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

[Hans K. Rosbach]

> > 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.

I assume that this is not as valid when re-encoded to lower resolution
and/or
with some kind of filtering before re-encoding..?

-=Dead2=-