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--the previous frame--was used for P-frames. Two reference frames (one past and one future) were used for B-frames
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 on which to base each macroblock
in the next frame. While the best frame for this purpose is usually the previous frame, the extra reference frames 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. Different reference frames can be chosen for each 8x8 partition of a macroblock. Another video format that supports multiple reference frames is Snow
, which can handle up to eight.
Multiple reference frames can 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.
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. The use of several reference frames also decreases locality of reference
, which might cause a speed impact.
- E. G. Richardson, Iain H.264 and MPEG-4 Video Compression: Video Coding for Next-generation Multimedia. Chichester: John Wiley & Sons Ltd..