[Radiance-general] Color definition in Plastic material
J. Alstan Jakubiec
alstan at jakubiec.net
Mon Jan 23 07:12:17 PST 2017
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
> <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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