[Radiance-general] Re: Strange problem using -vtl to cut sections

[Greg Ward]

> From: "Westling, Robert" <RWestling@mlp.com>
> To: <radiance-general@radiance-online.org>
> Subject: [Radiance-general] Strange problem using -vtl to cut sections
>
> Hi,
>
> I am cutting plan and sectional views through a model for radiance
> images, illuminance images using falsecolor, and point data output using
> rtrace and have found the following problem:  light is showing up inside
> of fully closed boxes.  For instance in plan view using -vtl and -o to
> get a plan cut 4' off the floor I find that light appears to be
> propagating inside of volumes with no holes.  Running rtrace in these
> areas also shows that a small amount of light is bouncing around inside
> these solid shapes.
>
> The solution I plan on using is to model a flat plane at the bottom of
> each of these spaces and give it a completely black plastic finish.
> However it is a little disconcerting that light is entering the inside
> of these boxes.
>
> Any ideas on the subject would be greatly appreciated.
>
> Thank you,
>
> Robert Westling
> steinerwestling

This is disturbing, but nothing really to worry about.  In most lighting 
calculations, there will be some small amount of light leakage, 
especially across infinitely thin boundaries.  And, as Carsten points 
out, the ambient term will give you light even where there is none.  If 
you want to avoid all such leakage problems, you would need to set -ar 0 
and -av 0 0 0.  However, the image would take longer to compute, and 
setting -av 0 0 0 rather than to a good guess of the ambient level 
results in a loss of accuracy over most of the scene.

(There is a script in the 3.4 release called compamb, which computes an 
ambient value for a rad input file automatically.)

-Greg