Difference in adding two images in numpy and opencv

Question

when i add two images in opencv, sum is limited to 255. (Both images are of uint8)

ie 175+100 is 255 in opencv.

but if we add it using numpy.add function, result is not a limited one.

ie 175+100 is 19 in numpy.

Question:

1) Why it happen so?

2) Is there a way for np.add to behave like cv2.add? ie limit sum to 255?

Thanks in advance.

Travis Oliphant · Accepted Answer · 2012-03-11 06:58:21Z

3

NumPy uses "modulo" arithmetic on overflow rather than clipping. This is the behavior of add on uint8 integers in C. So, 175+100 % 256 = 19 which is the result you are getting.

To get this clipping behavior, you will need to do some work:

Here are a couple of ideas:

1) Use a higher precision:

im1 = im.astype('u2')
im2 = im.astype('u2')
tmp = im1 + im2
result = tmp.clip(0,255).astype('u1')

2) Make sure the result is >= both the input values:

tmp = im1 + im2
mask = (tmp < im1) | (tmp < im2)
tmp[mask] = 255

answered Mar 11, 2012 at 6:58

Travis Oliphant

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1 Comment

Abid Rahman K Over a year ago

2nd method is a lot slower than first.

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