[Radiance-general] manipulate rtrace for desired outcome

[Andy McNeil]

I assumed a total surface reflectance of 50%, with a split of 30:70 ratio
of diffuse to specular ratio. The roughness was a wild guess. Though I see
now that I flipped your diffuse - specular ratio, but you get the point.

Andy

On Mon, Jun 27, 2016 at 7:21 AM, Mengdi Guo <mengdi.guo1988 at gmail.com>
wrote:

> Dear Andy,
>
> I have one more question about this part of your reply:
>
> *The relative weight of specular vs. diffuse rays is a result of the
> material applied to the surface. if you use a plastic material such as
> below (with 15% diffuse reflectance, 35% specular reflectance, and 0.05
> roughness):*
>
> How do you get the value: '*5% diffuse reflectance, 35% specular
> reflectance, and 0.05 roughness' *from the setting of 'specular weight
> 30% and diffuse weight 70% after reflection'.
>
> *void plastic test70-30*
> *0*
> *0*
> *5 0.15 0.15 0.15 0.35 0.05*
>
> *then the specular rays will account for 30% of the final ray value and
> the diffuse rays will account for 70% of the final ray value.*
>
> Thanks a lot.
>
> Cheers,
> Mengdi
>
> On 23 June 2016 at 05:53, Andy McNeil <mcneil.andrew at gmail.com> wrote:
>
>> Mengdi,
>>
>> Answers inline below...
>>
>> 1.       No matter what parameters I’ve set for rtrace function, there’s
>>> only one ray in the output. Yet, I’m expecting multiple rays emitted from
>>> the receiver points to the light source, as described in radiance manual.
>>
>>
>> By default rtrace outputs the accumulated value of the input ray. If you
>> want it to output information for rays traced to arrive at the reported
>> value you should use -o with either t or T.  See the rtrace manual page for
>> more output options:
>> http://radiance-online.org/learning/documentation/manual-pages/pdfs/rtrace.pdf
>>
>> 2.       I’d also like to know what parameters can control reflections
>>> characters of rays. For instance, I want the rays to partly diffuse after
>>> reflecting on the surface. The desirable condition is: for a bundle of rays
>>> hitting the surface, 1) 70% of the rays will follow specular reflection,
>>> 30% will be scattered after reflection. 2) for the scattered part, I want
>>> them to be reflected within an angle (e.g. 30 degree) to the specular
>>> reflected ray.
>>>
>>
>> The relative weight of specular vs. diffuse rays is a result of the
>> material applied to the surface. if you use a plastic material such as
>> below (with 15% diffuse reflectance, 35% specular reflectance, and 0.05
>> roughness):
>> void plastic test70-30
>> 0
>> 0
>> 5 0.15 0.15 0.15 0.35 0.05
>>
>> then the specular rays will account for 30% of the final ray value and
>> the diffuse rays will account for 70% of the final ray value. However the
>> actual number of rays spawned are controlled by the -ad parameter for
>> diffuse rays and -ss parameter for specular rays, with the caveat that only
>> one ray is traced for the specular reflections if the roughness is 0,
>> otherwise you'd be tracing the same ray several times.
>>
>> It seems to me that you want pure specular and scattered specular rays
>> together, so you'll probably need want to use a plasdata material
>> definition. The reference manual provides info on materials:
>> http://radsite.lbl.gov/radiance/refer/refman.pdf
>>
>>
>>
>>> 3.       Another question is: acknowledged that the emitted rays are
>>> not evenly distributed for hemispherical sampling. I'm still wondering if
>>> it's possible to evenly send rays with radiance.
>>
>>
>> The spawned rays are random, but are distributed such that each ray as
>> equal influence on the result. This produces a non-biased estimate. If by
>> even you mean that you don't want a cosine factor on the distribution of
>> rays, you should look at plasdata. If however by evenly distributed you
>> don't want them to be randomized, I'm not sure that's possible without
>> going into the Radiance code, and doing this will result in a biased
>> estimate.
>>
>> John Mardaljevic gives a good presentation on how radiance spawns rays in
>> order to find a result:
>>
>> http://radiance-online.org/community/workshops/2014-london/presentations/day1/Mardaljevic_AmbientCrash.pdf
>>
>> And chapter 12 in Rendering with Radiance gives a detailed overview of
>> how this all works:
>> http://radiance-online.org/learning/documentation/book/rwrcontents
>>
>> Best,
>> Andy
>>
>> On Mon, Jun 13, 2016 at 10:56 PM, Mengdi Guo <mengdi.guo1988 at gmail.com>
>> wrote:
>>
>>> Hi,
>>>
>>>
>>>
>>> Currently I'm using the ray tracing engine of radiance, trying to figure
>>> out the ray paths connecting designated view points and light resources (in
>>> cylinder and polygon type). In this process, I've encountered some
>>> difficulties that I'd appreciate your assistance.
>>>
>>> I'm mainly using light as primitive type, since I want to figure out the
>>> possible ray paths between receiver point and artificial light sources.
>>> However the ray paths is not what I’ve expected.
>>>
>>> In the testing condition where there is one receiver and one polygon
>>> light sources directly exposed to each other. The polygon light source is
>>> the only light in this scene. The source and receiver are placed in the
>>> middle of two building blocks. I’m controlling rtrace function with the
>>> parameters ‘ –ab’ ‘ –ad’ ‘–dj’ ‘– ds’ ‘-st’ ‘-ss’.
>>>
>>> 1.       No matter what parameters I’ve set for rtrace function,
>>> there’s only one ray in the output. Yet, I’m expecting multiple rays
>>> emitted from the receiver points to the light source, as described in
>>> radiance manual.
>>>
>>> 2.       I’d also like to know what parameters can control reflections
>>> characters of rays. For instance, I want the rays to partly diffuse after
>>> reflecting on the surface. The desirable condition is: for a bundle of rays
>>> hitting the surface, 1) 70% of the rays will follow specular reflection,
>>> 30% will be scattered after reflection. 2) for the scattered part, I want
>>> them to be reflected within an angle (e.g. 30 degree) to the specular
>>> reflected ray.
>>>
>>> Could you suggest me how to achieve the condition of 1 and 2 with
>>> radiance? Is there any other suggestions on the setting of model or scene?
>>>
>>> 3.       Another question is: acknowledged that the emitted rays are
>>> not evenly distributed for hemispherical sampling. I'm still wondering if
>>> it's possible to evenly send rays with radiance.
>>>
>>> Thank you very much with best regards,
>>>
>>> Mengdi Guo
>>>
>>>
>>> _______________________________________________
>>> Radiance-general mailing list
>>> Radiance-general at radiance-online.org
>>> http://www.radiance-online.org/mailman/listinfo/radiance-general
>>>
>>>
>>
>
>
> --
> Mengdi GUO
> PhD Candidate
> the University of Hong Kong
>
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