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Now again, I want to be completely honest with you. That previous Roto Brush example was deceptively simple. Very, very few shots will ever work out that well with that a small amount of work. The problem is many After Effects users who want to use Roto Brush don't bother learning the correct way to use it, expect they can do that small amount of work, and expect those good results. Well, they're sorely disappointed and they walk away convinced Roto Brush doesn't work. The reality is, Roto Brush does require a lot of work on your part and it works much better if you follow a very specific workflow.
And that's what we're going to be covering in the next several movies. The steps that workflow include: identifying the foreground that you wish to separate from the background, choosing a representative Base Frame with a maximum amount of foreground information visible, defining the Base Frame using a collection of foreground and background Roto Brush strokes, moving a few frames away from that base and tweaking the Roto Brush's Propagation parameters to optimize how Roto Brush tracks changes in the shot, returning to the Base Frame and then moving away from it one frame at a time adding corrective foreground-background strokes as needed, and finally tweaking the Matte parameters to refine the resulting alpha channel.
I've switched back to After Effects CS5.5. -- Roto Brush works in 5, 5.5, 6 or later -- and I'm going to close my previous example. And if you have access to the Exercise Files that came with this lesson, open up Roto Brush 2 Screen Replacement starter (RB2-Screen Replacement*starter). We've already done a lot of prep work for you in this shot. The original footage showed an actor sitting in front of a laptop with a fairly boring display. Note that the actor's hands are also in front of that screen.
What we would prefer to do is put a brand new, far sexier screen on the face of that laptop. I've already tracked this for you using Mocha, which was discussed in a previous lesson, and also did a little bit of blurring to make sure it matched the camera's depth of field blur that naturally existed in the scene. The next thing we need to do is cut out a copy of the actor's hands and arms using Roto Brush and put that copy in front of our new screen, that way it looks like it was behind his hands when the shot was originally taken. So with that in mind, in the next movie we're going to set up our project and go about identifying a good Base Frame to start our Roto Brush work.
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