[Radiance-general] window glass modeling

[Lars O. Grobe]

Hi Milan!

>> Perfect. Plus: if most ambient bounces are expected to happen in between
>> diffuse blinds, you need the high setting of -ab only for mkillum.
>
> Bravo Lars, I forgot to ask you about -ab for mkillum. But, when you say "high
> setting of -ab", what is high? Is that maybe -ab 8, or maybe more?

-ab sets the number of bounces in the diffuse indirect calculation after 
which Radiance will stop spawning further rays. When you run mkillum, 
this means that you need to set ab to a value that ensures that rays hit 
the sun or sky unless the number of reflections led to that little 
Radiance left that you can safely skip. I doubt that you will see much 
changes at ab higher then 3 in your case, but there is a safe way to 
find out. Run a couple of simulations varying -ab for mkillum, and plot 
irradiance against ab. Use a sunless sky for that. 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.

>> When running rtrace, you can probably stay at ab set to about 2, and that means
>> you can follow previous advices to calculate irradiance for more then one point

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.

>> By the way, you are looking at diffuse reflective blinds, right?
> Yes.

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

Cheers, Lars.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: smime.p7s
Type: application/pkcs7-signature
Size: 3740 bytes
Desc: S/MIME Cryptographic Signature
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20101230/6f535dc7/attachment.bin>