> Quote - izzy, this works, using yer suggestions to make it more elegant.  one inf. lite, no IDL.

I'm so glad - I was beginning to distrust my own thinking! 

My problem was I was using a spotlight with inverse square falloff and it just wasn't producing enough light and contrast in the first place for the Diffuse node to detect the shadows very well.

I made a slight modification to the shader to give a hard switch between normal and inverted textures, so there is no grey blending at all. You can see I used a spotlight, and the shader is reacting to the angular fall-off as well as the shadow.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)