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Video: Refining the matte

Now that you've finished all of this work of creating your segmentation boundary across your entire Roto Brush Span, you really don't want to lose this work so it's a good idea to do a Save or even better an Increment and Save so you can go back to an earlier version of your project in case you take a detour and decide maybe what's the right path to follow. Now if you've been following along and you have a segmentation boundary you're pretty proud of, you can continue with that composition. If you've not made it this far and if you have access to the Exercise Files that come with this lesson, go ahead and open up Comps_finished, RB2-Screen Replacement-Step 16.
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Watch the Online Video Course After Effects Apprentice 13: Paint, Roto, and Puppet
3h 11m Intermediate Dec 21, 2011 Updated Dec 12, 2012

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In this course, Trish and Chris Meyer introduce a series of creative tools inside Adobe After Effects. The centerpiece is Paint, where Trish demonstrates how to use the Brush, Eraser, and Clone Stamp tools to draw on a layer, remove portions of it, or repeat elements around a composition. These tools can be used for artistic purposes as well as to repair problem areas in footage. Chris shows off the Puppet tools for distorting layers, and the incredible Roto Brush, introduced in After Effects CS6, which allows you to separately define foreground and background elements so that you can replace backgrounds and selectively add special effects.

The After Effects Apprentice videos on lynda.com were created by Trish and Chris Meyer and are designed to be used on their own and as a companion to their book After Effects Apprentice. We are honored to host these tutorials in the lynda.com library.

Topics include:
  • Creating, erasing, and animating brush and clone strokes
  • Using Paint to reveal a layer over time
  • Creating animated distortions to flat artwork with the Puppet tools
  • Replacing backgrounds with the Roto Brush
Subject:
Video
Software:
After Effects
Authors:
Chris Meyer Trish Meyer

Refining the matte

Now that you've finished all of this work of creating your segmentation boundary across your entire Roto Brush Span, you really don't want to lose this work so it's a good idea to do a Save or even better an Increment and Save so you can go back to an earlier version of your project in case you take a detour and decide maybe what's the right path to follow. Now if you've been following along and you have a segmentation boundary you're pretty proud of, you can continue with that composition. If you've not made it this far and if you have access to the Exercise Files that come with this lesson, go ahead and open up Comps_finished, RB2-Screen Replacement-Step 16.

This is after I've created the segmentation boundary for you and I'll continue with this one as well and double-click Actor Roto to get it opened in my Layer panel. You notice that I don't have the little bars extending from my Base Frame that indicates that Roto Brush has not had an opportunity to go ahead and propagate that information out and calculate these frames. If you are happy with your segmentation boundary, a good strategy is to go ahead and click the Freeze button.

This says, go ahead and calculate all of the Roto Brush propagation for all the frames inside of your Roto Brush Span and allows you to save these along with your project. That will make working with Roto Brush, and even more important rendering Roto Brush, go much faster. So I'm going to go ahead and freeze this propagation boundary. You'll see the blue line indicating frozen frames. And if I decided to change this segmentation boundary later, I can always unfreeze it. But now that I've done this, I'll go ahead and Increment and Save again.

Now if I open up this project later, I would have these cached Roto Brush frames. The segmentation boundary is just a binary matte; on or off, are you foreground or background. It's come time to start saying just how good this matte is and to refine it. To do so, I'm going to go down to these extra buttons that were added in After Effects CS5 along with the introduction of Roto Brush. It gives you some additional options of how you view the Alpha channel of any layer. For example, if I turn off the segmentation boundary, I will now get my Alpha channel display for this layer.

I will now see my foreground and no background. If I want to look at the Alpha channel, I'll click on the Toggle Alpha button. If I like the Photoshop Style Red Overlay, here I can see my background but with a selectable color overlaying the background portion of my shot. I want to go ahead and just leave these all turned off, no segmentation boundary, and turn off my toggle transparency so I can see against the black background. This is the result of my binary matte, a little rough here and there. Let's go ahead and improve that. With my Actor Roto still selected, I'm going to press F3 to reopen my Effects Control panel, look at the Matte section and enable Refine Matte.

This engages a second section of Roto Brush where it smoothes out the matte, looks for areas of partial transparency and alters the Alpha channel, calculates motion blur and decontaminates color around the edges. Let's discuss what all of those mean. To see my shot in context, I'm going to bring my Composition panel forward again. Now I have my Actor Roto layer with the Roto Brush overlaid on top of my screen and my background. Turn it off and on and solo it, just to give you an indication of what these layers are.

First I'm going to work with these Smooth, Feather and Choke parameters. Roto Brush defaults to value of 2.0 for Smooth. I reduced it temporarily while I was creating my segmentation boundary. Let's see whether or not we need additional smoothing or less smoothing for that boundary. You'll notice that if I increase smoothing too much, you will start to see bits of the original footage creep into these gaps and blot out portions of my new control panel screen. On the other hand, if I take smoothing down too much, you'll see some fairly jagged lines around the edges of my Roto Brush matte.

So I want to scrub to pick a value where I'm not picking up any of the original shots where it's not supposed to be such as between his fingers but don't have too jagged of a boundary or get some ghosting like I do here. I'm going to say that right around here is a fairly good value. Let's increase a little bit here and I'm fairly happy with that compromise. Now this is me looking at one frame namely the Base Frame. Always check your work later and earlier in time.

For example, when I go earlier in time, I see that I have a nasty edge around here. This is a case where I might want a bit more smoothing to get rid of some this jaggieness. So let's crank up the smoothing a little bit and at least smooth out that profile. I need to find a compromised value that's going to work throughout this shot. In addition to smoothing, I can balance off the Feather and Choke. Feather is how soft the transitions between the foreground and background are.

If I have no Feather, I'm back to a fairly hard binary matte and you can see a bit of aliasing on these edges. If I have too much Feather, that will really soften my transition but I can get a little too puffy, a little bit too soft, I can introduce some unwanted areas again. So I need to find a balance between Feather, which creates some nice soft edges and Choke, which either says get rid of some artifacts around the edges, basically reduce the boundary, and I can go too tight on that, or go the other direction, expand the Alpha channel.

There I am creating some haloing that I don't like so that's obviously not very good. I find that increasing my Feather while keeping by smoothing down is getting rid of most of my artifacts. So let's see if we did a little bit of choking now, and get rid of the last of the artifacts at hand. That looks fairly good. I maybe eating away a little too much of his bracelet here, I could possibly need to go back and redo my segmentation boundary just for this portion of his wrist to make sure I don't lose too much of the bracelet.

Let's go ahead and look at some other points in time to see how well I'm balancing off by smoothing. I have got a little issue down here in the hand, with all of my other parameters and that's too little smoothing. Let's experiment with the Choke a little bit until I'm fairly happy, I have got a good compromise throughout this shot. Keeping in mind that this will also be in motion, I'll definitely need to -- yup, I got to increase my smoothing back a little bit to get rid of that artifact.

Keeping in mind I'm looking for a good compromise across all frames. A little more Choke, yeah. There's one more parameter in this section to discuss before we move on. It's called Reduce Chatter. There can be a problem if you have a somewhat stationary object but there's noise in your video or your film that is causing Roto Brush to think your edge is actually moving when it's not. If you see stationary or slow moving edges where the Alpha just seems to be bouncing and jittering too much, increase Reduce Chatter. On the other hand if you have moving edges where Roto Brush is missing subtle movement, you can decrease Reduce Chatter to go ahead and allow those edges back in and I find that actually decreasing Reduce Chatter is giving me a little bit of the wrist back at the expense of getting a little bit of an artifact back there.

So I'm going to try to find a compromise at a lower setting maybe a little bit more Choke that might give me a better solution. I'm going to go with minimal Reduce Chatter and just a little bit more Choke.

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