[Radiance-general] Color definition in Plastic material

[Greg Ward]

Xiaoming -- the calculation you performed based on the correct formula Germán gave you should yield a material with 50% diffuse and 10% specular reflectance.

The reason the Radiance material parameters are defined as they are, and "trans" is much more confusing than "plastic" or "metal", is so that there are clearly physical and non-physical ranges for each value.  As long as you are between 0 and 1 in the first four parameters for "plastic" or "metal," you know your material will not reflect more light than reaches it.

Cheers,
-Greg

> From: Germán Molina Larrain <germolinal at gmail.com>
> Date: January 23, 2017 5:20:22 AM PST
> 
> 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>:
> 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] 
> Sent: 20 January 2017 20:50
> To: Radiance general discussion <radiance-general at radiance-online.org>
> Subject: Re: [Radiance-general] Color definition in Plastic material
> 
>  
> 
> Hello Xiaoming,
> 
> If I understand the Materials documentation 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>:
> 
> 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
> 
> 
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