[Radiance-general] trans mat

[Greg Ward]

Wow, that's a great diagram, Schorsch.  I think I understand the 
"trans" type for the first time, myself!

Be sure also to refer to section 5.2.6 (pp. 325-6) in "Rendering with 
Radiance," which translates between more sensible physical quanitites 
and the parameters of the "trans" type.  I believe Rob G. has an Excel 
spreadsheet that implements these formulas nicely, if you'd like to 
request a copy.

By way of excuse, the reason the trans type is so baffling is my 
stubborn adherence to the principle that Radiance primitives have 
well-defined legal ranges.  All the material parameters (except 
roughness) have legal physical ranges of [0,1).  Since Radiance does 
not enforce these limits, you can specify values outside this range, 
but you should know that you are on shakey ground at that point.

That said, I freely admit that the obtuse derivation of the trans 
parameters in particular has caused a good deal more consternation than 
if I had used more usual values of Rd, Td, Rs, and Ts and simply noted 
that these coefficients must sum up to something less than 1.  Oh, 
well.

-Greg

> From: Georg Mischler <schorsch at schorsch.com>
> Date: March 10, 2005 8:47:19 AM PST
>
> Axel Jacobs wrote:
>
>> Dear all,
>>
>> I am trying to nail this one down, once and for all.
>> What is your experience/approach? Feedback welcome.
>>
>> refman:
>> "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."
>>
>> BIG QUESTION:
>> How do CAPITAL indeces (RADIANCE) relate to
>> lower case indices (PHYSICS)??? See below. Also for a lot more little
>> questions...
>
>   http://www.schorsch.com/rayfront/manual/transdef.html
>
> Scroll down a bit for the diagram.
>
> -schorsch