In animation and filmmaking, a key frame (or keyframe) is a drawing or shot that defines the starting and ending points of a smooth transition.
Supposing that an animator wants the beam size to change smoothly from one value to another within a predefined period of time, that could be achieved by using key frames.
This technique capitalizes on the fact that most video sources (such as a typical movie) have only small changes in the image from one frame to the next.
For that reason, it is beneficial to include key frames at arbitrary intervals while encoding video.
This drawback, however, does not produce significant compression loss when the bitrate is already set at a high value for better quality (as in the DVD MPEG-2 format).