code: def level(self, lev):
      if lev <= 0:
          lev = 0
      self._level = lev
      self.fshader["isolevel"] = float(lev)
  
imports: from .base_filter import Filter
  from ..shaders import Function, Varying
  from ...color import colormap, Color
label_desc: Use image node_id 3 for calling the IsolineFilter obj's underlying member method code with example usage: obj.level(lev) without return types