[Radiance-general] manipulate rtrace for desired outcome

[Mengdi Guo]

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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