POV-Ray : Newsgroups : povray.general : Absorption vs thickness Server Time: 23 Oct 2019 03:15:05 GMT
  Absorption vs thickness (Message 1 to 6 of 6)  
From: IGM
Subject: Absorption vs thickness
Date: 27 Sep 2019 13:15:00
Message: <web.5d8e0a843f3612417bf1a5370@news.povray.org>
Hi,
maybe this is an ancient (and stupid?) question, but after online search I still
don't find anything useful.

In PovRay, two semitransparent boxes with very different thickness but with same
rgbt values have the same appearance. The absorption/transmission doesn't depend
on box thickness. The same even using "media { absorption rgb <.,.,.> }". This
is a sample code:

box {<-250,0,0><250,500,100>
    texture {pigment {rgbt <.6,0,0,0.5>}}
    interior {ior 1.5 media { absorption rgb 0.0001*<1,1,1> } }
    hollow on
    rotate <0,0,0> translate <-250,0,100>}

box {<-250,0,0><250,500,1000>
    texture {pigment {rgbt <.6,0,0,0.5>}}
    interior {ior 1.5 media { absorption rgb 0.0001*<1,1,1> } }
    hollow on
    rotate <0,0,0> translate <250,0,100>}

This is not physical. Is there a way to simulate the real thickness dependence
of absorption/transmission? (Beer–Lambert law)

Thank you!
igmar


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From: IGM
Subject: Re: Absorption vs thickness
Date: 27 Sep 2019 14:15:00
Message: <web.5d8e19305a27978d7bf1a5370@news.povray.org>
I was wrong: media {absorption .. } do exactly what required.
Sorry!
igmar


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From: William F Pokorny
Subject: Re: Absorption vs thickness
Date: 27 Sep 2019 15:35:46
Message: <5d8e2c52$1@news.povray.org>
On 9/27/19 10:14 AM, IGM wrote:
> I was wrong: media {absorption .. } do exactly what required.
> Sorry!
> igmar
> 

Hi, You might find interior's fade_color, fade_distance and fade_power 
of use too.

Bill P.


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From: Alain Martel
Subject: Re: Absorption vs thickness
Date: 27 Sep 2019 15:58:20
Message: <5d8e319c$1@news.povray.org>
Le 2019-09-27 à 09:11, IGM a écrit :
> Hi,
> maybe this is an ancient (and stupid?) question, but after online search I still
> don't find anything useful.
> 
> In PovRay, two semitransparent boxes with very different thickness but with same
> rgbt values have the same appearance. The absorption/transmission doesn't depend
> on box thickness. The same even using "media { absorption rgb <.,.,.> }". This
> is a sample code:
> 
> box {<-250,0,0><250,500,100>
>      texture {pigment {rgbt <.6,0,0,0.5>}}
>      interior {ior 1.5 media { absorption rgb 0.0001*<1,1,1> } }
>      hollow on
>      rotate <0,0,0> translate <-250,0,100>}
> 
> box {<-250,0,0><250,500,1000>
>      texture {pigment {rgbt <.6,0,0,0.5>}}
>      interior {ior 1.5 media { absorption rgb 0.0001*<1,1,1> } }
>      hollow on
>      rotate <0,0,0> translate <250,0,100>}
> 
> This is not physical. Is there a way to simulate the real thickness dependence
> of absorption/transmission? (Beer–Lambert law)
> 
> Thank you!
> igmar
> 
> 

What you want is to add an interior block to that object that define 
light fading through that object :

interior{fade_colour Some_Colour fade_distance Distance fade_power 1001}

For realistic results, fade_power should be 1 or 1001.
fade_distance dictate the intensity of the fading, ow how fast the 
fade_colour affect the colour of what is behind your object.

In that same interior block, you can also set an index of refraction, or 
IOR, as well as the amount of spectral dispersion. Use real world IOR 
and dispersion values.

interior{
  ior 1.33 // water. Default value is 1
  dispersion 1.01 // default to 1
  dispersion_samples 10 // optional, have a default value
}

Also, as you discovered, the interior block can also be used to fill an 
object with some media.


Alain


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From: IGM
Subject: Re: Absorption vs thickness
Date: 27 Sep 2019 16:20:01
Message: <web.5d8e36625a27978de462584c0@news.povray.org>
Thank you both, Alain and William,
I didn't notice that fade_distance and fade_power were interior properties,
besides being light_source properties!

igmar


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From: Alain Martel
Subject: Re: Absorption vs thickness
Date: 28 Sep 2019 18:22:02
Message: <5d8fa4ca$1@news.povray.org>
Le 2019-09-27 à 12:18, IGM a écrit :
> Thank you both, Alain and William,
> I didn't notice that fade_distance and fade_power were interior properties,
> besides being light_source properties!
> 
> igmar
> 
> 
It's important to note that it don't work the same way for light_source 
and interior.

For the lights, it affect the intensity of the light only. The colour 
never change.

In an interior block it affect how the material affect the light going 
through and can change the colour.


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