[Radiance-general] Accurate definition of trans material

[Rob Guglielmetti]

Ben Masters wrote:
> 
> Just a quick one..
> 
> How do I use the trans material properly? It asks for both a colour and 
> a transmissivity value. I only have the transmissivity value which is 4.4%.

Welcome to trans, where the fun never ends.  Ben, the trans material is 
one of the more confusing ones in the Radiance suite, I'm afraid.  It 
requires seven different parameters, actually.  Do you have the 
"Rendering with Radiance" text?  There are some excellent formulas in 
there for deriving the seven trans parameters.  Here they are, in case 
you don't have the book:

A7=Ts / ( Td+Ts )
A6=( Td+Ts ) / ( Rd+Td+Ts )
A5=Sr
A4=Rs
A3=Cb / ( (1-Rs)*(1-A6) )
A2=Cg / ( (1-Rs)*(1-A6) )
A1=Cr / ( (1-Rs)*(1-A6) )

Where:
The red, green, and blue reflected components are Cr, Cg, and Cb. The 
photopic average of the RGB is Rd.

Rs = Reflected specularity

Rs = surface roughness

Td = Diffuse transmissivity (fraction of light that passes all the way 
through the surface diffusely).

Ts = Transmitted specularity (fraction of light transmitted as a 
beam—that is, the fraction of light not diffusely scattered).

You need to get the reflectance of your material, or guesstimate the 
value, before you can proceed.  For reference, here are trans values for 
a material that is 20% reflective and 4.4% transmissive, all of the 
transmitted component being diffusely scattered:

A1	0.244000
A2	0.244000
A3	0.244000
A4	0.000000
A5	0.000000
A6	0.180328
A7	0.000000

That was based on the following assumptions:

Diffuse reflectance, RED	Cr	0.200
Diffuse reflectance, GRN	Cg	0.200
Diffuse reflectance, BLU	Cb	0.200
Reflected Specularity		Rs	0.000
Surface Roughness		Sr	0.000
Diffuse Transmissivity		Td	0.044
Transmitted Specularity		Ts	0.000
Photopic Average refl.		Rd	0.200

I hope this helps. (?)

----

      Rob Guglielmetti

e. rpg at rumblestrip.org
w. www.rumblestrip.org