What preprocessing steps to be followed before image comparison? 1
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For example I am trying to find the similarity between two images using skimage - SSIM. The code block will be as follows

from skimage.measure import compare_ssim as ssim
from skimage import io
from skimage.transform import resize

a = io.imread("http://ecx.images-amazon.com/images/I/51PV4Dd8wAL._AC_UL246_SR190,246_.jpg",as_grey=False,dtype="float64")
b = io.imread("http://ecx.images-amazon.com/images/I/914ZHE6JolL._UY500_.jpg",as_grey=False,dtype="float64")

a = resize(a,b.shape)
ssim(a, b,multichannel=True)


I got the similarity score as follows

0.21009350738786017


Although both the images are same where one images in slightly oriented , I am getting very really low scores for this comparisons. Am I missing any preprocessing steps here before I compute compare images. If so what are the things I should consider before comparing images.

Thanks In Advance !!
There is not any fault! This is one of pitfalls of SSIM. It is very sensitive to the geometrical degradations such as orientation!