[Radiance-general] trans mat

[Greg Ward]

Sorry, Axel -- I should have read through your original posting more 
carefully.  I missed your questions and references at the end:

> Some Qs:
> --------
> The specular components (both diff and spec) are hard to measure.
> However, this is what the calculation in the radbook is based on.
> How would one make estimations?

I don't know of any *good* ways to measure these without some kind of 
photometer.  A crude way to measure total transmittance would be to 
measure illuminance near a large white card, then again, covering the 
illuminance probe completely with your material.  The same trick will 
work with a camera and Photosphere or hdrgen if you don't own a lux 
meter.  Use your camera in the same way as the probe.  Divide the 
average of the second image by the first using Photosphere (or hdrgen) 
and running "pvalue -h -H -d -b -o capture.hdr | total -m" (or "Edit -> 
Select All" in the image viewer of Photosphere).

For reflectance, you can try a similar trick if you know the 
reflectance of your white card, and this is described in RwR, I think.

Specular is more difficult to measure, but you can usually guess with 
reasonable accuracy.  If a surface is a single dielectric interface 
(plastic or glass), it's specular reflectance will be around 4%.  If it 
is a double interface, as in a pane of glass, it will be about twice 
that, unless the glass is very dark, where it will be somewhere between 
4% and 8% depending on the amount of absorption.

As for roughness, I have a nice little device with LEDs and a viewport 
that allow me to estimate this value, but I don't know how to offer it 
to anyone as it's a piece of hardware, and requires a lot of experience 
to use effectively.

> Is this only possible through visual comparision?
> What are the steps to A1..A7 if given:
> - reflectance (luxmeter + luminance meter)
> - transmittance (2x luxmeter)
> (practical scenario, assuming grey colour)
> if
> - Lambertion properties or
> - clearly see-through
> What is the plastic equivalent to a trans without transmittance?

I'm not sure I understand the question.  If you set A6 to 0, trans is 
the same as plastic.

> Would that be _wrong_ or just _stupid_ to use?

If you mean substituting trans for plastic to be general, there's no 
harm.

-Greg

> From: "Axel Jacobs" <a.jacobs at londonmet.ac.uk>
> Date: March 10, 2005 10:50:30 AM PST
>
>> Wow, that's a great diagram, Schorsch.  I think I understand the
>> "trans" type for the first time, myself!
>
> Couldn't agree more. Super! I might just like to your site, then.
>
>> 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 did, but called it radbook.
>
> How about real-world examples for those of us that don't have a
> goniphotometer in their kitchen cupboard?
>
> Cheers
> Axel