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Refining the matte

From: After Effects CS5 New Creative Techniques

Video: Refining the matte

I finished creating my segmentation boundary. I have created a couple of base frames, these gold bars and I've propagated that initial information throughout the link to this clip. Next I want to refine the matte outline and there is a few ways of going about it. You might have noticed that this pink outline, the segmentation boundary, doesn't exactly follow his hair. You can decide how closely or loosely to follow that hair. That's the smoothing parameter. For example, if I was to take smoothing down to 0, you can see now we have got some crinkles that follow his hair in much more detail, particularly as I go from frame to frame.

Refining the matte

I finished creating my segmentation boundary. I have created a couple of base frames, these gold bars and I've propagated that initial information throughout the link to this clip. Next I want to refine the matte outline and there is a few ways of going about it. You might have noticed that this pink outline, the segmentation boundary, doesn't exactly follow his hair. You can decide how closely or loosely to follow that hair. That's the smoothing parameter. For example, if I was to take smoothing down to 0, you can see now we have got some crinkles that follow his hair in much more detail, particularly as I go from frame to frame.

If I decided I didn't want that much detail, if I want a more feathered outline, I'll go ahead and increase smoothing to a higher value like 5 and now it just creates a basic smooth boundary. If you want to see what that looks like, you can turn off the segmentation boundary, the alpha boundary, and look at it. It either gets a checkerboard or against just a black background. In addition to having a smooth or sharp outline, you can decide how much feather there is around that outline. Basically, how wide the anti-aliasing falloff is.

Feather only really comes into play if Smooth has a value other than 0. So let me go to 1 where it's smoothed out little bit, increase Feather to 100%. Now, you see you have a much more anti- aliased falloff than if I was to put the Feather down to say 0, which is not anti-aliased at all. I'll go around 50 for now. Additionally, if you have got problems either with the edge being eroded away or if you are seeing part of the background when you shouldn't, you can go ahead and change the Choke percentage. I am just going to go ahead and do this until I no longer see that white contamination along the edge.

Turn around, there. That's a good looking edge. Alpha boundary, black background color. But these are but crude tools. The best tool of all is the Refine Matte checkbox. As soon as you turn that on, a few things go on. One, the edge is much better smoothed, particularly taking partial transparency and Motion Blur into account as he moves. You see on these frames, he is moving less, there is less blur around the edges.

And when he is moving more, there is more of a blurred edge. This is even more obvious when we view just the black and white alpha channel. Particularly if I go around here where he is moving particularly fast, you can really see how much blur is going on as he pulls his head back in this direction. This side of his face is being blurred in the direction he is moving. Since the collar lines up with the direction he is moving, it's not as blurred. So it's not an all around blur, it's not an all around feather. It is indeed a motion-predicted motion blur. Now, right around here, I am seeing just a little bit of strobing going on.

I probably need to use more Motion Blur samples. Go up to number like 16, and get a much smoother Motion Blur run through there. The Shutter Angle is at 360 right now, which is higher than the default and higher than normal. The typical film camera emulation is at 180 degrees of motion blur. But I had it set at 360 because I really want you to see that effect. I'll go back down to 180. There is a Higher Quality switch. You don't always need it. It does take more calculation time. But if you are finding you have got some problems with some edges, particularly very blurred edges, Higher Quality gets you a little bit more detail in those highly blurred edges.

I'll turn off my alpha channel, go back to my image for now. Now, in addition to smoothing out and automatically motion blurring that matte outline, there is a couple other things it does. One is Reduce Chatter and I mentioned this earlier. Quite often when you have noisy footage, film grain, a lot of smoke or dust in the air, what should be a straight, flat, not moving edge may seem to sizzle a little bit or chatter a little bit, just because the prediction is thrown off by all that noise or dust or whatever.

Reduce Chatter basically damps that down and says "don't make that edge move." See how much it's moving? If it doesn't cross that threshold, then keep it where it was. Again, you can adjust this to taste. Depending whether or not you are seeing edges getting eroded away, then you have got to reduce chatter crank too high. Or whether or not you are noticing that the edges are indeed sizzling when they shouldn't be. Then reduce chatter as too low. Another great feature here is Decontaminate Edge. Basically, this says remove colors spill.

Just like when you key, background color will wrap around an object and contaminate the color of your foreground. This is particularly an issue in motion blurred areas, because those are partially transparent and the color behind is the background, which you don't necessarily want to see. So, you almost always want to have Decontaminate Edge turned on. If you need to tweak it out, go ahead and view exactly what area is being decontaminated. This white area is how far After Effects' automatically motion estimating needs to be contaminated.

You can go ahead and increase the area being decontaminated. You can play around with the strength of the decontamination. And then there is this additional parameter, Extend Where Smooth, and I'll open up this compound wider so you can see that full wording there. This parameter only affects edges, which have been moved or stabilized to reduce edge chatter. When it's enabled, it's says decontaminate the background color a little bit more around these areas, because the edge is in a different place than we originally predicted.

And I'll turn off View map. Now, what's really interesting about this Refine Matte area is not only is it extraordinarily useful for cleaning up mattes that Roto Brush creates. It exists as an entirely separate plug- in you can apply to any layer with a matte. Whether you have created with paint, mask, keys, whatever, you can slap on that dedicated Refine Matte plug-in that uses all of these parameters to go ahead and clean up your edges. Do the predictive the motion blur, do the spill removal, etcetera.

It's really a nice touch. It came on free on the back of Roto Brush. Now, before we go, I do have to say not every piece of footage is going to be as easy to Roto Brush as this piece was. This one was particularly cooperative. Other shots will be more problematic, particularly when you have arms swinging and legs swinging. When you have to remove sections between arms and legs. It's going to take some more work. You are going to need to spend some more time making corrective strokes with Roto Brush to check what the good and bad areas are.

But what's important to note is you don't rely on Roto Brush alone. You can use other tools to augment Roto Brush. For example, if there were some problems with this black and white alpha matte, I could use my Paint tools to go ahead and alter this matte. Paint on just the alpha channel, paint on just this frame. If I paint with white, I will then be adding to the alpha channel. If I am painting with black, I'll then be removing from the alpha channel.

You can also use masking and other techniques. So Roto Brush should not be viewed as either works or doesn't work. It should be viewed as this is going to do a lot of my work for me and then I can refine my work later on, either playing awhile with parameters, additional corrective strokes or by using tools such as Paint to further clean up and finish off the matte.

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After Effects CS5 New Creative Techniques

35 video lessons · 17944 viewers

Chris Meyer and Trish Meyer
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