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The Roto Brush is a revolutionary new approach in After Effects CS5 for Isolating moving foreground elements from their backgrounds. It is largely based on the Video SnapCut system, a research prototype that was jointly developed by Adobe and University of Minnesota, and was presented at SIGGRAPH 2009. Roto Brush works much like the Quick Select tool in Photoshop: you draw foreground and background strokes on key frames to make an initial selection, and the system automatically propagates the selection to other frames, which provides a fast workflow for generating masks in video.

Here are some links to learn this feature: