[Radiance-general] perfect specular reflection in rtrace

[Greg Ward]

Hi Antoine,

It seems you are running up against the limitation of ray-tracing to find small sources such as the sun in specular surfaces.

There is a specific fix for this problem for flat surface in Radiance, which is the "virtual source" calculation.  However, you need to use a special "mirror" type rather than "plastic" to enable this:

void mirror M_Spec
0
0
3 1 1 1

If you use this as your material, I think you will get the results you expect.  

I should point out that a 100% specular material is not realistic.  Polished metallic surfaces may reflect as much as 99% of light, but 100% is difficult to achieve without resorting to a prismatic dielectric film or similar.

Best,
-Greg

> From: Antoine Bugeat <bugeatantoine at gmail.com>
> Date: October 12, 2017 10:22:28 AM PDT
> 
> Hello all,
> 
> I'm a civil engineering student and I'm trying to compute the impact of specular reflections in a street "canyon".
> My problem is:
> I use rtrace function with the basic parameters, the same as DAYSIM uses
> (-I -aa 0.1 -ab 5 -ad 1000 -ar 300 -as 20 -dj 0 -dr 5 -ds 0.2 -dp 512 -lr 6 -lw 0.004 -st 0.1 -oov)
> My sky and sun description come from gendaylit.
> My wall is defined by a perfect specular plastic and the ground is defined by a diffuse material.
> 
> When I do the simulation, I get illogical results as if the specular reflection of the walls had no impact.
> 
> So, I tried to simulate a really simple scene with only one specular surface:
> 
> void plastic M_Spec 
> 0
> 0
> 5 0 0 0 1 0
> 
> M_Spec	polygon	1
> 0
> 0
> 12	-5.0	0.0	-5.0
> 	-5.0	0.0	5.0
> 	5.0	0.0	5.0
> 	5.0	0.0	-5.0
> 
> My source is described as a sun perfectly perpendicular to the surface:
> 
> void light solar
> 0
> 0
> 3 1000 1000 1000
> 
> solar source sun
> 0
> 0
> 4 0.0 -1.0 0.0 0.5
> 
> And the sensor is between the surface and the source and is regarding right to the surface:
> 
> 0.0	-5.0	0.0	0.0	1.0	0.0
> 
> And in this case, I get 0 after my rtrace simulation.
> I tried to vary some parameters of rtrace (direct and specular parameters) but it change nothing I still get 0.
> 
> Thank you for your help.
> 
> Antoine Bugeat
> https://www.radiance-online.org/mailman/listinfo/radiance-general
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