[Radiance-general] Color definition in Plastic material
J. Alstan Jakubiec
alstan at jakubiec.net
Tue Jan 24 21:31:29 PST 2017
Hello everyone,
I am happy to report that the lighting materials website
<http://lighting-materials.com/> is back up, and specular plastics now
reflect the correct diffuse values. Thank you to everyone for the
helpful discussion, and I will also be uploading some new materials soon!
Picking a random, highly specular measurement (Greg's stainless steel
sink in 1995 <http://lighting-materials.com/materials/1444>) should show
that the diffuse characteristics are now correct.
R_diffuse = (0.3937*0.265 + 0.3734*0.670 + 0.3281*0.065)*(1-0.0821)
= 0.345
R_specular = 0.0821
R_total = R_diffuse + R_specular = 0.4271
Best wishes,
Alstan
On 1/24/2017 11:58 PM, J. Alstan Jakubiec wrote:
> Hi Andrei,
>
> The R, G, and B triplets on the website (at least those measured by
> me) are directly translated from explicit photometric measurements
> using a KM 2600d spectrophotometer. We use the same scripts that are
> in the Rendering with Radiance book for doing this. If I recall
> correctly, the X, Y and Z tristimulus values are calculated simply by
> using the spectral measurements with the color matching functions (via
> cieresp.cal), and then xyz_rgb.cal is invoked.
>
> The spectrophotometer takes measurements with specular component
> included (SCI) and with specular component excluded (SCE). Since
> plastic specularity is equal energy, specularity is simply
> Reflectance_SCI - Reflectance_SCE. The error in the database I
> referred to was something naive I wrote long ago and never noticed:
> the R, G and B values displayed currently (R_website) come directly
> from the SCI measurement; therefore, they contain the specular
> portions of the measurements. They should be based on SCE values
> (diffuse reflections) and be further scaled by 1/(1-Specularity) in
> order to be photometrically correct. Hence the correction equation I
> posted earlier in this thread. Come tomorrow morning (Singapore time),
> the issue should be fixed.
>
> In good news, for 99% of the materials on the website, this makes
> little or no difference. For highly specular plastics, the extra
> diffuse reflection which results can be moderately large.
>
> Roughness values are not calibrated for most materials, and if they
> have been, it is simply by an eye test.
>
> Best,
> Alstan
>
> On 1/24/2017 11:17 PM, Kolomenski, Andrei (JSC-SF311)[WYLE
> LABORATORIES, INC.] wrote:
>>
>> 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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