mirror and virtual sources

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?

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?

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?

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

Thanks,
Chris

ยทยทยท

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Hi Chris,

I'll respond inline, below...

From: "Christopher Rush" <[email protected]>
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
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