Reference frames are frames of a compressed video that are used to define future frames. As such they are only used in inter-frame compression techniques. In older video encoding standards, such as MPEG-2, only one reference frame was allowed, which was usually the last I-frame or P-frame. In simpler terms, this meant that each frame was encoded as either a stand-alone image (an I-frame) or as the difference from the previous frame.
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Multiple reference frames
Some modern video encoding standards, such as H.264/AVC, allow the use of multiple reference frames. This allows the video encoder to choose among more than one previously decoded frame to base each macroblock in the next frame off of. While the best frame for this purpose is usually the previous frame, the extra reference frames when used can improve compression efficiency and/or video quality. Note that different reference frames can be chosen for different macroblocks in the same frame. The maximum number of concurrent reference frames supported by H.264 is 16.
Mixed reference frames
H.264 also allows reference frames to be "mixed"; that is, the best reference frame to use is chosen individually for each partition, rather than each macroblock. This is considerably more efficient than the standard technique, as it allows decisions to be made on a more precise level.
Disadvantages
Encoding
Multiple reference frames considerably reduce encoding speed because many of the decisions, such as motion estimation, that are ordinarily carried out only on one reference frame have to be repeated on all of the reference frames. Heuristics can be used to reduce this speed cost at the cost of quality. Very high numbers of reference frames are rarely useful in terms of quality for live-action material because frames from farther back in time generally have less and less correlation with the current frame. This is not as true for animated sources, where repetitive motion can make high numbers of reference frames more useful.
Decoding
When decoding, reference frames must be stored in memory until they are no longer needed for further decoding. This can considerably raise the memory usage of the decoder for videos with large numbers of reference frames.
References
- E. G. Richardson, Iain (2003). H.264 and MPEG-4 Video Compression: Video Coding for Next-generation Multimedia. Chichester: John Wiley & Sons Ltd..
