In broadcast video, a small period of time elapses between video frames, during which a display device refreshes its display for the next frame. This period of time is called the Vertical Blanking Interval (VBI). Instead of sitting idle during the VBI, broadcast video encodes data in the first twenty-one scan lines of a video frame and sends these lines during the VBI. This data is often used for closed captioning or time-stamping, but can be used for other purposes.
DirectDraw video-port extensions enable you to divert data contained with the VBI to a surface, bypass scaling of VBI data, and automatically flip between VBI surfaces in a flipping chain. Once data is in a surface, you can directly access the surface's memory as needed.
For more information, see Auto-flipping.