Solved by KFrank in post #2 
              
              
                Hi Droid! 

Let me distinguish between a (single-label) multi-class problem and 
a multi-label, multi-class problem.  In both cases you have multiple 
classes (one of which might be a “background” or catch-all “other” 
class). 
In the single-label case, each pixel (or more generally, each item 
to b…