[Radiance-general] Color definition in Plastic material
J. Alstan Jakubiec
alstan at jakubiec.net
Tue Jan 24 07:58:26 PST 2017
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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