[theora] A comparison of VP3, and two MPEG-4 variants
chl at math.uni-bonn.de
Mon Mar 24 11:00:18 PST 2003
On Mon, 24 Mar 2003, Dan Miller wrote:
> good points, but I think constant Q is pretty much the standard for
> VBR/storage type applications. Doesn't Vorbis basically use a
> constant Q model for the most part?
> it's only when you have hard limits on transmission speeds that you
> need to employ rate control algorithms that vary Q over time.
Encoding with constant quantizer is almost impossible for storage if file
size has to be controlable. Even if size is allowed to vary, it's not true
that fixed quantizer means fixed quality (as you can see from PSNR plots).
Natural video with different scenes may have visual artefacts in one scene
and no artefacts in the next with same quantizer. Ratecontrol is difficult
business, and it's no wonder that most available high quality MPEG2 or
MPEG4 material is (at least) two-pass encoded.
<p>> > -----Original Message-----
> > From: Marco Al [mailto:marco at simplex.nl]
> > Sent: Monday, March 24, 2003 12:43 PM
> > To: theora at xiph.org
> > Subject: Re: [theora] A comparison of VP3, and two MPEG-4 variants
> > From: "Freun Laven" <FreunLaven at earthlink.net>
> > > >I think you would be much better off relying on subjective
> > > >measurements rather than PSNR.
> > >
> > > Considering the incredible vaguness in what's considered
> > "good enough",
> > > any decent testing method is going to *have* to do some sort of
> > > objective, reproducable measurments. (Unless, of course, people are
> > > going to be satisified with some group of 'experts' making
> > declarations
> > > of what is 'best'.)
> > Not from a group of experts, but a group of layman yes.
> > Experts can have
> > preconceptions based on objective measures and can tie them
> > to specific
> > codecs by recognising specific artifacts.
> > MOS is the benchmark to which all objective measures are
> > compared. To any
> > individual his subjective measure is the only one which
> > counts ... how then
> > can you look at the big picture and declare subjective
> > measures meaningless?
> > Obviously the average subjective impression is the only
> > measure which has
> > any meaning at all ...
> > > With video it's even worse. And for somebody like myself, who has
> > > eyesight problems, what I would consider to be 'good' would
> > probably be
> > > laughed at by others, simply because I have trouble
> > detecting the subtle
> > > differences.
> > That is a rather extreme example, on average over all
> > potential users these
> > kind of things even out. Although since with subjective tests
> > you usually
> > have a rather small group your opinion would indeed probably
> > not be usefull
> > to include :/
> > > A purely subjective comparison is worthless.
> > Actually it is the only comparison of value :) Indeed, the value of
> > objective measures themselves is measured by how well they
> > correlate with
> > subjective scores.
> > On a related matter, I dont quite see the relevance of
> > constant quantizer
> > measurements ... they are usefull as micro benchmarks during codec
> > development to compare a codec against its previous version,
> > but does anyone
> > actually use constant quantizer encoding in practice? If not
> > how are the
> > results relevant for comparing codecs against eachother?
> > Id find the results more relevant if the codecs were compared
> > as they would
> > be used. Which means seperate tests for streaming (CBR/ABR)
> > and storage
> > applications (VBR/2-pass encoding if available ... CBR/ABR
> > coding with the
> > rate set to what is needed for the required size if not).
> > Marco
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