Separating a foreground object, such as an actor, from a background is a crucial step in many visual effects and compositing workflows. When you’ve created a matte that isolates an object, you can replace the background, selectively apply effects to the foreground, and much more.
Conventionally, segmentation of a moving image into foreground and background elements has been accomplished through rotoscoping—defining mattes by manually drawing Bezier curves (masks) on most frames, with some interpolation. (See Rotoscoping introduction and resources.)
The Roto Brush tool provides an alternative, faster workflow for this segmentation and creation of a matte.
With the Roto Brush tool, you draw strokes on representative areas of the foreground and background elements, and then After Effects uses that information to create a segmentation boundary between the foreground and background elements. The strokes that you make on one area inform After Effects about what is foreground and what is background in adjacent areas and on adjacent frames. Various techniques are used to track regions across time, and this information is used to propagate segmentation forward and backward in time so that each stroke that you make is used to improve the results on nearby frames. Even if an object moves or changes shape from one frame to the next, the segmentation boundary adapts to match the object.
After you have created a segmentation boundary, you use the Refine Matte properties to improve the matte. The Refine Matte effect is also available separately for the improvement of mattes created using features other than the Roto Brush tool.