Calculate perimeter of numpy array

Count the number of edges in the interior and at the edges (assumes binary image):

n_interior = abs(diff(a, axis=0)).sum() + abs(diff(a, axis=1)).sum()
n_boundary = a[0,:].sum() + a[:,0].sum() + a[-1,:].sum() + a[:,-1].sum()
perimeter = n_interior + n_boundary

You can leave out n_boundary if the image is properly zero padded.

Do you mean, in the image, the total number of length-1 edges that separate blue-colored from red-colored tiles? In the picture above this number would be 28. In the example you give in code (which is slightly different, not having the 4 corners differ from the rest of the border tiles) it would be 20.

If that's what you want to compute, you can do something like:

numpy.sum(a[:,1:] != a[:,:-1]) + numpy.sum(a[1:,:] != a[:-1,:])