[Radiance-general] perfect specular reflection in rtrace

[Greg Ward]

Hi Antoine,

The "-lr 0" doesn't mean "zero reflections," but "no limit on reflections," so that's part of your problem.  Setting "-dr 0" has no effect if you don't have any non-glow light sources.  Offhand, I don't know how to compute what you're after other than temporarily replacing your mirror with a completely black surface.

Cheers,
-Greg

> From: Antoine Bugeat <bugeatantoine at gmail.com>
> Date: November 27, 2017 8:58:19 AM PST
> 
> Hello
> 
> Thanks again for your help Greg
> 
> I still have not been able to solve my problem ...
> 
> To summarize my case, I have an environment composed only of surface "mirror" and a light source "glow" (the sky). I want to get only the radiation that comes directly from the source without any reflection.
> It seems that even with the parameters -ab 1 -dr 0 -lr 0 (-ab 1 because the sky is modeled with "glow") the radiation coming from the sky produces reflections in the scene.
> 
> I modeled the environment with the material "plastic" and it works perfectly. I'm almost sure that the problem comes from the material "glow" combined with materials "mirror" but I want to keep this configuration.
> 
> Regards,
> 
> Antoine
> 
> 2017-10-24 20:46 GMT+02:00 Greg Ward <gregoryjward at gmail.com>:
> Hi Antoine,
> 
> Glad you got the basic mirror type working.
> 
> One thing you should take into consideration for your model is the Fresnel factor that affects how light reflects off glazing materials (building facades).  The "mirror" type does not change its reflectance with angle, but coated and uncoated glazing materials do.  There are exact calculations and approximations you can use, which you can apply as a "brightfunc" modifier to the "mirror" type.
> 
> To answer your question, though, the number of reflections or redirections from virtual source materials is controlled by the -dr rendering option.  Set this to "-dr 0" if you do not wish to include any specular source redirections.  You will still get reflections (even multiple reflections) of the sky, even if you set -ab 0.  That's because the specular reflections don't count in the diffuse interreflection (ambient) calculation.  To fully eliminate sky reflections, you'll need to set -lr 0.
> 
> Cheers,
> -Greg
> 
>> From: Antoine Bugeat <bugeatantoine at gmail.com>
>> Date: October 24, 2017 10:38:57 AM PDT
>> 
>> Hello all,
>> 
>> Thanks again for your quick reply. Now I can model my street walls as a perfect specular materials ("mirror" materials) and obtain the results I expected.
>> 
>> I come back with an other problem:
>> 
>> The main purpose of my work is to compute the impact of the specular reflections in a street canyon.
>> So, I would like to separate the light that comes from the reflections of the walls (only specular) and the light that comes directly from the sky (defined by "glow" material and "skyfunc"). I wanted to do a simulation with 0 reflection and an other simulation with 5 (or more) reflections. The results obtain with 0 reflection is directly the direct sky component and the substraction of 5 reflections - 0 reflections is the "environment" part.
>> 
>> I did the same process with a diffuse environment and it worked well. I changed the -ab parameter: 
>> -ab 1 to have the direct sky component (1 because of "glow" material);
>> and -ab 5 to have the entire reflections in the environment.
>> 
>> But in the specular environment (with "mirror" wall) there is no difference between my "0 reflection" case and my "5 reflections" case
>> I tried to change the parameters -ab -dr and -lr and it seems that all the reflections is computed whatever the parameters.
>> From what I know, -ab is for "diffuse inter-reflections", -dr is for "direct reflections", and -lr I don't know...
>> 
>> Obviously, I can replace the "0 reflection specular" case with the "0 refelction diffuse" case because it should be exactly the same, but I would like to understand where this problem comes from.
>> 
>> I do not know if I was clear and precise in my explanation. I am available if you have questions.
>> 
>> 
>> Regards,
>> 
>> Antoine
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