Maybe I missed some option. Inkscape 0.91 x64, win8.

At first I tried with photos and objects, to cut a part from a photo. Did not worked. Using the simplest scenario. I created 2 filled nontransparent circles, one over the other. When I try to activate "set clip" or "set mask" on the top circle - nothing happens. Also, from the context menu on the objects I'd like to use that way - "Release..." item is not active, as if indeed nothing was set to the object.

How to fix this? What I'm missing?

//edit: the same problem on second machine (32bit version).

Ok., solved. I assumed that it behaves differently.

Yes, masking works "backwards" in Inkscape, compared to how it works in raster graphics. Black and white are "flipped".