code: def _fracRoiSparse(self):
          
          self.frac_roi_sparse = np.min([self.mask_1.frac_roi_sparse,self.mask_2.frac_roi_sparse],axis=0)
          return self.frac_roi_sparse
docs: Calculate an approximate pixel coverage fraction from the two masks.
  
          We have no way to know a priori how much the coverage of the
          two masks overlap in a give pixel. For example, masks that each
          have frac = 0.5 could have a combined frac = [0.0 to 0.5]. 
          The limits will be: 
            max:  min(frac1,frac2)
            min:  max((frac1+frac2)-1, 0.0)
  
          Sometimes we are lucky and our fracdet is actually already
          calculated for the two masks combined, so that the max
          condition is satisfied. That is what we will assume...