[Radiance-general] Guidelines for trans Material

[Marcus Jacobs]

Hey Guys,

Thanks for the insight. First, concerning my oddly shaped lighting fixtures, 
the ones that I have modeled actually look like this:

http://i35.photobucket.com/albums/d178/marcdevon/bathroomview.jpg

So I think they are actually more complicated that the example that I showed 
to you before. Out of curiosity, just as many modeling software estimate a 
complex surface as a mesh of planes or triangles, could the same principle 
be used for the lighting fixtures themselves. In other words, instead of 
seeing each fixture as a single complex surface, can it be considered it as 
just a group of flat planes? One item that I need to have addressed is 
whether I should model the diffuse shades a single surface or if it 
acceptable to have inner and outer surfaces (you can think of it as having a 
thickness to it). See here:

http://i35.photobucket.com/albums/d178/marcdevon/luminaireview.jpg


The information in Rayfront's user manual and pages 325-326 from RwR were 
VERY useful to me in gaining insight as to how Radiance treats light. There 
was a word that was mentioned that has me worried. Measure. Aside from not 
having any experience with performing a empirical test of light 
distribution, measuring light sounds very expensive. May I ask, would 
purchasing the needed equipment require a second mortgage to be taken out or 
giving up one's first born? Is it required to use transfunc or transdata in 
lieu of trans to obtain acceptable accuracy?


Thanks,

Marcus


>From: "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
>


>From: Rob Guglielmetti <rpg at rumblestrip.org>
>
>Hi Marcus,
>
>Unfortunately this is one of those areas where the best course of action
>is some measurement, followed with some trial and error.  Trans can
>effectively model this material, however.  I did a bunch of measurements
>of a translucent glazing several years ago, to obtain the transmittance
>at the normal as well as every 15-degrees off-axis. A trans material was
>then created and progressively modified until the curves for the trans
>and the measurements aligned pretty well.  This will be fairly easy,
>assuming you can get a flat glass sample of those tulip-shaped shades.
>(Full disclosure: Greg helped me fit the data to a valid trans
>description, which is a large part of why it was fairly easy.)
>Measuring the actual shade itself would be a real bear, I'd expect.
>
> >
> > 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?
>
>The best references for trans are the flowchart at Schorsch's site,
>(http://www.schorsch.com/rayfront/manual/transdef.html) and pp 325-326
>in RwR.
>
>Good luck; I know Visarc Jack did some courthouse renderings a long time
>ago that featured these large alabaster pendant bowl-like shades, and
>they looked amazing; the trans really captured the soft lamp image quite
>well.  Maybe Jack can share what he did?  Even if he measured the actual
>shade, they had a fairly large radius to them.  Your tulip shades will
>be a real challenge, I think.
>
>- Rob
>
>