The Roto Brush is a new tool introduced in After Effects CS5 that helps automate creating a matte to separate a foreground from a background, such as isolating an actor from the room around him. To accomplish this, you draw brush strokes to teach After Effects the difference between the two. After Effects uses this information in conjunction with edge detection, motion tracking, and optical flow technologies to follow the changes in foreground and background over time. You can then touch up any remaining problem areas with the paint and masking tools. It’s not perfect, but it’s a lot easier than hand drawing and animating precise mask shapes.