Comparing two images pixel-wise with PIL (Python Imaging Library)

Question

I need a function which compares two PIL images of the same size. Let's call them A and B. The result is supposed to be a new image of the same size. If a pixels is the same in both A and B it's supposed to be set to a fixed color (e.g. black), otherwise it's supposed to be set to the same color as B.

Is there a library for implementing this functionality without an expensive loop over all pixels?

Aya · Accepted Answer · 2013-05-23 18:31:48Z

20

Not sure about other libraries, but you can do this with PIL, with something like...

from PIL import Image, ImageChops

point_table = ([0] + ([255] * 255))

def black_or_b(a, b):
    diff = ImageChops.difference(a, b)
    diff = diff.convert('L')
    diff = diff.point(point_table)
    new = diff.convert('RGB')
    new.paste(b, mask=diff)
    return new

a = Image.open('a.png')
b = Image.open('b.png')
c = black_or_b(a, b)
c.save('c.png')

answered May 23, 2013 at 18:31

Aya

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2 Comments

Maarten Over a year ago

Thanks a lot! I was almost going to write that part myself in C.

john k Over a year ago

what does this do?

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