As we learned earlier in the section "Scheduled Screen Updates," the Flash runtime updates the screen only when necessary (that is, when visual content has been changed or added). More specifically, when the Flash runtime updates the screen, it renders only those areas that have changed since the last update. For example, consider an animation with two frames, in which the first frame contains a circle, and the second contains the same circle but also adds a triangle. When the Flash runtime renders the second frame, it redraws the rectangular area containing the triangle but doesn't redraw the circle. The rectangular area encompassing all content that has changed is known as the redraw region (sometimes referred to in graphics programming as the dirty rectangle).
In debugging versions of the Flash runtime, the redraw region can be shown by right-clicking the Player window and choosing Show Redraw Regions.