Frames, Fields, Pictures (I, P, B)
MPEG 2 encodes video as a series of pictures.
For interlaced sequences the 2 fields of a frame may be encoded together as a frame picture. Alternatively they may be encoded separately as 2 field pictures. Both frame pictures and field pictures may be used together in a single interlaced sequence. High detail and limited motion favors frame picture encoding. Field pictures always occur in pairs (top - bottom or bottom - top). The output of the decoding process for an interlaced sequence is a series of reconstructed fields.
For progressive sequences, all pictures are frame pictures. The output of the decoding process for a progressive sequence is a series of reconstructed frames.
Encoded pictures are classified into 3 types: I, P, and B.
I Pictures
Intra Coded Pictures
All macroblocks coded without prediction
Needed to allow receiver to have a "starting point" for prediction after a channel change and to recover from errors
P Pictures
Predicted Pictures
Macroblocks may be coded with forward prediction from references made from previous I and P pictures or may be intra coded
B Pictures
Bi-directionally predicted pictures
Macroblocks may be coded with forward prediction from previous I or P references
Macroblocks may be coded with backward prediction from next I or P reference
Macroblocks may be coded with interpolated prediction from past and future I or P references
Macroblocks may be intra coded (no prediction)
Note that in P and B pictures, macroblocks may be skipped and not sent at all. The decoder then uses the anchor reference pictures for prediction with no error.
B pictures are never used as prediction references.
Introduction to MPEG 2 Video Compression
Frames, Fields, Pictures (I, P, B)