Pictures are coded and decoded in a different order than they are displayed. This is due to bidirectional prediction for B pictures. See example below which illustrates reordering for a 12 picture long GOP.

Source order and encoder input order:

I(1) B(2) B(3) P(4) B(5) B(6) P(7) B(8) B(9) P(10) B(11) B(12) I(13)

Encoding order and order in the coded bitstream:

I(1) P(4) B(2) B(3) P(7) B(5) B(6) P(10) B(8) B(9) I(13) B(11) B(12)

Decoder output order and display order (same as input):

I(1) B(2) B(3) P(4) B(5) B(6) P(7) B(8) B(9) P(10) B(11) B(12) I(13)

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