Welkin小窩

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

Encoding order and order in the coded bitstream:
(1) (4) (2) (3) (7) (5) (6) (10) (8) (9) (13) (11) (12)
 I   P   B   B   P   B   B   P    B   B   I    B    B

Decoder output order and display order (same as input):
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
 I   B   B   P   B   B   P   B   B   P    B    B    I