Video Compression Picture Types - Bi-directional Predicted Frames/slices (B-frames/slices)

Bi-directional Predicted Frames/slices (B-frames/slices)

• Require the prior decoding of other frame(s) to be decoded.
• May contain both image data and motion vector displacements or combinations of the two.
• 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.
• In older standard designs (such as MPEG-2), B-frames are never used as references for the prediction of other pictures. As a result, a lower quality encoding (resulting in the use of fewer bits than would otherwise be the case) can be used for such B-frames because the loss of detail will not harm the prediction quality for subsequent pictures.
• In H.264, may or may not be used as references for the decoding of other pictures (at the discretion of the encoder).
• In older standard designs (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.
• In H.264, can use 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.
• Typically require fewer bits for encoding than either I or P-frames.