B-frame

Bi-directional predicted (B) frames/slices (macroblocks)

• Require the prior decoding of subsequent frame(s) to be displayed.
• May contain image data and/or motion vector displacements. Older standards allow only a single global motion compensation vector for the entire frame or a single motion compensation vector per macroblock.
• Include some prediction modes that form a prediction of a motion region (e.g., a macroblock or a smaller area) by averaging the predictions obtained using two different previously decoded reference regions. Some standards allow two motion compensation vectors per macroblock (biprediction).
• In older standards (such as MPEG-2), B-frames are never used as references for the prediction of other pictures. As a result, a lower quality encoding (requiring less space) can be used for such B-frames because the loss of detail will not harm the prediction quality for subsequent pictures.
• H.264 relaxes this restriction, and allows B-frames to be used as references for the decoding of other frames at the encoder's discretion.
• Older standards (such as MPEG-2), use exactly two previously decoded pictures as references during decoding, and require one of those pictures to precede the B-frame in display order and the other one to follow it.
• H.264 allows for one, two, or more than two previously decoded pictures as references during decoding, and can have any arbitrary display-order relationship relative to the picture(s) used for its prediction.
• The heightened flexibility of information retrieval means that B-frames typically require fewer bits for encoding than either I or P-frames.