POV-Ray : Newsgroups : povray.newusers : Extinction value of scattering and mirrors : Re: Extinction value of scattering and mirrors Server Time
30 May 2024 11:55:12 EDT (-0400)
  Re: Extinction value of scattering and mirrors  
From: Alain
Date: 14 Jul 2016 11:47:39
Message: <5787b41b$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?
>
> The code:
> #version 3.7;
>
> global_settings { ambient_light 0
>     photons {spacing 0.1 max_trace_level 5 media 100,1 radius 0.5 autostop 0}
>     assumed_gamma 1.0
> }
This looks OK
I'd remove radius 0.5. In my experience, it tend to cause some problems 
if not set exactly right.
You don't need max_trace_level 5: It's the default value.

>
> //Container box
> box {<-11,-11,-31><11,11,31> hollow
>    texture {pigment {color rgb 1}}
>    interior { media { scattering {1, color rgb <0.5,0.5,0.5> extinction 0.1}
> method 3 intervals 1 samples 100 collect on} }
>    photons {pass_through}
> }
You may want to make the box thinner, or reduce the density of your 
media. If you get pure white with only the light, the reflected photons 
can't show.
method 3 is the default and don't need to be specified.
With method 3, intervals default to 1 and must be left unchanged. Beter 
to leave that one out.

>
> //camera
> camera {orthographic location <0,0,30.5> up <0,0,1> look_at <0,0,0> angle 50}
>
>
> //Source
> light_source { <0,0,0> rgb <1,1,1> fade_distance 1 fade_power 2
>     photons{reflection on refraction on}
> }
This photons block is useless. By default, all light_source, except 
shadowless, DO emit photons.
You need the photons block to turn on the area_light option or 
specificaly turn OFF some option for that light_source.
In this case, you don't need to use light fading. A plain non-fading 
light is OK.

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

Your clipping box is exactly the same hight as your cylinder. This leads 
to coincident surfaces, in some cases, even when using "open". Make your 
box a little smaller, or the cylinder a little taller.
  cylinder {<0,0,-30.001> <0,0,30.001> 10 open...

Make absolutely sure that there are no transparent object in front of 
the light. If there is any, it need photons{pass_through} to be added.

The extinction is specific to the scattering media. It controls the 
amount of incoming light that is absorbed by the media. It's default 
value is 1 and any other value is not physically correct. In case such 
as this one, it can be set lower, even down to zero, to ignore the 
absorption and to be able to show the full path of the light and 
photons. This affect ONLY light and photons, the background remains 
unaffected by this setting.

Just as a test, try this for your reflection:
reflection{rgb<1,0,0>}
This will make a reflector that reflect only the red, that should make 
it more visible. You may also unrealistically increase the reflection 
like this:
reflection{rgb<4,0,0>}


Alain


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