Page "Key frame" Paragraph 32

Because video compression only stores incremental changes between frames ( except for keyframes ), it is not possible to fast forward or rewind to any arbitrary spot in the video stream.

That is because the data for a given frame only represents how that frame was different from the preceding frame.

For that reason it is beneficial to include keyframes at arbitrary intervals while encoding video.

For example, a keyframe may be output once for each 10 seconds of video, even though the video image does not change enough visually to warrant the automatic creation of the keyframe.

That would allow seeking within the video stream at a minimum of 10 second intervals.

The down side is that the resulting video stream will be larger in size because many keyframes were added when they were not necessary for the visual representation of the frame.

This drawback, however, does not produce significant loss of compression when the bitrate is already set at a high value for better quality ( as in the DVD mpeg2 format ).