# File easy_vtk.rb, line 429
  def contour( num, min, max, options={} )
    opacity = false
    axes = false
    options.each{ |key, val|
      case key
      when 'opacity'
        opacity = val
      when 'axes'
        axes = val
      else
        raise "option (#{key}) is invalid"
      end
    }
    contour = Vtk::ContourFilter.new
    contour.SetInput( @grid )
    contour.GenerateValues( num, min, max )
    normals = Vtk::PolyDataNormals.new
    normals.SetInput( contour.GetOutput )
    mapper = Vtk::PolyDataMapper.new
    mapper.SetInput( normals.GetOutput )
    mapper.SetScalarRange( min, max )
    if @lookupTable
      mapper.SetLookupTable( @lookupTable )
    else
      @lookupTable = mapper.GetLookupTable
    end
    actor = Vtk::Actor.new
    actor.SetMapper( mapper )
    actor.GetProperty.SetOpacity( options['opacity'] ) if opacity
    @ren.AddActor( actor )
    draw_axes( contour, axes ) if axes
    return nil
  end