Thomas de Groot <tho### [at] degrootorg> wrote:
>
> This brings the following question: What is the difference between
> fade_power 2 and fade_power 1001, as both, in the docs/wiki, are
> described as 'exponential'?

I did a search through old newsgroups too, and found a useful comment by Alain
posted Dec 18 2017 (with my own comments in brackets, which I hope are correct):

"For a material [i.e., showing light attenuation inside a translucent object]
you should use fade_power 1 or fade_power 1001 to get a more realistic result.
Fade_power 2 is for light attenuation over a distance in empty space." [i.e., in
POV-ray's 'air' or external space, not *inside* an object.]

So there is a difference in how and when fade_power is used, apparently.

>
> Ive tells us: "note that this "fade_power 1001" statement is an old (and
> really dirty as it lacks any logic) hack that is part of POV-Ray since
> ages to turn realistic fade power calculation on."
>

I saw that one too. :-) And here's a later comment by Clipka, dated Oct 27 2012:

"...realistic fading for interiors is exponential rather than
linear; specify a fade_power value of 1000 or greater (1001 is often
used, but it doesn't really matter), POV-Ray will then enable
exponential fading."

It seems that fade_power 1001 has been used for quite a long time. It's in a
section of code in a post by Ross on September 11 2003 entitled "glass objects".

Personally, I have not yet run a comparative test of values <1000 vs. 1000 vs.
1001 vs. some higher value, so I can't comment on the expected effects. It seems
that a value of 1001 *or higher* turns on the 'exponential' fading, not
specifically 1001. What that exponent *is*, I don't know; I guess it's derived
by the in-built equation mentioned in the docs:
            Attenuation = exp(-depth/fade_dist)

??

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