Key Frame - Video Compression

Video Compression

In video compression, a keyframe, also known as an Intra Frame, is a frame in which a complete image is stored in the data stream. In video compression, only changes that occur from one frame to the next are stored in the data stream, in order to greatly reduce the amount of information that must be stored. 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. Whenever a drastic change to the image occurs, such as when switching from one camera shot to another, or at a scene change, a keyframe must be created. The entire image for the frame must be output when the visual difference between the two frames is so great that representing the new image incrementally from the previous frame would be more complex and would require even more bits than reproducing the whole image.

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).