

#### 16.2 Inter-Predicted Macroblocks                        {#h-16-02}


Otherwise (when the above bool is true), we are using inter-prediction (which of course only happens for interframes), to which we now restrict our attention.

The next datum is then another bool, `B( prob_last)`, selecting the reference frame. If 0, the reference frame is previous frame (last frame); if 1, another bool (`prob_gf`) selects the reference frame between golden frame (0) or altref frame (1). The probabilities `prob_last` and `prob_gf` are set in field J of the frame header.

Together with setting the reference frame, the purpose of inter-mode decoding is to set a motion vector for each of the sixteen Y subblocks of the current macroblock. This then defines the calculation of the inter-prediction buffer (detailed in Chapter 18). While the net effect of inter-mode decoding is straightforward, the implementation is somewhat complex; the (lossless) compression achieved by this method justifies the complexity.

After the reference frame selector comes the mode (or motion vector reference) applied to the macroblock as a whole, coded using the following enumeration and tree. Setting `mv_nearest = num_ymodes` is a convenience that allows a single variable to unambiguously hold an inter- or intraprediction mode.


~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
typedef enum
{
    mv_nearest = num_ymodes, /* use "nearest" motion vector 
                                for entire MB */
    mv_near,                 /* use "next nearest" "" */
    mv_zero,                 /* use zero "" */
    mv_new,                  /* use explicit offset from 
                                implicit "" */
    mv_split,                /* use multiple motion vectors */

    num_mv_refs = mv_split + 1 - mv_nearest
}
mv_ref;

const tree_index mv_ref_tree [2 * (num_mv_refs - 1)] =
{
 -mv_zero, 2,                /* zero = "0" */
  -mv_nearest, 4,            /* nearest = "10" */
   -mv_near, 6,              /* near = "110" */
     -mv_new, -mv_split      /* new = "1110", split = "1111" */
};
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
{:lang="c"}