[Radiance-general] mirror and virtual sources

[Greg Ward]

Hi Chris,

I'll respond inline, below...

> From: "Christopher Rush" <Christopher.Rush at arup.com>
> Date: April 8, 2008 2:12:50 PM PDT
>
> I am modeling a daylight redirection system, something like a light
> shelf type of geometry.  I have a two part question regarding the
> specular component and virtual sources.
>
> It's by no means a validated material definition, but I've been  
> testing
> with the following material meant to be something like brushed  
> aluminum.
> The intention is that the metal gives the best approximation of the
> surface characteristics, while the mirror adds the virtual sources of
> the specular reflection.
>
> void metal aluminum
> 0
> 0
> 5 .7 .7 .7 .5 .01
>
> void mirror aluminum_mirror
> 1 aluminum
> 0
> 3 .35 .35 .35
>
> My first question is the value of the mirror material.  I want to
> confirm that my value for the mirror of 0.35 is appropriate (0.35 =  
> 0.7
> * 0.5).  I've based this on my general estimate of 70% general
> reflectance and 50% specular reflectance for the actual material. Or
> should the value for the mirror be the full 0.5 to match the full
> specular component?

Your current value is best, as it corresponds to the amount of  
specular reflection from the alternate material, and the mirror type  
does not scatter light at all.

> The second part of the question is regarding the use of the alternate
> material in the calculation.  From an old post I turned up the  
> following
> from Greg:
>
>> If you are trying to reflect objects in a surface, the "mirror"
>> primitive allows you to specify any alternate material you choose  
>> when
>> it is not participating in the virtual source calculation.
>
> Does this mean that the alternate material will participate in the
> indirect calculation? Or is the alternate material only used to  
> produce
> the appearance of the surface in direct view?  Or something else?

Yes, the alternate material will be used during the diffuse  
interreflection calculation, with care not to over-count light source  
contributions.

> Also, is it reasonable that for a rough surface the mirror value  
> should
> be toned down a bit to account for roughness that is not part of the
> mirror definition?

Roughness affects the scattering of light (and therefore the  
sharpness of shadow boundaries) but not the total amount of light  
transferred.  Therefore, it's not a good idea to adjust the mirror  
value in an attempt to consider roughness.

> Any suggestions for appropriate definitions for actual metals found
> commonly in the built environment are welcomed.
>
> Thanks,
> Chris

Best of luck,
-Greg