[Radiance-general] window glass modeling

[Milan Cakanovic]

Hi Lars!

> ...In the case of open blinds, this will lead to a very low requirement for
> ab. In the case of mostly closed blinds, where the sky cannot be seen from
> the glass pane, ab would lead to an irradiance reading of 0, but will
> stabilize at higher ab. So the requirement will change according to the
> blinds tilt angle, and you will probably have to go with the worst case of
> almost closed blinds to determine the ab for your scene. I still doubt that
> you will see much changes when reaching high values for ab such as 8.

Let us assume that for the worst case of almost closed blinds, requirement for
-ab is 8. Is it right to use this set to ab (-ab 8) for the open blinds, or I
have to adapt (decrease) value for -ab?  I know that high value for -ab leads to
the longer time of calculation.

> Again, important to imagine what this means. The ab value used by rtrace
> after the mkillum surface has been generated needs to ensure that the mkillum
> generated source is hit during diffuse indirect calculation. As it is
> probably directly visible from the sensor location, most of the irradiance
> received will be directly transported from the source to the sensor, and only
> a very little fraction by one or more bounces over walls and ceilings. You
> will also be able to use relaced ad and as here, if the source surface is
> rather large and not hidden behind geometry.

I think that I understood this: If I use an appropriate high value of -ab for
mkillum, after that, I can use an appropriate low value of -ab for rtrace, and
thus, reduce a calculation time.
I have done some preliminary calculations for an empty room with completely
closed blinds, four luminaires turned on, and when I used rtrace with -ab 0 to
-ab 8 (for more accuracy), I got higher and higher values for an illuminance in
one sensor point (for -ab 0, illuminance~500 lux; for -ab 8 illuminance~800). I
assume that the higher value for an illuminance (-ab 8) is close to correct
value? (Similar situation is explained on p.43, figure 33, Basic Radiance
Tutorial, Axel Jacobs).

> Important detail, all the responses you got would not be true for highly
> specular blinds...

Material for the slats is define:

void plastic whit_0.9
0
0
5  .9 .9 .9  .015 .02

Specularity :  0 - matte;      0.07 - satin
Roughness :  0 - polished; 0.2   - low gloss
I used a value of .015 for specularity, but if you think that I need to use a
lower value, I will do that. Maybe a value of 0?

Cheers, Milan