POV-Ray : Newsgroups : povray.newusers : Extinction value of scattering and mirrors : Re: Extinction value of scattering and mirrors Server Time
30 May 2024 12:39:29 EDT (-0400)
  Re: Extinction value of scattering and mirrors  
From: Alain
Date: 18 Jul 2016 14:01:00
Message: <578d195c$1@news.povray.org>

> Hello,
> I don't know why my reflector doesn't reflect photons. With scattering and an
> extinction value I should be able to see the area inside the cilindrical
> reflector slightly brighter than the area outside. Furthermore, the image is the
> same despite the value of "reflection" I use.
> 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?

> Thanks

Thinking again about this case, I usualy prefer to get away with media 
and use a plane instead as follow:
#version 3.7;

global_settings { ambient_light 0
     photons {spacing 0.1 autostop 0}
     assumed_gamma 1.0

camera {orthographic location <0,0,30.5> up <0,0,1> look_at <0,0,0> 
angle 50}

light_source { <0,0,0> rgb <1,1,1>
  looks_like{sphere{ 0, 0.1 pigment{<rgb<0,1,0>}
  finish{emission 1}photons{pass_through}}}
//make the light's position visible
// Purely optionnal, good for debugging

cylinder {<0,0,-30> <0,0,30> 10 open clipped_by {box {<-30,-30,-40> 
inverse} }
     texture {pigment {color rgb<1,1,1>} finish {emission 0 ambient 0 
1 diffuse 0 } }
     photons {target reflection on refraction off collect on}

plane{z, -10
  pigment{rgb 0.5}
  finish{ambient 0 diffuse 0.5
    brilliance 0 // The "magic trick" is here

With brilliance set to zero, the illumination becomes independent on the 
incident angle of the incoming light. If the light can reatch a point, 
that point get full illumination.
I've set the pigment to 0.5 so that you don't get pure white everywhere. 
You can also reduce the diffuse value if you get an image that is to bright.

This is also much faster to render than using media.
If you still want to use media, you may try to add hollow to your 
reflector. Maybe it's considered as a non-hollow object, even if it's 
open, killing media inside and interfering with the photons.
It's just a guess as I only very rarely use clipped_by for anything.

Another thing that I tend to use in a similar case is to use a 
difference of 2 cylinders to get a shell:
  cylinder {<0,0,-30> <0,0,30> 10.15}// Slightly larger outer part
  cylinder {<0,0,-30.01> <0,0,30.01> 10}// Inner surface
  box{<-10.15, 0, -30.01><10.15, 10.15, 30.01>}// Keep upper half
  texture {pigment {color rgb<1,1,1>}
  finish {emission 0 ambient 0 reflection 1 diffuse 0 } }
  photons {target reflection on refraction off collect on}


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