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

 

P Pictures

 

B Pictures

 

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.

 

MPEG 2 Video Compression topics:

Introduction to MPEG 2 Video Compression

MPEG Coder/Decoder Diagram

Profiles and Levels

Frames, Fields, Pictures (I, P, B)

I P B Picture Reordering

Pixel Positioning

Compression tools

MPEG 2 Video Data Structures