code: def __change_opacity(self, slider):
      opacity = slider.value
      for contour in self.__actors:
          contour.GetProperty().SetOpacity(opacity)
  
file_path: dipy/dipy/viz/horizon/tab/roi.py
imports: from dipy.utils.optpkg import optional_package
  from dipy.viz.horizon.tab import HorizonTab, build_label, color_single_slider
label_desc: Use image node_id 2 for calling the ROIsTab obj's underlying member method code with example usage: obj.__change_opacity(slider) without return types