[Radiance-general] Re: Radiance-general Digest, Vol 52, Issue 19

[Greg Ward]

Dear Kam,

As John Mardaljevic points out, the change at the edges of the sky is  
due to the "mixing function" designed to reduce variance at the  
horizon between the sky and the ground.  You can also avoid its  
appearance without modifying skyfunc.cal by setting -g 1.0 in gensky,  
I believe.

As for the repeatability of the results, John's reference is a good one:

	http://www.radiance-online.org/pipermail/radiance-general/2006- 
December/004147.html

I'm grateful to him for digging this up -- I would have tried to  
answer it all over again...

Best,
-Greg

P.S.  It is better not to send attachments to the list.

> From: Kam Shing Leung <citykam at yahoo.com>
> Date: June 16, 2008 11:23:43 PM PDT
>
> Dear all,
> I am trying to obtain irradiance value on the surfaces of building  
> blocks under daylight but encounter the following problems.  Could  
> you shed some light on how they can be solved?
> 1.       Uneven distribution of the uniform sky
> I produced a uniform sky using the –u option under the gensky  
> function.  As measured from the fisheye view of the sky (both  
> angular and hemispheric), however, the radiance of the sky slightly  
> decreases towards the edge of the “circle” (please see attached).   
> Is there a way to improve this situation?  Here are the parameters  
> that I used for rendering the sky:
> > Rpict –vf view.vf –ab 0 –aa 0 –ar 128 –ad 4096 – as 1024 sky.oct
>
> 2.       Different value obtain in each run
> I tried to render some building blocks (with homogeneous color and  
> reflectance) and then measure the irradiance of a point on the  
> ground with rtrace.  Different values return even if the inputs are  
> identical.  The discrepancy can be as much as 5%.  Like the command  
> line printed above, ambient parameters had already been set at  
> their maximum values (except for -ab which is set at 1).  Is there  
> a way to obtain irradiance values with a smaller margin of error?
>
> Thank you very much for your help!
>
> Kam
>