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Alpha and Luma Matte problem

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Dustin Milner
Alpha and Luma Matte problem
on Nov 3, 2008 at 9:05:52 am

Hey guys,
Im currently trying to Alpha Matte some clouds into a static shot of a car pulling over for a short film using adobe after effects cs3.
Ive set up an image of some clouds to alpha matte through a duplicate layer thats above it.
The matte works but it places odd blue lines across the clouds.
I tried changing the blending mode and am able to get rid of it but the blending mode i want is the one that has the lines over it.
Any advice?
Cheers,
Dustin
(P.S im only a beginner with after effects so sorry if this is a really stupid question)


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Paul Conigliaro
Re: Alpha and Luma Matte problem
on Nov 3, 2008 at 2:19:24 pm

It sounds like it might just be a slightly off workflow. (Forgive me if it sounds like I'm talking down to you, but I'm not sure how much knowledge you have of AE.)

Remember that an alpha or luma matte must be applied like this:
1. Clouds
2. Car Shot -> Luma Matte

That is, your clouds are being used as an alpha channel for the car layer below. When setting this, your cloud layer should immediately turn off so it is no longer visible. Any blending mode should be applied to the car shot, not the cloud layer.

It sounds like you might have set the luma matte right, but then turned the cloud layer back on and applied the blending mode to that. If the clouds aren't quite working right as an alpha, you can apply a levels or curves effect to adjust the blacks and whites.

Does this help at all?



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Dustin Milner
Re: Alpha and Luma Matte problem
on Nov 4, 2008 at 3:09:23 am

Hey Mate,
Yeah your right thats exactly what i had done wrong.
I set the blending mode to the wrong layer and had it switched on...duh.
The clouds didnt quite look right so i decided to go for a grey solid (as all the other footage we shot was on an overcast day so i am trying to match it up as this shot has a bright blue sky) and it worked much better, however there are a few stray blue pixels around the edges of the trees.
I used a luma matte and coverted the car shot into a black and white image to define the transparency but there were still a few stray white pixels floating around in different parts.
Any suggestions on an easy way to soften the matte a bit?, im using the "dissolve" blending mode.
Thanks alot,
Dustin.





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Paul Conigliaro
Re: Alpha and Luma Matte problem
on Nov 4, 2008 at 3:59:15 pm

I'd recommend against using the dissolve transparency mode in this case as that only produces 1-bit transparency (IE, either a pixel is fully opaque or fully transparent, no in-between).

If switching to normal or another transparency mode still produces stray pixels (which I'm not sure why it would) use a combination of a mild blur (.5 to 1) and curves or levels on the layer being used as the matte. The blur will soften the edges a bit and the curves or levels can be used to fine-tine the edges.

Let me know if that helps.

(As a side note, I completely had the layer order reversed earlier. I thought you were using the clouds as a luma matte for the car layer, now I see that you are trying to do a sky replacement. Take a look at this tutorial from Video Copilot: Advanced Sky Replacement. It might help you.)



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Dustin Milner
Re: Alpha and Luma Matte problem
on Nov 5, 2008 at 3:12:42 pm

Hey Paul,
Yeah i figured that was a typo but i did actually have the layer switched on and another one in the wrong stacking order.
I'll take a look at the tutorial now,
Thanks alot for your insight,
Dustin




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