[Radiance-general] manipulate rtrace for desired outcome

[Andy McNeil]

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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20160622/a80e5459/attachment.html>