[Radiance-general] Shadow above highlight?

[Greg Ward]

Hi John,

Since Radiance uses an azimuth/altitude breakup of the hemisphere,  
odd things can happen at the zenith in the indirect calculation,  
which is why it's generally good to have a "direct" calculation as  
well.  You can eliminate this problem by changing your window to an  
illum via mkillum or even applying the sky distribution function to  
the window directly as demonstrated on page 32 of the introductory  
chapter of RwR:

	http://radsite.lbl.gov/radiance/book/ch1/ch1.pdf

You might also try setting -aa 0, which would avoid indirect  
interpolation and perhaps help matters by itself (though it will take  
longer).

Best,
-Greg

> From: schwaj3 at rpi.edu
> Date: July 27, 2009 3:41:40 PM PDT
>
> I am fairly stumped as to what is going on here. I have a box with a
> square hole, 1/3 x 1/3 the width of the box, to allow sunlight into  
> my room.
>
> I am using the following gensky to attempt to only light the room  
> using
> sunlight.
> !gensky 6 21 12:00EST -a 40.77 -o 73.967 -g 0.0 +s
> skyfunc glow sky_glow
> 0
> 0
> 4 0 0 0 0
>
> sky_glow source sky
> 0
> 0
> 4 0 0 1 180
>
> This picture was rendered using -ab 1
> http://i639.photobucket.com/albums/uu111/radiancebox/radiance.jpg
> You'll note on this image, that there is a strange shadow on the  
> ceiling.
> I am unable to figure out why.
>
> If I instead place a light in the room, also 1/3x 1/3 the size of the
> box, I do not have this problem. This picture is taken from the  
> "back" of
> the box, with the light on the ceiling instead of the floor. There  
> is no
> gensky used on this picture.
>
> http://i639.photobucket.com/albums/uu111/radiancebox/Light.jpgThis  
> picture was rendered using -ab 0
>
> Changing the camera angle doesn't effect the shadow seen in the first
> picture. I have determined that the shape of the window is not causing
> the shadow, instead the highlight on the floor causes the shadow on  
> the
> ceiling.
>
> If anyone can help clear this up for me, I would appreciate it.
>
> --John