import Image

lena = Image.open('lena.png')
width, height = lena.size
numpixels = width * height
image = [0] * (numpixels * 3 / 2)

i = 0
ui = numpixels
vi = numpixels + numpixels / 4

#r, g, b, _ = lena.getpixel((128, 128))
#print r, g, b

print width, height

for y in range(height):
 for x  in range(width):
  r, g, b, _ = lena.getpixel((x, y))
  image[i] = (( (66*r + 129*g + 25*b + 128) >> 8) + 16) & 0xFF
  if x % 2 == 0 and y % 2 == 0:
   image[ui] = (((-38*r - 74*g + 112*b + 128) >> 8) + 128) & 0xFF
   image[vi] = (((112*r - 94*g - 18*b + 128) >> 8) + 128) & 0xFF
   ui += 1
   vi += 1
  i += 1
#

with open('lena.yuv420p', 'wb') as f:
 f.write(''.join(chr(x) for x in image))