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pythonarraysopencvnumpysubtraction

Python array subtraction loops back to high number instead of giving negative value

Here is my code: I'm using numpy and opencv

q = np.array(image)
q = q.reshape(-1, q.shape[2])
r = np.subtract(q,p)
print r

Basically what happens is if the value in my q array is greater than p the subtraction loops back up to 256 and subtracts whats left from there. I'd rather get a value of 0 if the subtraction goes negative. Does anybody know a good way to do this?

Solution

  • You could change to int16 which supports negative integers and set neg values to 0, your values are wrapping because you have uint8's:

    arr1 = np.array([100, 200, 255],dtype=np.int16)
arr2 = np.array([180, 210, 100],dtype=np.int16)

sub_arr = np.subtract(arr1, arr2)
sub_arr[sub_arr < 0] = 0
print(sub_arr)
[  0   0 155]

    To change you array you can use array.astype(np.int16) to change from uint8 to np.int16 and use the same to change back again after subtracting.

    arr1 = np.array([100, 200, 255],dtype=np.uint8)
arr2 = np.array([180, 210, 100],dtype=np.uint8)
_arr2 = arr2.astype(np.int16)
sub_arr = np.subtract(arr1, _arr2)

sub_arr[sub_arr < 0] = 0
sub_arr = sub_arr.astype(np.uint8)
print(sub_arr)

    Or also use np.clip:

    arr1 = np.array([100, 200, 255],dtype=np.uint8)
arr2 = np.array([180, 210, 100],dtype=np.uint8)

sub_arr = np.subtract(arr1, arr2.astype(np.int16)).clip(0, 255).astype(np.uint8)
print(sub_arr)
[  0   0 155]