text_src/17.01__vp8-bitstream__coding-of-each-component.txt

#### 17.1 Coding of Each Component                         {#h-17-01}


Each component is a signed integer V representing a vertical or
horizontal luma displacement of V quarter-pixels (and a chroma
displacement of V eighth-pixels).  The absolute value of V, if non-
zero, is followed by a boolean sign.  V may take any value between
-1023 and +1023, inclusive.

The absolute value A is coded in one of two different ways according
to its size. For 0 <= A <= 7, A is tree-coded, and for 8 <= A <=
1023, the bits in the binary expansion of A are coded using
independent boolean probabilities.  The coding of A begins with a
bool specifying which range is in effect.

Decoding a motion vector component then requires a 19-position
probability table, whose offsets, along with the procedure used to
decode components, are as follows:


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
typedef enum
{
    mvpis_short,         /* short (<= 7) vs long (>= 8) */
    MVPsign,             /* sign for non-zero */
    MVPshort,            /* 8 short values = 7-position tree */

    MVPbits = MVPshort + 7,      /* 8 long value bits
                                    w/independent probs */

    MVPcount = MVPbits + 10      /* 19 probabilities in total */
}
MVPindices;

typedef Prob MV_CONTEXT [MVPcount];    /* Decoding spec for
                                          a single component */

/* Tree used for small absolute values (has expected
   correspondence). */

const tree_index small_mvtree [2 * (8 - 1)] =
{
 2, 8,          /* "0" subtree, "1" subtree */
  4, 6,         /* "00" subtree, "01" subtree */
   -0, -1,      /* 0 = "000", 1 = "001" */
   -2, -3,      /* 2 = "010", 3 = "011" */
  10, 12,       /* "10" subtree, "11" subtree */
   -4, -5,      /* 4 = "100", 5 = "101" */
   -6, -7       /* 6 = "110", 7 = "111" */
};

/* Read MV component at current decoder position, using
   supplied probs. */

int read_mvcomponent( bool_decoder *d, const MV_CONTEXT *mvc)
{
    const Prob * const p = (const Prob *) mvc;
    int A = 0;

    if( read_bool( d, p [mvpis_short]))    /* 8 <= A <= 1023 */
    {
        /* Read bits 0, 1, 2 */

        int i = 0;
        do { A += read_bool( d, p [MVPbits + i]) << i;}
          while( ++i < 3);

        /* Read bits 9, 8, 7, 6, 5, 4 */

        i = 9;
        do { A += read_bool( d, p [MVPbits + i]) << i;}
          while( --i > 3);

        /* We know that A >= 8 because it is coded long,
           so if A <= 15, bit 3 is one and is not
           explicitly coded. */

        if( !(A & 0xfff0)  ||  read_bool( d, p [MVPbits + 3]))
            A += 8;
    }
    else    /* 0 <= A <= 7 */
        A = treed_read( d, small_mvtree, p + MVPshort);

    return A && read_bool( r, p [MVPsign]) ?  -A : A;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}