code: def sc_zoom_coarse(self, viewer, event, msg=True):
          
          if not self.canzoom:
              return True
  
          zoom_accel = self.settings.get('scroll_zoom_acceleration', 1.0)
          # change scale by 20%
          amount = self._scale_adjust(1.2, event.amount, zoom_accel, max_limit=4.0)
          self._scale_image(viewer, event.direction, amount, msg=msg)
          return True
docs: Interactively zoom the image by scrolling motion.
          This zooms by adjusting the scale in x and y coarsely.