text_src/17.02__vp8-bitstream__probability-updates.txt

#### 17.2 Probability Updates                                {#h-17-02}


The decoder should maintain an array of two `MV_CONTEXT`s for decoding row and column components, respectively. These `MV_CONTEXT`s should be set to their defaults every key frame. Each individual probability may be updated every interframe (by field [[ "J" ]] of the frame header) using a constant table of update probabilities. Each optional update is of the form [[ B? P(7) ]], that is, a bool followed by a 7-bit probability specification if true.

As with other dynamic probabilities used by VP8, the updates remain in effect until the next key frame or until replaced via another update.

In detail, the probabilities should then be managed as follows.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/* Never-changing table of update probabilities for each 
   individual probability used in decoding motion vectors. */

const MV_CONTEXT vp8_mv_update_probs[2] =
{ 
  {
    237,
    246,
    253, 253, 254, 254, 254, 254, 254,
    254, 254, 254, 254, 254, 250, 250, 252, 254, 254
  }, 
  {
    231,
    243,
    245, 253, 254, 254, 254, 254, 254,
    254, 254, 254, 254, 254, 251, 251, 254, 254, 254
  }
}; 

/* Default MV decoding probabilities. */

const MV_CONTEXT default_mv_context[2] =
{
  {                       // row 
    162,                    // is short
    128,                    // sign
      225, 146, 172, 147, 214,  39, 156,      // short tree
    128, 129, 132,  75, 145, 178, 206, 239, 254, 254 // long bits
  },

  {                       // same for column
    164,                    // is short
    128,
    204, 170, 119, 235, 140, 230, 228,
    128, 130, 130,  74, 148, 180, 203, 236, 254, 254 // long bits 

  }
};

/* Current MV decoding probabilities, set to above defaults 
   every key frame. */

MV_CONTEXT mvc [2];     /* always row, then column */

/* Procedure for decoding a complete motion vector. */

typedef struct { int16 row, col;}  MV;  /* as in previous chapter */

MV read_mv( bool_decoder *d)
{
    MV v;
    v.row = (int16) read_mvcomponent( d, mvc);
    v.col = (int16) read_mvcomponent( d, mvc + 1);
    return v;
}

/* Procedure for updating MV decoding probabilities, called 
   every interframe with "d" at the appropriate position in 
   the frame header. */

void update_mvcontexts( bool_decoder *d)
{
    int i = 0;
    do {                      /* component = row, then column */
        const Prob *up = mv_update_probs[i];    /* update probs 
                                                   for component */
        Prob *p = mvc[i];                  /* start decode tbl "" */
        Prob * const pstop = p + MVPcount; /* end decode tbl "" */
        do {
            if( read_bool( d, *up++))     /* update this position */
            {
                const Prob x = read_literal( d, 7);

                *p = x? x<<1 : 1;
            }
        } while( ++p < pstop);              /* next position */
    } while( ++i < 2);                      /* next component */
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}


This completes the description of the motion-vector decoding procedure and, with it, the procedure for decoding interframe macroblock prediction records.