bench.py

#!/usr/bin/env python

from PIL import Image
from timeit import repeat

def prepare_test_images(dim):
    """Plese, be careful with dim > 32. Result image is have dim ** 4 pixels
    (i.e. 1Mpx for 32 dim or 4Gpx for 256 dim).
    """
    i1 = bytearray(dim ** 4 * 2)
    i2 = bytearray(dim ** 4 * 2)
    res = 255.0 / (dim - 1)
    rangedim = range(dim)
    pos = 0
    for l1 in rangedim:
        for l2 in rangedim:
            for a1 in rangedim:
                for a2 in rangedim:
                    i1[pos] = int(res * l1)
                    i1[pos + 1] = int(res * a1)
                    i2[pos] = int(res * l2)
                    i2[pos + 1] = int(res * a2)
                    pos += 2
        print '%s of %s' % (l1, dim)

    i1 = Image.frombytes('LA', (dim ** 2, dim ** 2), bytes(i1))
    i2 = Image.frombytes('LA', (dim ** 2, dim ** 2), bytes(i2))
    return i1.convert('RGBA'), i2.convert('RGBA')

im1, im2 = prepare_test_images(8)
im1.save('im1.png')
im2.save('im2.png')

im1, im2 = Image.open('im1.png'), Image.open('im2.png')
im1.load(), im2.load()

print repeat(lambda: Image.alpha_composite(im1, im2), number=1)

out2 = Image.alpha_composite(im1, im2)
out2.save('out2.png')

diff.py

#!/usr/bin/env python
import sys
from PIL import Image, ImageMath

def chanel_diff(c1, c2):
    return ImageMath.eval('127 + c1 - c2', c1=c1, c2=c2).convert('L')

def highlight(c):
    return c.point(lambda c: c if 126 <= c <= 128 else 127 + (c - 127) * 10)

im1 = Image.open(sys.argv[1])
im2 = Image.open(sys.argv[2])

diff = map(chanel_diff, im1.split(), im2.split())

if len(sys.argv) >= 4:
    highlight(Image.merge('RGB', diff[:-1])).save('%s.png' % sys.argv[3])
    highlight(diff[-1]).convert('RGB').save('%s.alpha.png' % sys.argv[3])

def stats(ch):
    return sorted((c, n) for n, c in ch.getcolors())

for ch, stat in zip(['red ', 'grn ', 'blu ', 'alp '], map(stats, diff)):
    print ch, '  '.join('{}: {:>5}'.format(c, n) for c, n in stat)
print