A source picture is a contiguous rectangular array of pixels. A picture may be a complete frame of video ("frame picture") or one of the interlaced fields from an interlaced source ("field picture").

A field picture does not have any blank lines between its active lines of pixels.  If viewed on a normal display it would appear "short and fat".

A coded picture (also called a video access unit) begins with a start code and a header.

The header carries -

picture type (I, B or P)

temporal reference information

motion vector search range

optional user data

As stated above, a picture may be a frame picture or a field picture.

A frame picture consists of:

a frame of a progressive source or

a frame (2 spatially interlaced fields) of an interlaced source

A field picture consists of only one field from an interlaced source.

In a progressive sequence, all pictures are frame pictures, and all macroblocks are frame DCT coded.

In an interlaced sequence, any frame may be coded as a frame picture or as two field pictures.  In a frame picture from an interlaced sequence, individual macroblocks may be either field DCT coded or frame DCT coded. In a field picture, all macroblocks are field coded by default since all pixels are from only one field.

MPEG 2 Video Data Structures Topics:

- PIXEL

- BLOCK

- MACROBLOCK

- Field DCT Coding and Frame DCT Coding

- SLICE

- PICTURE

- GROUP of PICTURES (GOP)

- SEQUENCE

- PACKETIZED ELEMENTARY STREAM

Up to MPEG 2 Video Data Structures