Question

What is the python module to count number of ones in a binary image ?

to rephrase,

I have a matrix that has only ones and zeros, it’s of numpy array type and I want to know how many ones are there.

Answer 1

You can simply use sum:

>>> import numpy
>>> n = numpy.random.randint(0, 2, size=(3,3))
>>> n
array([[1, 0, 1],
       [0, 1, 1],
       [1, 1, 1]])
>>> n.sum()
7

Since bools have integer values of 0/1 for False/True, even if the array had elements that weren’t 0 or 1 you could use a variant of this trick:

>>> n = numpy.random.randint(0, 3, size=(3,3))
>>> n
array([[2, 2, 0],
       [0, 2, 0],
       [1, 1, 0]])
>>> n == 1
array([[False, False, False],
       [False, False, False],
       [ True,  True, False]], dtype=bool)
>>> (n == 1).sum()
2