clipka <ano### [at] anonymousorg> wrote:
>
> As for not giving an advantage with JPEG, that's intentional: Dithering
> is high-frequency low-amplitude noise, which is exactly the type of
> signal JPEG compression tends to throw away. With POV-Ray's standard
> JPEG quality settings you wouldn't see a difference between an image
> that had dithering applied and an image that didn't. You'd need
> top-quality settings, at which image size probably wouldn't differ much
> from a PNG. Therefore, JPEG output currently doesn't support dithering
> at all (though I might change that).
>

Thanks for reminding me of this :) Another way of explaining this (which I'd
prefer) is that JPEG basically uses a Fourier transform, cuts at a specified
frequency (depending on the quality setting) and transforms back. Dithering
noise is of course high-frequency, causing it to be cut off. This is moreover
done separately for gray-scale and color information, as a stronger color
compression is usually not noticed by the human eye :) The latter fact causes
the chromatic noise.

I think implementing the dithering for JPEG would be labor lost. Having a
"guide" on how to achieve a good dithering using PNG output, and maybe info on
what the user needs to consider with MPEG to not restore the artifacts might be
much more useful.

> Please elaborate why you say the Bayer dithering "increases" the
> problem. In my own experiments I do still see concentric artifacts (*),
> but IMO they're less obvious than without dithering.
>

Somehow I couldn't reproduce this today, so please cancel what I said. I think
it was because I still had JPEG output enabled plus some pseudo-artifacts which
were only in my head ;)

>>
>> Moreover, it ruins text rendering :(
>Dithering or mp4?
>How so?
>

I was using a quite high setting of antialiasing to get a sharp text (for title,
subtitles and such). It is created directly in POV-Ray. But with the dithering
there is of course a noticable noise at the edges of the latters, which is
moreover not constant from frame to frame. In consequence I think dithering can
generally not be used when rendering the text directly in POV-Ray. Am I wrong?

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