[Radiance-general] Generating a black and white pic

[Christopher Kings-Lynne]

> Be careful with thresholding, since the if() function takes any non-zero 
> positive value as "true".  For this reason, it's generally a good idea 
> to use some small epsilon to avoid errors, like:
> 
>     if(li(1)-1e-7, 1, 0);

In my case, the areas are in total shadow (ab=0), and hence have an 
input brightness of exactly zero.

So in your example above, you're also including areas that have a very 
slight amount of brightness, right?

Chris