[Radiance-general] irradiance vs. radiance / luminance vs. illuminance

[Greg Ward]

Hi Marija,

>> Surfaces made of these materials pass as if they aren't there.
>
> What does this exactly mean when values on the image are traced in  
> ximage?
> For example if I have a window made of some glass material or some  
> curtain on it made of trans material. On irradiance image when I  
> try to trace value of some pixel on the window surface, what do I  
> get? Is it irradiance of the objects behind the window (outside  
> ground, buildings etc)? Does window/curtain transmittance has an  
> influence on reported value?

If you query a pixel on a window, the window acts as if it's not  
there, so it's transmittance has no effect.

> What happens for surfaces made of transparent BRTfunc materials  
> (like double glazing)? Are they ignored too, or their irradiance  
> value is reported?

The BRTDfunc isn't one of the materials on the list, so it shows up  
with an irradiance value like other materials.  I'm sorry if this is  
confusing, but you can usually tell simply by looking at the image  
which materials are transparent and which aren't.  If you want all  
materials to be treated the same and irradiance to be computed for  
everything, you can use the following in place of the rpict -i option:

vwrays [view options] -ff -x xres -y yres -pa 0 | rtrace -h -ff -opN  
octree | rtrace -ffc -I -x xres -y yres > irrad.pic

The first trace comes up with the intersection points and surface  
normals for the second trace, which computes irradiance at those  
points.  You need to be sure that your view has no distant light  
sources in it, since those don't have intersection points or surface  
normals.

-Greg