code: def showcutoffs(self, redblue = False):
          
          
          highmask = self.numpyarray > self.z2
          lowmask = self.numpyarray < self.z1
          if redblue == False :
              if self.negative :
                  self.drawmask(highmask, colour = 255)
                  self.drawmask(lowmask, colour = 0)
              else :
                  self.drawmask(highmask, colour = 0)
                  self.drawmask(lowmask, colour = 255)
          else :
              
              self.drawmask(highmask, colour = (255, 0, 0))
              self.drawmask(lowmask, colour = (0, 0, 255))
docs: We use drawmask to visualize pixels above and below the z cutoffs.
          By default this is done in black (above) and white (below) (and adapts to negative images).
          But if you choose redblue = True, I use red for above z2 and blue for below z1.