#returns copy of picture at 50% scale
def scale_half(pic):
  w=getWidth(pic)
  h=getHeight(pic)
  pic2=makeEmptyPicture(w/2,h/2)
  for px in getPixels(pic2):
    x=getX(px)
    y=getY(px)
    sourcepx=getPixel(pic,2*x ,2*y )
    c=getColor(sourcepx)
    setColor(px,c)
  return pic2
    

#returns copy of picture at 200% scale
def scale_double(pic):
  w=getWidth(pic)
  h=getHeight(pic)
  pic2=makeEmptyPicture(2*w ,2*h )
  for px in getPixels(pic2):
    x=getX(px)
    y=getY(px)
    sourcepx=getPixel(pic,(x+1)/2 ,(y+1)/2 )
    c=getColor(sourcepx)
    setColor(px,c)
  return pic2

#the demo selects a picture, displays several
#successive reductions, and then several successive
#enlargements.    
def scale_demo():
  p=makePicture(pickAFile())
  show(p)
  p=scale_half(p)
  show(p)
  p=scale_half(p)
  show(p)
  p=scale_half(p)
  show(p)
  p=scale_double(p)
  show(p)
  p=scale_double(p)
  show(p)
  p=scale_double(p)
  show(p)