# This Python code uses OpenCV to create a circular disk at the centre of an
# image. The radius of the disk is the minimum of width and height of image.
#-------------- ----------------- ---------------- ----------------- ----------
# References: 
# stackoverflow.com/.../how-to-use-opencv-to-crop-circular-image
# stackoverflow.com/.../what-does-bitwise-and-operator-exactly-do-in-opencv
import numpy as np
import cv2

image = 'image.png'
out = 'maskedImg.png'

# Read image as coloured channels: w x h x 3 uint8
img = cv2.imread(image)

# Create white but coloured image that is with 3 channels: w x h x 3 uint8
mask_w = np.full_like(img, fill_value=(255,255,255))

# Get height and width of the image, define centre and radius of the disk
h = img.shape[0]
w = img.shape[1]
centre = (w//2, h//2)  #Note 'Centre' is ordered in image coordinate system
radius = min(centre)

# Create black but coloured image that is with 3 channels: w x h x 3 uint8
mask_b = np.zeros_like (img[:,:], dtype='uint8')

# Create a white disk at the centre of the black image created in previous step
img_x = cv2.circle(mask_b, centre, radius, [255]*3, thickness=cv2.FILLED)
cv2.imwrite("maskIntermediate1.png", img_x)

# Create white image with single channels: w x h x 0 uint8. This is the mask,
# note that mask has to be 8-bit single channel array. Image created next is
# same as previous image (img_x) except it has single channel.
mask_b_1_chan = img_x[:,:,0]

# Extract pixels disk using white disk single channel. This will result in a
# black image with cropped disk at the centre. This is the desired image but 
# with black background beyond circular disk at the centre
masked = cv2.bitwise_and(img, mask_w, mask=mask_b_1_chan)
cv2.imwrite("maskIntermediate2.png", masked)

# Invert the black mask to white mask: image with white background and black
# disk at the centre
mask_b_inv = ~mask_b

# Add white background to the original image. Note that with bitwise_or:
# white + anycolour = white, black + anycolour = anycolour
result = cv2.bitwise_or(img, mask_b_inv)

# Save the output image
cv2.imwrite(out, result)