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Back when I was demonstrating the new Roto Brush tool in After Effect CS5, I mentioned the Refine Matte section. This gives you a lot of additional power to clean up the edges of a matte. It basically does a motion estimation to see where the edges are moving from frame-to-frame. And then it will try to reduce chatter, noise, buzz, jitter in the edges. It can apply a motion blur to those matted edges, rather than being a hard edged matte, even when something is moving very quickly and is very blurred, and it can decontaminate background color from those edges.
So if you have anything bleeding in from the background or bleeding through a motion blurred section, you could remove that color contamination so you have a more pure composite. As it turns out, Refine Matte is not limited just to Roto Brush. It's a standalone effect as well that you can apply to any matte you create. For example, let me bring up this simple green screen example. Earlier in the shot this was pretty easy to key because the character's not moving at all, but as we get further into this shot where there is a lot of action, you can see there is a lot of motion blur and a lot of activity happening in this shot.
This would normally make it harder to get a good key, because you'd be worried about how do you deal with these motion blurred areas, how do you deal with these edges? You want this to be blurrier, and this, so just a simple feather amount is not going to cut it. Let's go ahead and key this and then play around Refine Matte. I bring up the old Keylight effect, which is still bundled with After Effects. It's a great keyed. Drag it on to my footage, take my Eyedropper and pick some green close to this action. Now Keylight does give me a pretty good start to the image.
But as I turn on Toggle Transparency, you see I've got some problems. The Key is not yet perfect. I got some partial transparency in those motion blur areas like his face. I would need to do a lot more work to help refine this key and make it look good. Let's go ahead and add to this Refine Matte. Drag that on, on top and bang! See how much better my matte is automatically? And not only that, look how it's retaining the motion blurred sections around these arms, around the head.
The key did a good job to begin with, but if I was to turn off Refine Matte's Motion Blur you can see there is some problems, some hard edges here and there. Using the Motion Blur section of Refine Matte helps preserve the partial transparency we want in those motion blurred areas. The other issue I have is with color contamination of the edges. This was shot against the green background after all. Let me pull this wider so you can read these names a bit better. If I turn off Decontaminate Edge Colors, you'll see I've got some problems with black and other colors creeping into these motion blurred edges.
When I say go ahead and decontaminate, now I've got nice partially transparent skin tones where his hand is moving fast instead of getting other colors such as black or green from the background mixed in. Now back in the Roto Brush section I went to these parameters in a little bit more detail. I'm not going to repeat myself here, but let me tell you it's just really nice to be able to simply spread or choke my matte, decide the amount of feathering, how soft I want to make it, reduce the edge chatter in case I've got a very noisy key or I have got a lot of film grain that's causing edges to be appeared to be chewed up.
You can just do a lot with Refine Matte effect after any matte, be it keyed, hand masked, rotoscoped by traditional means, whatever. So this is another wonder addition to the arsenal.
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