[x264-devel] out-of-range motion vectors

[List, Peter]

> Von: Foxy [mailto:foxyshadis at hotmail.com]


> Gabriel Bouvigne wrote:
> > List, Peter a écrit :
> >
> >> So, do you think the drift to enormous motion values is not an issue?
> >>
> >
> > Don't get me wrong: I think that in some cases it is an issue, and that
> > fixing this issue could potentially improve compressibility.
> >
> >
> Not necessarily. I've been looking for the Doom9 thread, but it was
> discussed in the context of interframe interpolation that the truest
> motion of any given block may not correspond to the least residue or
> most efficient coding. It's better for x264 to choose the most efficient
> way to code a frame (and those it depends on) than to make the vectors
> look pretty.

Two comments to these arguments:

1. Don't forget that these huge vectors are most likely NOT the best 
vectors and coding them can be pretty inefficient. That is because if 
you start at an offset of 125 full pels, but have a search range of 
only +-32 you would not be able to check the vector range of 0...+-93!
That's why I talk about "drift". Once far enough off from the true 
minimum we might never find "the way back" and have in fact VERY bad vectors. That hopefully does not happen in the x264 implementation though, 
because we always check the (0,0) vector in parallel as I just learned.  
One point here could be the following: 
If we start from a very large offset, because the predictor has a very 
large value, the (0,0) prediction might get a very high RD-weight, and 
get rejected for just this reason. Probably one could experiment with 
varying (reducing) the motion-vector RD-weight for the (0,0) case... 


2. Even if these vectors are the most efficient from an RD point of view 
and in fact produce the highest SNR. Still, the SUBJECTIVE quality 
might be better with the "true motion". The "true motion" usually has SAD 
values very close to the mathematical minimum anyways. 
 
Regards
Peter

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