Why PIL.ImageChops.difference and np.array difference have different results?

Question

Why PIL.ImageChops.difference and np.array absolute difference have different results? Pillow document says ImageChops.difference acts just like absolute difference(https://pillow.readthedocs.io/en/3.1.x/reference/ImageChops.html).

tamp_image = Image.open(tamp_file_path).convert("RGB")
orig_image = Image.open(orig_file_path).convert("RGB")
diff = ImageChops.difference(orig_image, tamp_image)
diff.show() #1
Image.fromarray(abs(np.array(tamp_image)-np.array(orig_image))).show() #2

results(top:#1, bottom:#2):

Interestingly, if I convert diff to np.array and then Image object again, it shows like #1.

Answer 1

I had a similar problem. The solution is quite simple, the converted numpy arrays have the datatype uint8. By subtracting large values, the bits will flip entirely, and you get some weird looking result.

So the solution is to convert the images to a datatype with an appropriate range, like int8.

img1 = np.array(tamp_image, dtype='int8')
img2 = np.array(orig_image, dtype='int8')
diff = np.abs(img1 - img2)
Image.fromarray(np.array(diff, dtype='uint8')).show()