Draw a partially opaque butterfly image on the background

Then Listing 9 uses the current value of the slider

Listing 9. Draw the butterfly and repaint. 
          //Set the opacity of butterfly and copy it onto
          // the display. Then repaint the display.
          butterfly =
                  setOpacity(butterfly,slider.getValue());
          drawPictureOnPicture(
                  butterfly,
                  display,
                  backgroundWidth/2 - butterflyWidth/2,
                  backgroundHeight/2 - butterflyHeight/2);
          
          display.repaint();
        }//end stateChanged
      }//end new ChangeListener
    );//end addChangeListener
    //--------------------------------------------------//
  }//end constructor
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