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#### 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
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" */