OpenCV error img is incompatible with cv::Mat-CodePudding

Just realized of a surprising thing when using the following code:

import cv2
import numpy as np

a = np.zeros((720, 1280, 2), dtype=np.uint8)
b = np.zeros((720, 1280), dtype=np.uint8)

cv2.circle(b, (100,100),3,1,-1)   # works
cv2.circle(a[..., 0], (100,100),3,1,-1)  # does not work

Calling exactly same function with exactly same arguments is not working. Is this related with how numpy deals with arrays internally?

CodePudding user response：

Try it:

cv2.circle(a[..., 0].astype(np.uint8), (100,100),3,1,-1)

CodePudding user response：

when translating between numpy arrays and OpenCV cv::Mat, OpenCV's bindings require the numpy array to be contiguous.

Numpy slicing commonly produces non-contiguous arrays because it merely adjusts strides and offsets, yet still refers to the entire original data.

The simplest way to "fix" this is to make a copy of the numpy array: a[..., 0].copy()

BUT!!! that's a copy, so changes to it won't be reflected in the original array. If you do the copy, you need to take care to copy the data back into your source array.

plane = a[..., 0].copy() # copy makes contiguous
cv2.circle(plane, (100,100), 3, 1, -1)
a[..., 0] = plane