# numpy - Implementing gradient operator in Python

I'm working on a Computer Vision system and this is giving me a serious headache. I'm having trouble re-implementing an old gradient operator more efficiently, I'm working with numpy and openCV2.

This is what I had:

``def gradientX(img):rows, cols = img.shapeout = np.zeros((rows,cols))for y in range(rows-1):Mr = img[y]Or = out[y]Or[0] = Mr[1] - Mr[0]for x in xrange(1, cols - 2):Or[x] = (Mr[x+1] - Mr[x-1])/2.0Or[cols-1] = Mr[cols-1] - Mr[cols-2]return outdef gradient(img):return [gradientX(img), (gradientX(img.T).T)]``

I've tried using numpy's gradient operator but the result is not the same

For this input

``array([[ 3, 4, 5],[255, 0, 12],[ 25, 15, 200]])``

Using my `gradient` returns

``[array([[ 1., 0., 1.],[-255., 0., 12.],[ 0., 0., 0.]]),array([[ 252., -4., 0.],[ 0., 0., 0.],[-230., 15., 0.]])]``

While using numpy's `np.gradient` returns

``[array([[ 252. , -4. , 7. ],[ 11. , 5.5, 97.5],[-230. , 15. , 188. ]]),array([[ 1. , 1. , 1. ],[-255. , -121.5, 12. ],[ -10. , 87.5, 185. ]])]``

There are cleary some similarities between the results but they're definitely not the same. So I'm missing something here or the two operators aren't mean to produce the same results. In that case, I wanted to know how to re-implement my gradientX function so it doesn't use that awful looking double loop for traversing the 2-d array using mostly numpy's potency.

I've been working a bit more on this just to find that my mistake. I was skipping last row and last column when iterating. As @wflynny noted, the result was identical except for a row and a column of zeros.

Provided this, the result could not be the same as np.gradient, but with that change, the results are identical, so there's no need to find any other numpy implementation for this.

Answering my own question, a good numpy's implementation for my gradient algorithm would be

``````import numpy as np
def gradientX(img):
return np.gradient(img)[::-1]
``````

I'm also posting the working code, just because it shows how numpy's gradient operator works

``````def computeMatXGradient(img):
rows, cols = img.shape
out = np.zeros((rows,cols))
for y in range(rows):
Mr = img[y]
Or = out[y]
Or[0] = float(Mr[1]) - float(Mr[0])
for x in xrange(1, cols - 1):
Or[x] = (float(Mr[x+1]) - float(Mr[x-1]))/2.0
Or[cols-1] = float(Mr[cols-1]) - float(Mr[cols-2])
return out
``````