[XviD-devel] Coefficient thresholding

Christoph Lampert xvid-devel@xvid.org
Sun, 18 Aug 2002 13:17:57 +0200 (CEST)


On Sun, 18 Aug 2002, Radoslaw 'sysKin' Czyz wrote:

> > However I have one trailer which I constantly use for
> > checking different motion estimations with fixed quant 3 and MPEG
> > quantization. So I tested it with coeff thresholding enabled and there
> > IS a drop in filesize compared to older builds - about 600 kb. To get
> > to the point, after watching this trailer I noticed that too much
> > details were removed, which was the most noticable on human faces.
> 
> I have to admit - I didn't really check any video with this. However,
> it's quite obvious that not coding some coefficients will result in
> smaller filesize *and* worse quality. The problem is, if in fixed
> filesize/bitrate the general quality is better or worse.
> I trusted PSNR on this. I didn't check myself.

Yes, you can't compare this stuff at fixed quant. 
You'll have to use bitrate mode and compare PSNR or visual quality. 


> It's very good that coeff thresholding is a flag, because - now when I
> think of it - it really removes some details, while savings in bitrate
> should remove some blocks. A step in a direction of h.263 quant type.
> 
> > It looks like TOOSMALL_LIMIT is set too high (or is that too low?).
> 
> If I remember correctly, TOOSMALL_LIMIT of 2 gave me a reasonable PSNR
> improvement, too. It was something like 0.07 (compared to 0.10 for 3).
> 
> I don't know if it's too late, but I recommend not to make coeff
> thresholding a flag but an integer of 1, 2, or 3. Users could choose
> what they like. (oh great now I remember the discussion how users get
> confused when they have too many options available......)

Problem is, for an integer, you need a new variable. For a flag, 
API doesn't have to be changed. 

Christoph 

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