Dirac Bitstream Format
by Anuradha Suraparaju (anuradha@rd.bbc.co.uk)
v0.1: 18-Aug-2004

[ This document is a temporary document. It will be replaced by full-fledged
  bitstream syntax and decoding process document in alpha release 0.5.0 ]

1. Introduction

   A Dirac bitstream is made of up of a sequence of one or more frames. Each
   frame is either intra or inter-coded. Intra frames are compressed without
   references to any other frame. Inter frames are motion compensated with
   respect to one or more reference frames. 
  
2. Sequence structure

   The start of a sequence is the only random access point in this version of
   the Dirac bitstream. A random access point is from where a decoder can 
   start decoding the bitstream without prior information.

2.1 Start codes

   Start codes are unique bit patterns that do not otherwise occur in the
   bitstream. They identify specific points in the bitstream e.g. start of
   frame, start of sequence, etc. A start code is made up of a 4 byte
   start code prefix and a one byte code. The start code prefix is

   0x42 0x42 0x43 0x44 (BBCD)

   Due to the nature of the arithmetic coded output, the first four bytes can
   be output as part of the component data and therefore an additional code is
   required to indicate that a header does not follow. The different start 
   codes are

   Code                  Description
   0xD7                  Start of Sequence
   0xD6                  Start of an I Frame
   0xD5                  Start of an L2 Frame
   0xD4                  Start of an L1 Frame
   0xD0                  End of Sequence
   0xFF                  Not a header code treat code prefix and next byte as
                         data

2.2 Sequence data

   A sequence commences with a start of sequence code followed by 
   the sequence header which is followed by one or more coded frames. 
   The order of the coded frames in the coded bistream is in the order 
   in which the the decoder processes them but not in the display
   order. The sequence is terminated by a sequence end code.

   TODO: list sequence header fields

2.3. Frame structure

   Each frame commences with a start of frame code (I or L1 or L2 depending
   on the type of frame) and is followed by a frame header. A frame can be 
   either an intra frame (I frame) or inter frame. There are two types of 
   inter-coded frames: Level 1 (L1) frames which are also used as temporal 
   references for other pictures and Level 2 (L2) pictures which are 
   bi-directionally predicted and are not used as temporal references. The 
   frame header is followed by motion vector data (for L1 and L2 frames only) 
   and component data.

   TODO: list frame header fields and frame data format

3. Bitstream syntax

   Field                 Value

   stream-id             8 byte identfier set to KW-DIRAC
   sequence start code   0x42424344D7
   sequence header       Golomb coded sequence header 
   frame start code      0x42424344D6 (I) or 0x42424344D5(L2) or
                         0x42424344D5 (L1)
   frame header          Golomb coded frame header
   frame data            Arithmetic coder output
   .
   .                     
   .
   sequence end code    0x42424344D0