[vorbis] Video codec

Tue Sep 12 02:54:12 PDT 2000

Ralph Giles wrote:
> 
> On Mon, 11 Sep 2000, Chrissy and Raul wrote:
> 
> > >Totally agreed. Let's start making list and try to agree upon it?
> 
> I assume these are for purposes of comparison and guidelines for common
> source types, rather than requirements. 

I'd suggest as guidelines to verify if we have a codec yet that
satisfies some type of users yet.

> I was hoping we also "already
> agree" to allow arb. frame size and rate. 

I agree that would be wise, considering the 'alternatives' do so too,
MPEG2/4, H.263v2. The only place where limits come into view is
application. So there are MPEG2 video's that don't display well on TV
screens because of their parameters. It's up to the applications to
choose wise values I think.

> One of my pet peeves with the
> DVD spec is the way they include the letterbox mask in the frame they
> compress, just to keep things within the spec'd size and aspect ratio.

Could you please explain what you mean by this. I don't know exactly
what you mean here.

I do believe it would be smart to have some method to allow some
parameter variation across the screen. MPEG4 does it with objects, and I
believe H.263 allows abritrary shapes to define the screen region where
to decompress the video, plus allows occasional variations in resolution
and frame rate.  These features allow encoders, to very efficiently
encode the 'talking head in front of overhead sheet' for example, where
the sheets have maximum sharpness and low update frequency, and the
talking head is overlayed (bluescreened) on top of that with a higher
frame rate but less sharpness. I'm thinking of traning video's for
example.

> > Frame Rates:
> >
> > - 15 fps (half 30 fps)
> > - 24 fps (Film native rate.  I heard that in Europe 24 fps movie DVDs are
> > presented at 25fps which make them end sooner, good, more time for the
> > family, is this true?)
> > - 30 fps
> 
> Likewise, we're not worrying that 30 fps isn't really 30 fps in NTSC
> video?
> 
> I'd also suggest 16 fps for early cinema, and 60 for action sequences
> (ask someone who plays first person shooters). Rates of 8 and 12 fps are
> sometimes used for animation.
> 
> > SIF (320x240 resolution)
> > [...]
> > QSIF (160x120 resolution)
> 
> We also need other frame sizes for film. Common aspect ratios are:
> 
> 1.33:1  (4x3)
> 1.78:1  (16x9)
> 1.85:1  (US standard for 'just ordinary movies')
> 2.35:1  ("cinemascope")
> 
> Here are some examples based on multiples of 240, by way of example:
> 
> vert.   x1.33   x1.78   x1.85   x2.35
> 240     320     426     444     564
> 480     640     854     888     1128
> 720     960     1280    1332    1692
> 960     1280    1706    1776    2256
> 
> Note that block-oriented codecs like mpeg want the sizes to be multiples
> of 8, 16, or 32, of which every few of these qualify. For a given aspect
> ratio one can usually find something though. For example, the "Lord of the
> Rings" trailer (quicktime) is available in 240x104, 320x136, 480x204, and
> 640x272. All divisible by 8 and all but the smallest at 2.3529:1.
> 
> As Monty pointed out, the proposed wavelet codec doesn't care about
> blocks. I don't know anything about the proposed non-power-of-2 transform
> though. Are things like small prime factors important like with the fft?
> 
> The US HDTV standard also suggests some standard frames:
> 
> 640x480   (4x3)
> 1280x720  (16x9)
> 1920x1080 (16x9)
> 
> All but the largest support 24,30, and 60 fps (actually 60/1.001).
> 
> IMO we should be planning to target content production and a replacement
> for DVD-video as well as web streaming. That means large formats. The
> acknowledged digital-equivalent vertical resolution of 35mm film is 4k
> lines (these days; that's about equivalent to the 70mm epics of 25+ years
> ago). Most special effects work is done at 2k lines and some folks claim
> to see the difference. That's not to say you always get that, of course:
> it depends on film stock, exposure, lens quality, development, etc. At
> some point, you're just resolving the grain. OTOH, rumour says the new
> "digital film" camera being used to shoot the next "Star Wars" are only 1k
> lines (960, I think) and they're still having to be more careful with set
> painting because they give sharper detail than film.
> 
> See this recent slashdot article for further comments:
> 
> http://slashdot.org/article.pl?sid=00/09/11/1710204
> 
> Speaking of square pixels, the advantage is obvious to most software
> people, but we'll need to be ready to defend this one. One thing I didn't
> realize is that effectively *all* 35mm film work uses an anamorphic
> (cylindrical) lens to squeeze the director's chosen aspect ratio onto a
> standard-sized area of film. (The process is reversed in projection.) So
> people are used to this, and from the point of view of hardware
> implementations, it makes a lot of sense to have a fixed frame-size plus
> (even arbitrary) horizontal scaling.
> 
> > I am using three scenes for the video compression experiments: hockey,
> > mongolian rock concert, mongolian flag and plaza.
> 
> Do you own the copyright on these clips? Would you be willing to make them
> available in uncompressed form (on the net, or mail someone a cd) so we
> can make a start on a test suite?
> 
> Cheers,
>  -ralph
> 
> --
> giles at ashlu.bc.ca
> 
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