Color definition in Plastic material

radiance-general
1

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

2

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 <[email protected]>:

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

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

3

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:[email protected]]
Sent: 20 January 2017 20:50
To: Radiance general discussion <[email protected]>
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 <[email protected]<mailto:[email protected]>>:
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

_______________________________________________
Radiance-general mailing list
[email protected]<mailto:[email protected]>
http://www.radiance-online.org/mailman/listinfo/radiance-general

4

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 <[email protected]>:

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:[email protected]]
*Sent:* 20 January 2017 20:50
*To:* Radiance general discussion <[email protected]>
*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 <[email protected]>:

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

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

5

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 <[email protected]>
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 <[email protected]>:
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:[email protected]]
Sent: 20 January 2017 20:50
To: Radiance general discussion <[email protected]>
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 <[email protected]>:

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

6

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 <[email protected] <mailto:[email protected]>>:

    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:[email protected]
    <mailto:[email protected]>]
    *Sent:* 20 January 2017 20:50
    *To:* Radiance general discussion
    <[email protected]
    <mailto:[email protected]>>
    *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
    <[email protected] <mailto:[email protected]>>:

        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

        _______________________________________________
        Radiance-general mailing list
        [email protected]
        <mailto:[email protected]>
        http://www.radiance-online.org/mailman/listinfo/radiance-general
        <http://www.radiance-online.org/mailman/listinfo/radiance-general>

    _______________________________________________
    Radiance-general mailing list
    [email protected]
    <mailto:[email protected]>
    http://www.radiance-online.org/mailman/listinfo/radiance-general
    <http://www.radiance-online.org/mailman/listinfo/radiance-general>

_______________________________________________
Radiance-general mailing list
[email protected]
http://www.radiance-online.org/mailman/listinfo/radiance-general

7

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:[email protected]]
Sent: Monday, January 23, 2017 9:12 AM
To: [email protected]
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 <[email protected]<mailto:[email protected]>>:
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:[email protected]<mailto:[email protected]>]
Sent: 20 January 2017 20:50
To: Radiance general discussion <[email protected]<mailto:[email protected]>>
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 <[email protected]<mailto:[email protected]>>:
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

_______________________________________________
Radiance-general mailing list
[email protected]<mailto:[email protected]>
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________
Radiance-general mailing list
[email protected]<mailto:[email protected]>
http://www.radiance-online.org/mailman/listinfo/radiance-general

_______________________________________________

Radiance-general mailing list

[email protected]<mailto:[email protected]>

http://www.radiance-online.org/mailman/listinfo/radiance-general

8

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:[email protected]]
*Sent:* Monday, January 23, 2017 9:12 AM
*To:* [email protected]
*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
    <[email protected] <mailto:[email protected]>>:

        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:[email protected]
        <mailto:[email protected]>]
        *Sent:* 20 January 2017 20:50
        *To:* Radiance general discussion
        <[email protected]
        <mailto:[email protected]>>
        *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
        <[email protected]
        <mailto:[email protected]>>:

            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

            _______________________________________________
            Radiance-general mailing list
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            <mailto:[email protected]>
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9

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:[email protected]]
*Sent:* Monday, January 23, 2017 9:12 AM
*To:* [email protected]
*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
    <[email protected] <mailto:[email protected]>>:

        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:[email protected]
        <mailto:[email protected]>]
        *Sent:* 20 January 2017 20:50
        *To:* Radiance general discussion
        <[email protected]
        <mailto:[email protected]>>
        *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
        <[email protected]
        <mailto:[email protected]>>:

            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

            _______________________________________________
            Radiance-general mailing list
            [email protected]
            <mailto:[email protected]>
            http://www.radiance-online.org/mailman/listinfo/radiance-general

        _______________________________________________
        Radiance-general mailing list
        [email protected]
        <mailto:[email protected]>
        http://www.radiance-online.org/mailman/listinfo/radiance-general

    _______________________________________________

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    [email protected]
    <mailto:[email protected]>

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