def grayScale(redWeight,greenWeight,blueWeight):
  pic=makePicture(pickAFile())
  show(pic)
  for p in getPixels(pic):
    r = getRed(p)*redWeight #0.299
    g = getGreen(p)*greenWeight #0.587
    b = getBlue(p)*blueWeight #0.114
    lum = r+g+b
    setColor(p,makeColor(lum,lum,lum))
  repaint(pic)