POV-Ray : Newsgroups : povray.newusers : Extinction value of scattering and mirrors : Re: Extinction value of scattering and mirrors Server Time
30 May 2024 11:52:20 EDT (-0400)
  Re: Extinction value of scattering and mirrors  
From: clipka
Date: 14 Jul 2016 11:31:36
Message: <5787b058$1@news.povray.org>
Am 14.07.2016 um 16:46 schrieb fsv2712:

> Another question. Please, could someone explain me what the extinction value
> means?. Is it related to the Beer-lambert law, as I have read at the newsgroups
> or only a percentage of the scattered light?

The rationale behind the extinction value is that a physically realistic
scattering medium not only has a brightening component to it (the light
normally not travelling in the direction of the camera, but scattered
towards it as it passes the medium), but also a darkening component (the
light normally travelling in the direction of the camera, but scattered
away from it).

I can't say off the top of my head to what detail this effect is modeled
in POV-Ray. It definitely includes (A) attenuation of the scenery behind
the medium, and IIRC it also includes (B) attenuation of scattered light
by the medium between the point of scattering and the camera, but I
think it does not include (C) attenuation of scattered light by the
medium between the light source and the point of scattering. The reason
is that while (B) can be computed pretty much along with (A) without
much additional effort, (C) would increase the effort significantly, and
should have a comparatively low impact for the use cases the media
mechanism was designed for.

If you need realistic modeling of high-density media, SSLT is probably
the better alternative.

The default for the extinction parameter, 1, results in a realistic
balance between the brightening component and the darkening component
for purely scattering media; lower values are physically unrealistic,
while higher values can be used for media with both a scattering and
absorbing component to avoid having to explicitly specify an additional
absorbing media.


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