Inter Prediction MV Derivation

Totally three modes are used to derive inter prediction motion vector for MB partitions/Sub MB partitions.
* P skip mode
* B skip/direct mode (spatial/temporal)
* Others

The mv's could be directly calculated in the first two modes while they are derived with mvp and mvd in the third mode. There are two issues which are very important in the course of the derivation, i.e. how to locate the colPartition/colBlk in the colPic and how to locate the neighboring partition/blk in the current pic.

* ColPartition/colBlk
Note: The basic processing unit for B_skip, B_16X16_direct and B_8X8_direct are 4X4 blk. For P skip it is 16X16.
Three parameters are needed for temporal direct mode: colPic, colPart/colBlk, and refIdxL0
- colPic: different combinations of fld, frm and afrm. Table 8-6
- colPart/colBlk: The basic unit is 4X4 block in current MB and colMB. If the partition in colMB is larger than this unit, the motion inforamtion would be copied on all the composited blocks. When 8X8_direct_flag is enabled, the basic units in one 8X8 sub partition of current MB share the same mv and refidx. Otherwise, 16 blocks have their own mv and refidx. To save memory, mv and refidx are needed for only several units in colMB for the derivation. The basic unit mapping could be defined with Table 8-8. LUT could be used to implement this kind of mapping.
- refIdxL0: Keep in mind the picture structure of colRef might be different. The current MB is field MB in afrm is always a special case.

* Neighboring partition/blk
The basic unit is still 4X4 blk as the above and it is possible that multiple units share the same mv information due to copy operation. It is not necessary to store all the blk information for the derivation. For example, only 4 units are needed for B and C. It is special for the processing of A and D, however. And all the blk information is needed in the current MB.