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two tricky alpha channel questions

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Caspar Stracke
two tricky alpha channel questions
on Apr 22, 2010 at 5:17:38 am

Dear After Effectitos,

FCP editor here, new to this particular AE forum.
So here are my two questions:


The first:
Dave Rockeby, a Canadian video artist and developer wrote once a genius video filter for Max/Jitter (live video) An alpha-mask that reveals only parts of an image that contain movement (or a change of particular pixels compared to the ones from previous frame at the same location..)
Say, we see a fixed image of a sidewalk and a car passes by, only the car movement would reveal white silhouette, while the rest (static) portion of the image would remain black.
Anybody ever created such a movement mask in AE maybe utilizing the "DIfference " blending mode or some plug-on?!?

my second question is related:
Can I apply an alpha mask (or better an independent layer in ho contrast black &white) on two layers simultaneously? The black parts revealing layer 1 and the white parts revealing Layer 2? Like in old analog video mixer?

Applying Keying effects, I have managed to do it in two passes - however, I have lots of giant files and would prefer to manage it in one pass. (Even if it required nesting one Comp into another..)

Any help GREATLY appreciated!

Caspar Stracke







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Tudor "Ted" Jelescu
Re: two tricky alpha channel questions
on Apr 22, 2010 at 7:34:53 am

"Can I apply an alpha mask (or better an independent layer in ho contrast black &white) on two layers simultaneously?"
apply to both layers:Filter- Channel- Set Matte- select the layer for the alpha, check the "invert" option on one of the layers.

Difference - or Image Lounge and Composite Wizard had plug-ins like that if I remember well.

Tudor "Ted" Jelescu
Senior VFX Artist
Bucharest, Romania
http://www.ennstudio.ro


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Caspar Stracke
Re: two tricky alpha channel questions
on Apr 23, 2010 at 12:34:07 am

Thanks a lot Ted.

Didn't know that this type of "Set-Matte" keying is such a piece of cake!

As for the WIzard People: Could find almost everything there, incl. Spill Killer, Super Alpha Cleaner and what not, but unfortunately nothing that stencils this kind of Difference... anyhow, the set matt already solved half of my problems in less than a minute!!! :-)



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Roland R. Kahlenberg
Re: two tricky alpha channel questions
on Apr 22, 2010 at 7:44:09 am

The Difference Matte should give you what you want for the first part. It's got a great algorithm though, so be prepared to tweak the source to pre-process it and also some post processing of the resultant matte.

For the second part, Set Mate applied to each fill layer should do the trick.

HTH
RoRK

Sell your AEPs with broadcastGEMs' DVD series of templates. Click here for more


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Caspar Stracke
Re: two tricky alpha channel questions
on Apr 23, 2010 at 12:59:12 am

Robert Thanks a lot!
I am *ALMOST* there...
Still would need to know for one TINY advice for tweaking:

In Difference Matte, it seems that all movement of moving objects perfectly register when displaying alpha only. Except humans (no idea of its the contrast in my image of what? )
Here's the image of the alpha only view,
the left lower corner is a human walking by, in front of a fence, in differnce matte creating not a clear white area, but lot of abstract garbage inside the shape. Any idea how to get this clean without manual rotoscoping?
Excessive Luma/color keys are to harsh, I probably would need kind of an equivalent to Photoshop's "grow"....

thanks, best,

Caspar


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Roland R. Kahlenberg
Re: two tricky alpha channel questions
on Apr 23, 2010 at 4:06:39 am

Set your project to 16bit to have better fluency over tweaking the matte. Then, use an animated garbage matte to quickly roto (isolate) the human figure. Then apply Difference Matte. Use Alpha Levels to tweak the resultant matte.

HTH
RoRK

Sell your AEPs with broadcastGEMs' DVD series of templates. Click here for more


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Caspar Stracke
Re: two tricky alpha channel questions
on Apr 23, 2010 at 5:25:16 am

rrright on Roland.

Of course garbage mattes ...that's how they're called. Not so coincidentally I used this word already in my prev question. Another helpful term, I just learned: "negative choking"! :-)

Thanks again,

Caspar



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