text_src/09.05__vp8-bitstream__token-partition-and-partition-data-offsets.txt

#### 9.5 Token Partition and Partition Data Offsets        {#h-09-05}


VP8 allows DCT coefficients to be packed into multiple partitions,
besides the first partition with header and per-macroblock prediction
information, so the decoder can perform parallel decoding in an
efficient manner.  A two-bit `L(2)` is used to indicate the number of
coefficient data partitions within a compressed frame.  The two bits
are defined in the following table:


| Bit 1   |  Bit 0  |  Number of Partitions
| :-----: | :-----: | :--------------------:
| 0       | 0       | 1
| 0       | 1       | 2
| 1       | 0       | 4
| 1       | 1       | 8


Offsets are embedded in the bitstream to provide the decoder direct
access to token partitions.  If the number of data partitions is
greater than 1, the size of each partition (except the last) is
written in 3 bytes (24 bits).  The size of the last partition is the
remainder of the data not used by any of the previous partitions.

The partitioned data are consecutive in the bitstream, so the size
can also be used to calculate the offset of each partition.  The
following pseudocode illustrates how the size/offset is defined by
the three bytes in the bitstream.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Offset/size  =  (uint32)(byte0) + ((uint32)(byte1)<<8) 
  + ((uint32)(byte2)<<16);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~