BSDF material interaction with -ab

Hello Radiance community,

I was wondering in my reasearch how did -ab parameter interact with bsdf
based material. Specifically, how would an -ab 0 behave. Would it let me
keep the direct portion of the calculation? If so what would be the direct
portion, the transmitted ray that has the same direction as the incident
one ? I'd guess such assumption wouldn't hold up very well in some cases.

Sincerely,
Marouane.

Hello Marouane,

There is a separate "direct" calculation associated with all of the Radiance bsdf materials, which queries the reflectance function for the eye ray and source ray directions. All bsdf materials include some notion of diffuse contributions, sometimes adding the non-source directional diffuse in with the truly Lambertian component. Only the "BSDF" type sends additional rays in the non-source directions, keeping the Lambertian part separate.

I hope this helps.

-Greg

···

From: [email protected]
Date: April 20, 2018 1:57:44 AM PDT

Hello Radiance community,

I was wondering in my reasearch how did -ab parameter interact with bsdf based material. Specifically, how would an -ab 0 behave. Would it let me keep the direct portion of the calculation? If so what would be the direct portion, the transmitted ray that has the same direction as the incident one ? I'd guess such assumption wouldn't hold up very well in some cases.

Sincerely,
Marouane

Hi Marouane,

First, it is best not to reply to the digest-form offered by the mailing list, as it tends to create long and unthreaded content. (I.e., the subject is not preserved.) I have cleaned up this response.

I don't think there is any proper documentation of how BSDF materials behave in Radiance. The closest we have are presentations from past workshops, which you may find useful if you track them down. For example, the 2011 workshop had a few different presentations covering BSDF materials:

  https://radiance-online.org/community/workshops/2011-berkeley-ca

There are two ways to get a Lambertian component in a BSDF. The primary way is automatic, where the BSDF library finds a minimum global value in the data, and separates that from the rest. You can also manually specify an additional Lambertian component via floating point arguments to the BSDF primitive. This is explained in the reference manual (page 11):

  http://radsite.lbl.gov/radiance/refer/refman.pdf

As for the direct component, this is the value applied to "shadow test" rays sent directly to light sources. It is nothing more or less than an evaluation of the BSDF for incident and source directions as I said earlier.

Best,
-Greg

···

From: [email protected]
Date: April 24, 2018 10:30:31 AM PDT

Hello Greg,

Thank you very much for your answer! I tried my very best to understand it but unfortunately I've hit a wall.
I believe something about my understanding of BSDF material is lacking. genBSDF for example, produces klems BSDF which is a 145x145 matrix (for front/back,transmittance/reflectance) or tensor tree which a BSDF with variable resolution.
I don't see how for example how "bsdf materials include some notion of diffuse contribution" separately from the rest of the calculation. I failed to understand how the direct calculation is separate from the rest. I tried to check for documentation online about the BSDF material without much success.

Sincerely,
Marouane.

------------------------------

Date: Fri, 20 Apr 2018 07:15:09 -0700
From: Greg Ward <[email protected]>

Hello Marouane,

There is a separate "direct" calculation associated with all of the Radiance bsdf materials, which queries the reflectance function for the eye ray and source ray directions. All bsdf materials include some notion of diffuse contributions, sometimes adding the non-source directional diffuse in with the truly Lambertian component. Only the "BSDF" type sends additional rays in the non-source directions, keeping the Lambertian part separate.

I hope this helps.

-Greg

> From: [email protected]
> Date: April 20, 2018 1:57:44 AM PDT
>
> Hello Radiance community,
>
> I was wondering in my reasearch how did -ab parameter interact with bsdf based material. Specifically, how would an -ab 0 behave. Would it let me keep the direct portion of the calculation? If so what would be the direct portion, the transmitted ray that has the same direction as the incident one ? I'd guess such assumption wouldn't hold up very well in some cases.
>
> Sincerely,
> Marouane

Hello Greg,

Thank you very much for your answer! I tried my very best to understand it
but unfortunately I've hit a wall.
I believe something about my understanding of BSDF material is lacking.
genBSDF for example, produces klems BSDF which is a 145x145 matrix (for
front/back,transmittance/reflectance) or tensor tree which a BSDF with
variable resolution.
I don't see how for example how "bsdf materials include some notion of
diffuse contribution" separately from the rest of the calculation. I
failed to understand how the direct calculation is separate from the rest.
I tried to check for documentation online about the BSDF material without
much success.

Sincerely,
Marouane.

···

Von: [email protected]
An: [email protected]
Datum: 20.04.2018 21:04
Betreff: Radiance-general Digest, Vol 170, Issue 7

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Today's Topics:

   1. BSDF material interaction with -ab
      ([email protected])
   2. Re: BSDF material interaction with -ab (Greg Ward)

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Date: Fri, 20 Apr 2018 10:57:44 +0200
From: [email protected]
To: [email protected]
Subject: [Radiance-general] BSDF material interaction with -ab
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Hello Radiance community,

I was wondering in my reasearch how did -ab parameter interact with bsdf
based material. Specifically, how would an -ab 0 behave. Would it let me
keep the direct portion of the calculation? If so what would be the direct

portion, the transmitted ray that has the same direction as the incident
one ? I'd guess such assumption wouldn't hold up very well in some cases.

Sincerely,
Marouane.
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Message: 2
Date: Fri, 20 Apr 2018 07:15:09 -0700
From: Greg Ward <[email protected]>
To: Radiance general discussion <[email protected]>
Subject: Re: [Radiance-general] BSDF material interaction with -ab
Message-ID: <[email protected]>
Content-Type: text/plain; charset="us-ascii"

Hello Marouane,

There is a separate "direct" calculation associated with all of the
Radiance bsdf materials, which queries the reflectance function for the
eye ray and source ray directions. All bsdf materials include some notion
of diffuse contributions, sometimes adding the non-source directional
diffuse in with the truly Lambertian component. Only the "BSDF" type
sends additional rays in the non-source directions, keeping the Lambertian
part separate.

I hope this helps.

-Greg

From: [email protected]
Date: April 20, 2018 1:57:44 AM PDT

Hello Radiance community,

I was wondering in my reasearch how did -ab parameter interact with

bsdf based material. Specifically, how would an -ab 0 behave. Would it let
me keep the direct portion of the calculation? If so what would be the
direct portion, the transmitted ray that has the same direction as the
incident one ? I'd guess such assumption wouldn't hold up very well in
some cases.

Sincerely,
Marouane

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