I hope I'm just missing the obvious somewhere, and will gladly be labelled an idiot as long as someone points me to an easy solution... But I can't find a simple node solution for "invert" (make the black areas of an image white and the white areas black, and perform similar reversals on colors in between).

I see just about every other option possible under the math functions node, but no simple "invert".

Please tell me I'm just missing something very simple somewhere.... or do I really have to hack out some kind of multi-node trickyness to get this to work? 

Answered my own question... I used the subtract  math node with "1" in the value 1 place and the image plugged into value 2.

Still seems like there ought to be an "invert" selection among the standard math functions, though.

I use this technique, click the material that is inverted, and check the box (in the materials room), box is at the bottom, Normals forward.  That should work just fine.  Sharen

I appreciate the suggestion, but I think you've misunderstood my need here:

I'm talking about inverting colors in a node (like a photographic negative) in the process of building a shader tree.   Useful for things like making a bump map out of a color image where you want the dark areas raised instead of the light areas (just to give one example).

Your answer suggests I was trying to fix something that was in error and/or geometry related, when in actuality I'm trying to modify existing images and/or other shader nodes before they get to the final output shader where your checkbox is.

Probably my bad for not explaining the problem better.
Anyway, my solution above works for my needs.

Hi servo, Yup, that is the way to do it.

To make it more intuitive, try using a colour-math node. Subtract your texture from white...
It does exactly the same thing, just names it better..

But for some colour operations, the colour-math node works better than the math node, although I couldn't give examples. Something to do with the way RGB values get translated to floating point IIRC....

Cheers,
Diolma