Subtract two histograms

Question

I am trying to find the residual left behind when you subtract pixel distribution of two different images(the images are in a 2D array format).

I am trying to do something like the below

import numpy as np
hist1, bins1 = np.histogram(img1, bins=100)
hist2, bins2 = np.histogram(img2, bins=100)
residual = hist1 - hist2

However, in my above method the problem is that both the images have different maximum and minimum and when you do hist1-hist2 the individual bin value of each element in hist1-hist2 is not the same.

I was wondering if there is an alternative elegant way of doing this.

Thanks.

Answer 1

You can explicitly define bins in np.histogram() call. If you set them to the same value for both calls, then your code would work.

If your values are say between 0 and 255, you could do following:

import numpy as np
hist1, bins1 = np.histogram(img1, bins=np.linspace(0, 255, 100))
hist2, bins2 = np.histogram(img2, bins=np.linspace(0, 255, 100))
residual = hist1 - hist2

This way you have 100 bins with the same boundaries and the simple difference now makes sense (the code is not tested but you get the idea).

Answer 2

import numpy as np
nbins = 100
#minimum value element wise from both arrays
min = np.minimum(img1, img2)
#maximum value element wise from both arrays
max = np.maximum(img1, img2)
#histogram is build with fixed min and max values
hist1, _ = numpy.histogram(img1,range=(min,max), bins=nbins)
hist2, _ = numpy.histogram(img2,range=(min,max), bins=nbins)

#makes sense to have only positive values 
diff = np.absolute(hist1 - hist2)