Is there a lot I'm missing out on?


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James KumorekRe: Mask drifting from its points
by on May 30, 2015 at 9:41:33 am

Can you describe more specifically what you did to track the TV and apply the track to your mask? And, is this the first time you've done this, or have you done this successfully in the past?


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Adrian Graham-SmithRe: Mask drifting from its points
by on May 30, 2015 at 11:19:00 am

I solved it myself so will add for posterity. I had scaled the image and set a couple of position key frames on it too. This was the source of the problem.

To solve it I had to precomp the shot, draw the mask again and reapply the mask tracker in there. I then applied the scaling and position to the precomp and everything worked as it should again.

Seems like an After Effects quirk to me rather than a deliberate feature.


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Richard HerdRe: Mask drifting from its points
by on Jun 2, 2015 at 10:42:30 pm

[Adrian Graham-Smith] "Seems like an After Effects quirk to me rather than a deliberate feature"

It's a deliberate feature in the render order: SMET -- source, mask, effects, transform.

Imagine if the masks were not first! Yikes!


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Adrian Graham-SmithRe: Mask drifting from its points
by on Jun 3, 2015 at 7:50:14 am

Oh how I wish I'd learned that acronym years ago! Thanks Richard.


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