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Neil Burns
difference matte problem
on Aug 12, 2016 at 10:30:31 pm

I am trying to composite some footage of a model landscape.

It consists of three layers. The background layer is a shot of a swamp. The middle layer will be a sunbeam. The top layer is the swamp again, but with a card just behind the trees which frame the foreground. The final shot would look like a sunbeam in a swamp that is framed by trees in the foreground. The card is too unevenly lit to get anything like a decent key.

I had hoped to be able to matte out the parts of the foreground shot which are different from the background shot, leaving just the frame of trees, but when I tried to do so with a difference matte I find it eliminates the part that is the same, and leaves the card, which is the exact inverse of what I need.

Is there a way to invert the part that the difference matte eliminates, or to use something like a combination of a difference matte and luma matte to get the desired result, or some other solution that hasn't occurred to me?


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Dave LaRonde
Re: difference matte problem
on Aug 12, 2016 at 10:36:38 pm
Last Edited By Dave LaRonde on Aug 12, 2016 at 10:40:12 pm

Why don't you use Mocha to track the card, then use it as a track matte? That is, if the footage is moving. If it isn't, it should be easy to make a mask to do the job.

Unless the two layers involved in the Difference Matte are dead-solid-perfect identical -- with the exception of the part of the image that's the difference -- the effect is often a Right Royal Pain to use.

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


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Neil Burns
Re: difference matte problem
on Aug 12, 2016 at 10:58:08 pm

The two shots are locked off, and identical with the exception of the card, (and possibly a tiny bit of reflective spill from the card which should be easy to mask out). They were shot with a DSLR so they are very high quality and noise shouldn't be a problem.

I am unfamiliar with mocha.

If you were to use another method to make the mask what would you choose? The foreground trees are very fiddly with lots of twigs and leaves, so painting a mask would be a pain, and is compounded by the fact that the final product will be in stereo 3D, so I would need to be able to make a second mask that paints out the exact same elements from a slightly different angle or the 3D will cause headaches in the viewer.


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Dave LaRonde
Re: difference matte problem
on Aug 12, 2016 at 11:07:52 pm

If it's a white card, you could try a Luma Key. If it's a color card, you could try a chroma key. Use Keylight. If you've never used it before, look for Keylight tutorials. There are a couple right here on the COW.

Avoid YouTube Keylight tutorials made by high school kids as you would avoid the Ebola virus.

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


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Neil Burns
Re: difference matte problem
on Aug 13, 2016 at 12:18:46 am

Thanks very much, for your suggestions.


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