[Radiance-general] Color definition in Plastic material

Tue Jan 24 07:17:14 PST 2017

Hello All,

Thank you for sharing a link to the http://www.lighting-materials.com/ database. This is a useful reference to have.

I want to understand how R_reflectance, G_reflectance, B_reflectance are calculated, as defined for a material in the online database. Can someone please explain how these were calculated? Is the Radiance RGB triplet (as obtained from xyz_rgb.cal) simply scaled by total reflectance to obtain these?

Radiance 5.0 official release contains a script, colorcal.csh, does this script convert spectral reflectance straight to a RGB material primitive definition, or does the resulting RGB triplet need to be scaled by total or diffuse reflectance?

As I understand from previous posts for the red channel, R_material definition = (R_website - Specularity_website) / (1 - Specularity_website). Can someone please clarify how R_website was determined.

I want to clearly understand how a Radiance material definition is generated from spectral reflectance.

Thank you for the help,
Andrei Kolomenski

From: J. Alstan Jakubiec [mailto:alstan at jakubiec.net]
Sent: Monday, January 23, 2017 9:12 AM
To: radiance-general at radiance-online.org
Subject: Re: [Radiance-general] Color definition in Plastic material

Hi Germán, Xiaoming,

Thank you both for pointing this out and having me go back to check the work on specular materials. I indeed made a pretty critical mistake by momentarily forgetting the tricky procedural nature of Radiance materials... and then never noticing. This is rather a shame after spending a long time getting a trans material just perfect earlier in the week. First of all, big apologies for the mistake. All diffuse materials, metals, and the specialized PV materials should be correct on the website. Things with specularity will have some errors, and I will fix this soon.

In the interim, the specularity and total reflectance values on the website are correct, and you can fix individual R, G or B coefficients with a little math:

(R_Website - Specularity_Website) / (1-Specularity_Website) = R
So in the case of the material referenced earlier<http://www.lighting-materials.com/materials/648>,

R = (0.3517 - 0.0218) / (1 - 0.0218) = 0.3373
G = (0.3480 - 0.0218) / (1 - 0.0218) = 0.3335
B = (0.3313 - 0.0218) / (1 - 0.0218) = 0.3164

Again, I apologize for the mistake. I will post a public message here when all is corrected on the website.

Germán, I owe you a separate e-mail and will reply soon afterwards :).

Xiaoming, to answer your question: If you want a 50% diffuse and 10% specular material, then indeed the R/G/B coefficients are 0.5556 and the specularity is 0.1 as far as I understand the functioning of the material models. Germán, R_specular is simply equal to the specularity in the plastic material. In that sense, Red, Green and Blue are not the total reflection (that's the mistake I made). Rather they represent the percentage of the leftover light after specular reflection that is then reflected diffusely.
R_diffuse = 0.5556*(1-0.1) = 0.5
R_specular = 0.1
R_total = R_diffuse + R_specular

Best,
Alstan
On 1/23/2017 9:20 PM, Germán Molina Larrain wrote:
Hi Xiaoming,

mmm... I would say you are looking for a 60% reflective material (10% specular, 50% diffuse).

In that case, I would choose a1, a2, a3 = 60% and the specularity would be (C-rho_d)/C = 16,666%. (16.6666%*60% = 10%).
It seems that I have the same issue as you, though.... my calculation of total reflectance is not correct when comparing to Lighting Materials website.
Anyone actually know these things?
Kind regards,

2017-01-23 8:24 GMT-03:00 Xiaoming Yang <xiyang at fosterandpartners.com<mailto:xiyang at fosterandpartners.com>>:
Hi Germán,

Thank you very for your reply and suggestions. I am still not sure what values I should put in the material definition. According to the definition in:
rho_d = p*C*(1-specularity)

For a material with 50% of incoming light reflected diffusely and 10% specularly then

C= 0.5/(1-0.1) = 0.555

Shall I define the material as follow?

Void plastic diffuse_50
0
0
5 0.555 0.555 0.555 0.1 0

Regards,
Xiaoming

From: Germán Molina Larrain [mailto:germolinal at gmail.com<mailto:germolinal at gmail.com>]
Sent: 20 January 2017 20:50
To: Radiance general discussion <radiance-general at radiance-online.org<mailto:radiance-general at radiance-online.org>>
Subject: Re: [Radiance-general] Color definition in Plastic material

Hello Xiaoming,
If I understand the Materials documentation<http://radsite.lbl.gov/radiance/refer/materials.pdf> correctly, I would say that the Red, Green and Blue are total reflection. If specularity is non-zero, the diffuse reflection is reduced by this amount:
rho_d = p*C*(1-specularity)

At least, that would be my guess.

Best!

2017-01-20 16:28 GMT-03:00 Xiaoming Yang <xiyang at fosterandpartners.com<mailto:xiyang at fosterandpartners.com>>:
Hi,

I have a rather basic question about color definition for plastic material and hope someone could help me.

mod plastic id
0
0
5 red green blue spec rough

Does the red, green and blue define diffuse reflection or total reflection?

For example, if a material reflect 40% light diffusely and 10% specularly, is it correct to define this material as:
Void plastic testMaterial
0
0
5 0.4 0.4 0.4 0.1 0

I was checking with the spectrum data from SUTD http://www.lighting-materials.com/materials/648

And it seems they are using the other approach which defines R G and B with total reflection and the material will be
Void plastic testMaterial
0
0
5 0.5 0.5 0.5 0.1 0

Xiaoming

Foster + Partners Limited | Registered in England and Wales | CRN + 01644989

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