Python script to put a rainbow overlay in an image

rainbow.py

#!/usr/bin/env python
import Image, colorsys, sys

if len(sys.argv) < 2:
    print 'usage: {} <file> [<output>]'.format(sys.argv[0])
    sys.exit(0)

img = Image.open(sys.argv[1])
width, height = img.size

for i in xrange(width):
    for j in xrange(height):
        pixel = img.getpixel((i,j))
        rbow = colorsys.hsv_to_rgb((j/float(height)-0.05)%1.0, 1, 1)

        img.putpixel((i, j), tuple(int((a+b*255)/2) for a, b in zip(pixel, rbow)))

outfile = sys.argv[2] if len(sys.argv) >= 3 else 'rainbow_' + sys.argv[1]
img.save(outfile)