[XviD-devel] Re: XviD 1.0 Feature Request: autodetect iDCT decoder(fwd)

[Michael Militzer]

Hi,

Quoting Ivan Kalvachev <ivan at cacad.com>:

> Christoph Lampert said:
> >
> > Hi,
> >
> > I got this by private mail.
> > Would it be possible to autodetect this?
> >
> > gruel
> >
> >
> > ---------- Forwarded message ----------
> > Date: Thu, 20 Nov 2003 19:33:21 +0100 (CET)
> > From: ------- <uldead at freemail.hu>
> > To: gruel at xvid.org
> > Subject: Re: XviD 1.0 Feature Request: autodetect iDCT decoder
> >
> >
> > Hi
> >
> > I have an  idea  for feature of XviD 1.0: You must know that some
> > versions used Simple iDCT instead of Walken.  You also know that if
> > you played back it with Walken iDCT versions artifacts would appear.
> > So my idea would be to implement some kind of autodetect that would
> > determine that if the codec should use Simple or Walken iDCT for
> > decode. It would be essential for backwards compatibility i think,
> > because a lots of ppl still using the Simple iDCT versions, and i have a
> > pretty good archive of videos encoded with Simple iDCT ones, and they
> > look very bad played back with Walken, and i dont like the idea of trash
> > em.
> >
> > So what do you think ? plz answer back even if you think its a lots of
> > bullshit
> >
> >
> > Martin
> 
> I thought that old no IEEE-1180 complaint IDCT are threated
> as encoder bug.
> You have an counter and you write it into the stream,
> so it is quite easy to read it back, and workaround it.

Both Simple and Walken idct are IEEE-1180 compliant. Never was there a non
IEEE compliant idct included in XviD. On the other issue: unfortunately
it's not possible to autodetect whether Walken or Simple idct had been
used for a coded XviD stream (at least I can't think of a way how to do
this). You're right that there is a bitstream counter that allows to detect
which XviD cvs version was used to encode a stream and to automatically
activate workarounds for older bugs. However XviD cvs _always_ used Walken
idct, Simple idct code is included in XviD cvs but was never used. There
are builds available that use the Simple idct code, however these are just
unofficial modifications of the XviD cvs version. Since the bitstream
counter of these modified versions is/was not different to the vanilla
cvs versions, it's impossible to distuingish them :-((

bye,
Michael