Am 07.11.2017 um 22:39 schrieb cbpypov:
> 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.

Actually POV-Ray chooses to not use chroma subsampling in JPEG output.

> I think implementing the dithering for JPEG would be labor lost.

Not really. When I implemented dithering support, I concocted a nice
little framework for it; enabling it for JPEG output would essentially
be a one-liner.

> 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.

I guess the key to achieving well-dithered PNGs is to just enable
default dithering (`+th`). Ive might have a thing or two to add though.

As for MPEG, I have far too little experience to give any advice on it
at all; but I wouldn't be surprised if colour banding artifacts were
virtually inevitable there.


>>> 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?

Are you sure you weren't seeing JPEG/MPEG artifacts?

POV-Ray's dithering noise is at a magnitude of 0.4% in output colour
space (presuming 8 bit per colour channel). /Especially/ in the vicinity
of high contrast edges this noise should be imperceptible by human vision.

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