text_src/13.00__vp8-bitstream__dct-coefficient-decoding.txt

### Section 13: DCT Coefficient Decoding                   {#h-13-00}


The second data partition consists of an encoding of the quantized
DCT (and WHT) coefficients of the residue signal.  As discussed in
the format overview (Section 2), for each macroblock, the residue is
added to the (intra- or inter-generated) prediction buffer to produce
the final (except for loop filtering) reconstructed macroblock.

VP8 works exclusively with 4x4 DCTs and WHTs, applied to the 24 (or
25 with the Y2 subblock) 4x4 subblocks of a macroblock.  The ordering
of macroblocks within any of the "residue" partitions in general
follows the same raster scan as used in the first "prediction"
partition.

For all intra- and inter-prediction modes apart from `B_PRED` (intra:
whose Y subblocks are independently predicted) and `SPLITMV` (inter),
each macroblock's residue record begins with the Y2 component of the
residue, coded using a WHT.  `B_PRED` and `SPLITMV` coded macroblocks
omit this WHT and specify the 0th DCT coefficient in each of the 16 Y
subblocks.

After the optional Y2 block, the residue record continues with 16
DCTs for the Y subblocks, followed by 4 DCTs for the U subblocks,
ending with 4 DCTs for the V subblocks.  The subblocks occur in the
usual order.

The DCTs and WHT are tree-coded using a 12-element alphabet whose
members we call "tokens".  Except for the end-of-block token (which
sets the remaining subblock coefficients to zero and is followed by
the next block), each token (sometimes augmented with data
immediately following the token) specifies the value of the single
coefficient at the current (implicit) position and is followed by a
token applying to the next (implicit) position.

For all the Y and chroma subblocks, the ordering of the coefficients
follows a so-called zig-zag order.  DCTs begin at coefficient 1 if Y2
is present, and begin at coefficient 0 if Y2 is absent.  The WHT for
a Y2 subblock always begins at coefficient 0.