text: @mfunction("skyMask, I, dilate, erosion")
  def findSky(input=None):
  
      # Summary - find sky part in image
  
      # threshold to blue space
      I = input(mslice[:], mslice[:], 3)
      I = imbinarize(I, 0.7)
  
      # do dilation -> erosion
      sq = ones(3, 3)
      dilate = imdilate(I, sq)
      erosion = imerode(dilate, sq)
  
      # choose the biggest part as sky
      J = bwlabel(erosion, 4)