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Why is SET MATTE effect stretching the matte?

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Mickey NationWhy is SET MATTE effect stretching the matte?
by on Jan 7, 2010 at 7:54:45 pm

Very new to program so chances are I'm overlooking the obvious.

I've masked a moving person that I need to isolate in order to composite to a new background. I'm at the stage where I'm trying to apply the matte to the video clip. I've used the Color Difference Key to get the black and white traveling matte but now I'm stuck where I select the footage and add the Set Matte effect. Here's the problem: the matte appears over my footage but it's stretched horizontally (more and more left and right the further it moves away from the center) as to not match up with the character in the video clip which appears normal.

I see the checkboxes next to "If Layer Sizes Differ" which are:

* Stetch Matte To Fit
* Composite Matte With Origin
* Premultiply Matte Layer

None of these checkboxes have any effect at all on this stretching matte. I've tried everything. Does anyone have any ideas? (I'm on CS3, Mac).


Regards,


Mickey Nation


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Dave LaRondeRe: Why is SET MATTE effect stretching the matte?
by on Jan 7, 2010 at 10:00:47 pm

Well, that's just plain weird.

If you're having success with a difference key, you're darned lucky. Very few people do: it an effect that looks good on paper, but because most footage isn't absolutely perfect, rarely works as billed.

For AE weirdness like this, there are two common causes. One is using Open GL for rendering. If it's on, turn it off. It's basically useless, and frequently detrimental.

Second, check the type of footage you're using. Read On:

Dave's Stock Answer #1:

If the footage you imported into AE is any kind of the following -- footage in an HDV acquisition codec, MPEG1, MPEG2, mp4, m2t, H.261 or H.264 -- you need to convert it to a different codec.

These kinds of footage use temporal, or interframe compression. They have keyframes at regular intervals, containing complete frame information. However, the frames in between do NOT have complete information. Interframe codecs toss out duplicated information.

In order to maintain peak rendering efficiency, AE needs complete information for each and every frame. But because these kinds of footage contain only partial information, AE freaks out, resulting in a wide variety of problems.


Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mickey NationRe: Why is SET MATTE effect stretching the matte?
by on Jan 7, 2010 at 10:16:48 pm

I should have mentioned that I didn't achieve the matte using just a Color Difference Key. I rotoscoped the person with masks and keyframes, then I used the Color Difference Key to get separation from the garbage background.

I'll try to decipher the information you gave me -- it might take a while. Thanks!


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Dave LaRondeRe: Why is SET MATTE effect stretching the matte?
by on Jan 7, 2010 at 11:02:35 pm

[Mickey Nation] "I rotoscoped the person with masks and keyframes, then I used the Color Difference Key to get separation from the garbage background. "

Ah! Then by the very fact that you have isolated the subject by rotoscoping, you have already removed the subject from the background! There is no Set Matte stuff necessary! If you precomp the subject, you will discover that it makes its own matte through the use of the Alpha Matte selection for track mattes.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Mickey NationRe: Why is SET MATTE effect stretching the matte?
by on Jan 8, 2010 at 3:44:57 am

Perfect!

It worked. It's amazing how much things make sense once you learn them. Thanks, guy.


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