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to achieve more similar / better results, you need to:

1: flip the kernel

instead of:

1  2  1
0  0  0
-1 -2 -3

use:

-1 -2 -1
0  0  0
1  2  3

2: choose a different output depth

assuming your input img is uchar, and the result is the same (-1), negative values will get saturated to 0.

(your numpy code silently converts to float64)

all in all that would make it:

filtered = cv2.filter2D(img, cv2.CV_64F, flipped_kernel)

p.s: like this, the anchor point is still at the kernel center, while your code has it top-left, but imho this is the correct version