How to Mask an image using Numpy/OpenCV?

Question

I have an image I load with:

im = cv2.imread(filename)

I want to keep data that is in the center of the image. I created a circle as a mas

Answer 1

circle is just a 2D array with 1.0s and 0.0s. Numpy needs help to understand what you want to do with the third dimension of your im so you must give it an extra axis and then your line would work.

masked_data = im * circle[..., np.newaxis]

But note that the masking is simply setting the color to (0, 0, 0) for things outside the circle according to your code if the image lacks an alpha-channel.

However you have another potential problem: circle will be of the default data-type (which probably will be float64 or float32. That's not good for your image, so you should change the line where you create circle to

circle = np.zeros((height, width), dtype=im.dtype)