[Radiance-general] Generating a black and white pic

[Christopher Kings-Lynne]

Hi,

I wish to render a scene and output a black and white image where the 
black areas have 0 irradiance and everything else is white.

I then want to pcomb this over another image so that on that image all 
the black parts of the mask appear in black, but wherever the mask is 
white, the underlying picture shows through.

How can I do this?  I can see it should be possible, but the devil is in 
the details it seems...

Thanks,

Chris