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%.
I am ignoring specularity and roughness at this stage.
Thanks,
Ben
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. (?)
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Hi Rob - and others,
Please can you clarify the following re- trans material, as myself and my
colleagues have been arguing over whether to use transmission or
transmissivity for diffuse transmission and/or specular transmission when
setting the parameters for the trans material based on radiance tests.
Do you consider transmissivity of the diffuse transmittance only, and use
the pure transmission of specular component? The reason I ask is that we
set up a small test which measures the luminance of a trans material and the
overall light transmission through a trans material with different settings.
Using a specular transmission (Rs) of 0.5 (50%) - i.e. not converting to
tranmissivity, and NO diffuse transmission/transmissivity, we get exactly
half the source luminance on the side opposite the light source on the trans
material using rtrace. i.e. tranmissivity not used for specular component.
Working out the diffuse transmission is a little more tricky, but looking at
the maths behind the settings for the trans material, I don't quite
understand whether transmissivity or transmittance should be used - it
depends on how the internal workings of radiance interprets the parameters
and therefore I can't be sure.
(As I understand, transmissivity is the proportion of light that travels
through the medium before inter-reflection within the medium).
So - the question could be written :
Do I use specular transmission with diffuse transmissivity when setting the
trans parameters?
Also - if transmissivity is used for the transmitted light, why is a factor
(such as transmissivity) not applied to the reflected part also (due to
inter-reflections)?
Thanks for any insight - I know there's been many questions and answers on
this subject, but I'm still confused!
Tim
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Tim Hanson wrote:
Please can you clarify the following re- trans material...
Do I use specular transmission with diffuse transmissivity when setting the
trans parameters?
Hi Tim,
To be honest, the one project I've done so far that made heavy use of trans was approached in a different manner. I took front-side illuminance & back-side luminance readings of my samples, shining a theatrical spotlight at them. These numbers were sent to Radiance Central High Command (Albany, CA) via secret email address ([email protected]), and the Radiance Grand Poobah himself derived proper values for trans. He did this by old-fashioned trial & error, trying various trans values and plotting the results against my datasets, tweaking until the curves more or less matched up. I summarized this process at radiance-online.org recently, even included sample code that Greg used to create the plots. Here's the post:
http://www.radiance-online.org/pipermail/radiance-general/2004-January/001415.html
I never could get that shell script to work, but the bgraph input file works if you feed it directly to bgraph. That input file also contains the Rosetta Stone you're looking for, I think: (excuse the wrapping)
B(x)=Td/PI+Ts/(PI*sq(alpha)*sqrt(cos(theta)*cos(PI/180*x)))*exp(-
sq(tan(PI/180*x-theta)/alpha))
That's where Greg mathematically described the Radiance trans behavior, to be overlaid with my dataset from the actual sample measurements. You will see the variables Ts and Td in there; those are Ts & Td from trans. I should add that these samples did not have very much of a specular transmitted component. I really don't know if this method would work if there is a higher amount of specular transmission...
Hmmm, after all that, I didn't even answer your question! In essence, we worked backwards to arrive at a valid description. To definitively answer your speculodiffusetransmissionivity question will require more brain power (cue lights; Chas enters stage left, Schorsch from stage right, and Greg arrives, deus ex machina-style, in a large winged chariot from above). =8-)
Hi Tim,
I think your answer is transmittance all the way.
If you take a look at the reflection equation, there's a term there for the transmitted indirect component, Tau(a)/pi*A(t). A(t) is the indirect irradiance on the opposite side of the material (W/m2). It's a similar story for diffusely transmitted direct components, except with Tau(d). So, the luminance of the transmitted component is determined by Tau, with no additional factors relating transmissivity to transmittance. This is the same Tau (Td) that was referred to in Rob's message from yesterday.
I don't know whether this makes any sense for anyone, but the result is that you're looking at transmittance, not transmissivity.
Phil.
[email protected] 10/03/2004 1:26:26 am >>>
Hi Rob - and others,
Please can you clarify the following re- trans material, as myself and my
colleagues have been arguing over whether to use transmission or
transmissivity for diffuse transmission and/or specular transmission when
setting the parameters for the trans material based on radiance tests.
Do you consider transmissivity of the diffuse transmittance only, and use
the pure transmission of specular component? The reason I ask is that we
set up a small test which measures the luminance of a trans material and the
overall light transmission through a trans material with different settings.
Using a specular transmission (Rs) of 0.5 (50%) - i.e. not converting to
tranmissivity, and NO diffuse transmission/transmissivity, we get exactly
half the source luminance on the side opposite the light source on the trans
material using rtrace. i.e. tranmissivity not used for specular component.
Working out the diffuse transmission is a little more tricky, but looking at
the maths behind the settings for the trans material, I don't quite
understand whether transmissivity or transmittance should be used - it
depends on how the internal workings of radiance interprets the parameters
and therefore I can't be sure.
(As I understand, transmissivity is the proportion of light that travels
through the medium before inter-reflection within the medium).
So - the question could be written :
Do I use specular transmission with diffuse transmissivity when setting the
trans parameters?
Also - if transmissivity is used for the transmitted light, why is a factor
(such as transmissivity) not applied to the reflected part also (due to
inter-reflections)?
Thanks for any insight - I know there's been many questions and answers on
this subject, but I'm still confused!
Tim
Phil,
What!? You mean 'The Book' is wrong! Sacrilege!
Well - I tried to stick up for the book, but Giulio (otherwise known as
Pillo in the radiance world), was arguing for transmission also.
It always helps to have another view from someone with a different
perspective (i.e. from beneath, being in Australia :o)). In fact, I've just
re-read the definition it gives which says 'Td - Diffuse Transmissivity :
Fraction of light that passes all the way through the surface diffusely' -
which is a contradiction anyway!
Thanks for the clarification,
Tim
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oi oi
I an lost!
like superman, now they know who I am!!!
···
-----Original Message-----
From: Tim Hanson [mailto:[email protected]]
Sent: 10 March 2004 12:15
To: '[email protected]'
Subject: [Radiance-general] Accurate definition of trans material
Phil,
What!? You mean 'The Book' is wrong! Sacrilege!
Well - I tried to stick up for the book, but Giulio (otherwise known as
Pillo in the radiance world), was arguing for transmission also.
It always helps to have another view from someone with a different
perspective (i.e. from beneath, being in Australia :o)). In fact, I've just
re-read the definition it gives which says 'Td - Diffuse Transmissivity :
Fraction of light that passes all the way through the surface diffusely' -
which is a contradiction anyway!
Thanks for the clarification,
Tim
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I just wanted to confirm that the term "transmissivity" is used incorrectly on page 325 of "Rendering with Radiance." It should be diffuse transmittance (a.k.a. "transmission coefficient") rather than transmissivity. However, the equations for these values are correct. The term transmissivity is used properly in relation to the "glass" material type as given in the trans.cal code on page 238.
Google pointed me to these terms better defined on Schorsch's website:
http://www.schorsch.com/kbase/glossary/transmittance.html
For further information, you can also refer to the document that I believe Pillo is referring to:
http://radsite.lbl.gov/radiance/refer/materials.pdf
-Greg
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