[x264-devel] Re: about motion compensation

[Mathieu Monnier]

> I think we only need as many half-pels as image width.

No. H264 allows motion vectors to point outside the reference frame. The 
reference frame is extended by padding ( border pixels are replicated ). 
Hpel positions, however, are interpolated from the whole padded 
reference frame. So, on the left border for example, it looks like that :

Originally :
  -5 -4 -3 -2 -1  0  1  2  3  4
  x  x  x  x  x  a1 a2 a3 a4 a5

After padding :

  a1 a1 a1 a1 a1 a1 a2 a3 a4 a5

Now, if i want to interpolate a hpel position between -1 and 0, you'll 
use a1 a1 a1 a1 a2 a3, and if you interpolate between -4 and -3, you'll 
use a1 a1 a1 a1 a1 a1 : the hpel planes must be interpolated on a larger 
area before being padded.

Regards,

Mathieu

-- 
This is the x264-devel mailing-list
To unsubscribe, go to: http://developers.videolan.org/lists.html