[Radiance-general] daylight factor and rtrace cmd

[Greg Ward]

Yes, the -oo option combined with -I doesn't really work properly in  
rtrace, as it "fakes" an intersection with an ideal diffuse surface a  
tiny amount (about 1e-6) in front of the point you specified, while  
setting the ray origin arbitrarily one unit before it.  No actual  
intersection calculation is done, so no worries about intervening  
rays.  This is something I should fix, but since you give the origin  
on input, it hasn't come up very often where people want the same  
origin on output.

-Greg

> From: Thomas Bleicher <tbleicher at googlemail.com>
> Date: July 16, 2009 9:45:58 AM PDT
>
> As Chris wrote, -I implies an observer point 1m above the 'point of  
> interest'.
> But it also turns around the view vector (your output shows  
> -1.000000e+000).
>
> Basically rtrace introduces a virtual plane of lambertian  
> properties and looks
> down onto this plane to calculate the irridance from the reflection  
> (which is
> identical).
>
> The offset of 1m is only a problem if you want to use the output to  
> anchor
> your results in a model or if your workplane is close to an  
> obstruction that's
> lower than 1m above the plane. Then the rtrace rays would hit the back
> of the obstruction and not the ideal reflector.
>
> But then, perhaps Greg has anticipated this case and added some  
> magic to
> avoid it ...
>
> Thomas