[x264-devel] Re: H.264 motion vector predictor?

[Loren Merritt]

On Mon, 21 Feb 2005, Tuukka Toivonen wrote:

> As far as I know, the MV predictor is obtained as median of MVs of 
> (sub)partitions left, above, and above left of the current (sub)partition. 
> For 16x8 and 8x16 partitions it's different, but I think I understand that 
> case too.
>
> Let's suppose that a macroblock (MB) is divided into four 8x8 partitions,
> and the upper-right partition is further divided into four 4x4 partitions.
>
> According to my understanding, when motion estimation is performed for the
> lower right 8x8 partition, one of the MVs from which the median is computed 
> is the lower-left 4x4 subpartition of the top-right partition of the same 
> macroblock. But this is impossible, because x264 estimate the MV for that
> subpartition later than the MV for the lower-right partition.

In this case, x264 hasn't yet decided to split the top-right partition 
into 4x4, so it uses the MV from the top-right 8x8 block instead.
Yes, we should update the predictor after deciding sub-partitions, and 
again after performing Qpel motion estimation. But the whole motion 
estimation system is currently being overhauled (including the search 
order), so fixing it will wait for the new system.

--Loren Merritt

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