POV-Ray : Newsgroups : povray.newusers : Transparent Objects Become Black : Re: Transparent Objects Become Black Server Time
9 Dec 2021 08:12:08 EST (-0500)
  Re: Transparent Objects Become Black  
From: Thomas de Groot
Date: 17 Nov 2021 02:42:35
Message: <6194b26b$1@news.povray.org>
Op 16/11/2021 om 20:57 schreef Kenneth:
> 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):
> 
Hey! Thanks Kenneth! This does answer some questions indeed. I never 
paid much attention to the 'issue', and always mindlessly used the 1001 
value when necessary. The query from bubble_person made me wonder 
suddenly, however.

> "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.
> 
Good. This is an additional information of importance it seems. If 
confirmed by 'the powers that be', I would be grateful if this info 
could be added to the wiki and/or official documentation.

>>
>> 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".
> 
He he! I had not come across those two. I believe the comment by Clipka 
should also find its way, one way or another, to the documentation.

> 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)
> 
Yes, I expect that is the case.

Thanks again for your sleuth work! :-)

-- 
Thomas


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