Neighboring Location Derivation

P6.4.9 is used to define the address of neighboring MB/Partition/Sub MB partition, given one location coordinates. The derivation would be much more complicated when MBAFF is enabled and Table 6-4 would be used. Under this circumstance, the following points should be kept in mind.

- The basic unit in one slice would be a MB pair instead of a MB. Therefore the indicing order for MB is different.
- If MBAFF is disabled, only PAFF is applicable, which means slices/MBs structure is the same for the whole picture. When MBAFF is enabled the neighboring MB may be either field or frame MB.
- Only top MbAddr could be derived by using p6.4.7.
- Field MB and frame MB have different starting points and steps vertically. Keep in mind that two fields are interleaved in the FRAME grid. The starting point for top field would be the most left point of first line while bottom field be the most left point of the second line. The unit step for fields would be two instead of one for frames.
- When coordinates are negative, only one value is possible: -1.
- Note this clause just defines the derivation of neighboring MB index since MaxW and MaxH are for MB boundary. For some cases of calculating neighboring sub MB partitions, it might be possible that neighboring partition C is located on the top MB. Here the key parameter is the difference of luma/chrma location: xD, yD for MB, MB partition, subMB partition, luma8X8Blk, luma4X4Blk, chroma4X4Blk.
- The predPartWidth of xD is special for these cases: P skip, B skip, B direct 16X16 and B direct 8X8, it would be 16. Otherwise, it would be SubMbPartWidth/MbPartWidth.
- Generally the MB could be divided into 16 blocks each of which stores the motion information. The motion information might be the same for one partition or sub MB. Given one MB/Partition/Sub MB Partition, the neighboring ones could be located with the first block of this partition with the help of x, y and predPartWidth, according to Table 6-3/6-4.