Re[2]: [XviD-devel] xvid + bframes and mplayer/ffmpeg decoding

[Michael Militzer]

----- Original Message -----
From: "Christoph Lampert" <chl@math.uni-bonn.de>
To: <xvid-devel@xvid.org>
Sent: Saturday, August 17, 2002 12:46 PM
Subject: Re: Re[2]: [XviD-devel] xvid + bframes and mplayer/ffmpeg decoding


> On Sat, 17 Aug 2002, peter ross wrote:
>
> > quants 1, 2 & 3 can cause large coefficients;
> > sometimes these coefficients are to big and must be clamped/saturated
such
> > that they can be written in the bitstream.
>
> Is this still done, although we switched from [-128,127] to [-2048,2047]
> a while ago? (We did that, didn't we?)

well, our main problem with these large coefficients was that we didn't clip
the coeffs after quantization (or at least we didn't clip them anymore, I
think I remember an older XviD version did this) but instead before mb
coding. This had the bad effect that the clipping resulted in visual
artifacts that not only affected the current frame but also all following
p-frames.

The MPEG-4 VM 18 requires the coeffs to be clipped into the range [-128,
127] (8 Bit) but the VM 18 seems outdated and the reference codecs (momusys
and f-dam) use 12 Bits to code coeff data in fixed length mode. Therefore
clipping is not necessarily needed at all. Latest XviD versions don't clip
the coeffs anymore, so all clamping artifacts should be gone...

bye
Michael