[XviD-devel] Xvid v's H.264/AVC

[Christoph Lampert]

On Tue, 6 Jul 2004, Daniel Larkin wrote:

> Thanks Christoph for clarifying a number of matters.
> 
> If Xvid is a mpeg4-asp implementation does this mean that Xvid is
> constrained by all the patenting issues of MPEG-4 (mpegla etc)?

Yes. That's why there's only source code on the homepage, no binaries. 
XviD is intended for learning about MPEG-4, not for creating 
MPEG-4 based products. 

 
> I'm also curious about your comment:
> > The same for
> > QPel (e.g. for MPEG-2 input like DVDs it's known not to help not much, if
> > at all).
> Is this because the lossy compression is reducing the performance of the
> interpolation process?

I'm not the Qpel expert, but IIRC, MPEG-2 uses bilinear subpel 
interpolation and MPEG-4 does pure bilinear filtering only in non-Qpel 
mode. In QPel mode, it uses some 6-tap filtering process for the halfpel 
positions (and bilinear on top of that for the Qpels) or so. 
So, one explanation for MPEG-4s rather poor performance in Qpel mode would 
therefore be that MPEG-2 images are more smoothed/blurred level than 
MPEG-4 can directly reproduce with it's sharpness preserving filter. It 
therefore needs to spend additional texture bits to just simulate the 
smoothness of MPEG-2. 
Also, if the input doesn't contain real Qpel motion (e.g. because they 
are quantized away due to MPEG-2s halfpel vectors), the qpel won't help 
anything, and just costs extra bits because vectors become longer (in 
units). 
But all this is just speculation. To my knowledge, there are no real 
studies available on reencoding MPEG-2 to MPEG-4. 

chl