text_src/11.02__vp8-bitstream__luma-modes.txt

#### 11.2 Luma Modes                                         {#h-11-02}


First comes the luma specification of type `intra_mbmode`, coded using the `kf_ymode_tree`, as described in Chapter 8 and repeated here for convenience:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
typedef enum
{
    DC_PRED, /* predict DC using row above and column to the left */
    V_PRED,  /* predict rows using row above */
    H_PRED,  /* predict columns using column to the left */
    TM_PRED, /* propagate second differences a la "True Motion" */

    B_PRED,  /* each Y subblock is independently predicted */

    num_uv_modes = B_PRED,  /* first four modes apply to chroma */
    num_ymodes   /* all modes apply to luma */
}
intra_mbmode;

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


For key frames, the Y mode is decoded using a fixed probability array as follows:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
const Prob kf_ymode_prob [num_ymodes - 1] = { 145, 156, 163, 128};
Ymode = (intra_mbmode) treed_read( d, kf_ymode_tree, kf_ymode_prob);
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


`d` is of course the `bool_decoder` being used to read the first data partition.

If the Ymode is `B_PRED`, it is followed by a (tree-coded) mode for each of the 16 Y subblocks. The 10 subblock modes and their coding tree as follows:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
typedef enum
{
    B_DC_PRED,  /* predict DC using row above and column
                   to the left */
    B_TM_PRED,  /* propagate second differences a la
                   "True Motion" */

    B_VE_PRED,  /* predict rows using row above */
    B_HE_PRED,  /* predict columns using column to the left */

    B_LD_PRED,  /* southwest (left and down) 45 degree diagonal
                   prediction */
    B_RD_PRED,  /* southeast (right and down) "" */

    B_VR_PRED,  /* SSE (vertical right) diagonal prediction */
    B_VL_PRED,  /* SSW (vertical left) "" */
    B_HD_PRED,  /* ESE (horizontal down) "" */
    B_HU_PRED,  /* ENE (horizontal up) "" */

    num_intra_bmodes
}
intra_bmode;

/* Coding tree for the above, with implied codings as comments */

const tree_index bmode_tree [2 * (num_intra_bmodes - 1)] =
{
 -B_DC_PRED, 2,                   /* B_DC_PRED = "0" */
  -B_TM_PRED, 4,                  /* B_TM_PRED = "10" */
   -B_VE_PRED, 6,                 /* B_VE_PRED = "110" */
    8, 12,
     -B_HE_PRED, 10,              /* B_HE_PRED = "11100" */
      -B_RD_PRED, -B_VR_PRED,     /* B_RD_PRED = "111010",
                                     B_VR_PRED = "111011" */
     -B_LD_PRED, 14,              /* B_LD_PRED = "111110" */
       -B_VL_PRED, 16,            /* B_VL_PRED = "1111110" */
         -B_HD_PRED, -B_HU_PRED   /* HD = "11111110",
                                     HU = "11111111" */
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


The first four modes are smaller versions of the similarly-named 16x16 modes above, albeit with slightly different numbering. The last six "diagonal" modes are unique to luma subblocks.