text_src/16.01__vp8-bitstream__intra-predicted-macroblocks.txt

#### 16.1 Intra-Predicted Macroblocks                      {#h-16-01}


For intra-prediction, the layout of the prediction data is
essentially the same as the layout for key frames, although the
contexts used by the decoding process are slightly different.

As discussed in Section 8, the "outer" Y mode here uses a different
tree from that used in key frames, repeated here for convenience.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
const tree_index ymode_tree [2 * (num_ymodes - 1)] =
{
 -DC_PRED, 2,           /* root: DC_PRED = "0", "1" subtree */
  4, 6,                 /* "1" subtree has 2 descendant subtrees */
   -V_PRED, -H_PRED,    /* "10" subtree:  V_PRED = "100",
                           H_PRED = "101" */
   -TM_PRED, -B_PRED    /* "11" subtree:  TM_PRED = "110",
                           B_PRED = "111" */
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}

The probability table used to decode this tree is variable.  As
described in Section 11, it (along with the similarly treated UV
table) can be updated by field J of the frame header.  Similar to the
 coefficient-decoding probabilities, such updates are cumulative and
 affect all ensuing frames until the next key frame or explicit
 update.  The default probabilities for the Y and UV tables are:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Prob ymode_prob [num_ymodes - 1] = { 112, 86, 140, 37};
Prob uv_mode_prob [num_uv_modes - 1] = { 162, 101, 204};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


These defaults must be restored after detection of a key frame.

Just as for key frames, if the Y mode is `B_PRED`, there next comes an
encoding of the `intra_bpred` mode used by each of the sixteen Y
subblocks.  These encodings use the same tree as does that for key
frames but, in place of the contexts used in key frames, these
encodings use the single fixed probability table.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
const Prob bmode_prob [num_intra_bmodes - 1] = {
    120, 90, 79, 133, 87, 85, 80, 111, 151
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


Last comes the chroma mode, again coded using the same tree as that
used for key frames, this time using the dynamic `uv_mode_prob` table
described above.

The calculation of the intra-prediction buffer is identical to that
described for key frames in Section 12.