All of the common editing and special effects packages allow you to create Keys – the removal of a coloured background – by adding an Effect to the clip in question.
In After Effects, there are numerous filters but they deal with the removal of one colour only, usually blue or green. When the question arises of removing a more variegated background, it gets tricky. Up to now (ignoring third-party plug-ins) the only sure way to extract a foreground element from its background would be to mask the subject frame by frame. Very time-consuming.
However, Adobe has addressed this problem with the Roto Brush tool. You still have to get your hands dirty now and again, but the process is programmed to do as much of the extraction as automatically as possible.
1. Importing When using the Roto Brush tool, your composition must be the same frame rate as that of the clip you are going to rotobrush. After importing your movie you can check the frame rate in the Project Viewer (top left, Project palette). You can reset it in the Composition Setting box (Composition > Composition Settings).
2. Identifying your subject Place the video in the Composition window and set in and out points as necessary. To extract, find the part of the clip where the subject is shown against the background with as much contrast and definition as possible. Drag the playback head along the timeline until you reach a suitable point.
3. Enter the layer To use the Roto Brush we must enter the Layer window. Double click on the clip in the Composition window. A window opens in the Composition window with its own timeline. Select the Roto Brush from the toolbar. When the brush is over the clip it is represented by a green circular icon with a ‘+’ inside.
4. Create the selection Draw roughly inside the object you wish to extract, tracing its outline inside. A thick green line reminiscent of Photoshop’s Extract Tool traces your path. In a high- contrast clip a single line is enough to catch the object. When you let go, the Roto Brush tries to define the area you wish to extract.