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The Roto Brush is probably the most important new feature in After Effects CS5. With Roto Brush you draw just general brushstrokes, saying here's the foreground, here's the background, and Roto Brush will automatically detect the edge between that foreground and background. You don't have to carefully draw a mask edge. You don't have to carefully paint along the edge. It will find the difference between foreground and background for you. What's really important is that it does it across multiple frames. You don't need to do this every single frame yourself.
It looks the foreground and background, does motion prediction following things as it moves from frame-to- frame and adjusts the map boundaries to follow that motion in the shot. It's pretty easy to make corrections in case it makes a mistake. Now Roto Brush is not magic. It's not going to give you a perfect matte the first time. You are going to need to do a little bit of work and cleaning up edges and correcting the way it doesn't follow, and you will probably need to clean some stuff up later on post. Add paint strokes or add mask shapes. But it's going to be a lot faster than having a hand-cut a mask or hand-paint an object every single frame of a long shot.
So let me show you how you use Roto Brush. I kind of break the process into three steps. One, come up with a good base reference frame. Really define the foreground and the background to give Roto Brush good information to work from. Two, propagate that information across all the frames in the clip, following all the motion that takes place in the clip. Then three, refining that matte, cleaning up the edges, taking motion blur on edges into account, decontaminating colors spill around the edges, and other cleanups you need to do. And after you've done that, you're going to have a lot of fun.
So let's dive in!
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