5. Refine the selection The area the brush selected is outlined by a purple path, and unless you are very lucky there will be areas to add and remove from the selection. To add to the selection, just brush or click over that area. To remove areas, press the Option key and similarly brush or click over the areas you wish to remove.
6. The timeline The frame the Roto Brush is applied to is called the Reference Frame and as soon as the brush is applied After Effects creates a span of frames either side of this under the timeline of about two seconds. The selection within this span is based on the referenced frame both backwards and forwards in time.
7. Evaluate the selection Press the spacebar: After Effects will calculate the selection for the right half of the span. The selection is based on the reference frame and is propagated forwards. For backwards, move the playback head left of the frame, or to the beginning and press spacebar to render automatically.
8. Fine-tune the selection Regions where the edge does not match the object can be edited frame by frame using the Roto Brush to add or subtract as in Step 5. To resize the brush, hold down the Cmd key and drag on the screen. As you work your way through the frames the span will automatically get longer.
9. The views The buttons left of the timeline let us choose to see the Alpha channel, the Selection Boundary or with a Rubylith overlay. They also toggle on or off the selection’s visibility. You can paint on the mask in any of the views to refine your selection, as well as select the brush size from the Brushes palette.
10. Propagation When the Roto Brush is applied, its controls appear in the Effect Controls palette. The Propagation controls regulate how the program selects the boundary. Reducing the Search Boundary from the default 30 tightens overspill. Check View Search Region to see it represented on screen as a yellow border.