Compression can be of two kinds: intraframe and interframe.
Intraframe compression compresses the data within one frame relative only to
itself. Key frames are compressed with intraframe compression because they must
reconstruct an entire image without reference to other frames.
Interframe compression compresses the data in one frame relative to others. These
relative frames are called delta frames. If the
information in delta frames is encoded relative only to previous frames, then
that is unidirectional compression--the codec always looks
back in order to decompress the video image.
Bidirectional prediction is a form of compression in which the codec uses
information not only from frames that have already been
decompressed, but also from frames yet to come. The codec looks in two directions:
ahead as well as back. This helps avoid large spikes
in data rate caused by scene changes or fast movement, improving image quality.
Bidirectional prediction adds a certain amount of
playback overhead; for a clip without much movement, such as a talking head,
it may not significantly improve visual quality.
