[theora] A comparison of VP3, and two MPEG-4 variants

[Dan Miller]

2-pass encoding may still imply constant Q.  The problem is to find the right Q; it's not necessary to vary it over the whole file.

 ___  Dan Miller
(++,) Founder, CTO, On2 Technologies

<p>> -----Original Message-----
> From: Christoph Lampert [mailto:chl at math.uni-bonn.de]
> Sent: Monday, March 24, 2003 2:00 PM
> To: theora at xiph.org
> Subject: RE: [theora] A comparison of VP3, and two MPEG-4 variants
> 
> 
> 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.
> 
> Christoph
> 
> 
> > > -----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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