[Radiance-general] Guidelines for trans Material

[Gregory J. Ward]

Hi Marcus,

Welcome to the wonderful, confusing world of "trans".

I don't know of any good way to measure oddly-shaped translucent  
materials.  Perhaps others on the list will have suggestions for  
that....

> I have some additional questions concerning the trans material  
> type. In a Radiance reference manual, it is stated:
>
> "Trans is a translucent material, similar to plastic. The  
> transmissivity is the fraction of penetrating
> light that travels all the way through the material. The  
> transmitted specular component is the fraction of transmitted light  
> that is not diffusely scattered. Transmitted and diffusely  
> reflected light is modified by the material color. Translucent  
> objects are infinitely thin."
>
> First, should the transmissivity + transmitted specular component =  
> 1 in order for the material to be physically correct? Also, since  
> the transmitted specular component is the fraction of transmitted  
> light that is not diffusely scattered, does this mean that this is  
> apart of the direct calculation or is this too apart of the  
> indirect calculation.

The reason the trans parameters are so peculiar is so their valid  
range is easy to understand -- between 0 and 1 for everything except  
roughness.  (For physical value ranges, you may also consult the file  
"ray/doc/notes/materials".)  Pages 325 and 326 from Chas' chapter in  
"Rendering with Radiance" are also invaluable.  There have been a  
number of threads on trans over the years, and this is definitely one  
for the Radiance FAQ, should it ever materialize -- no pun intended.

-Greg