# 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.
#-------------- ----------------- ---------------- ----------------- ----------
# This code uses NumPy to change the red-green-blue channels of the image.

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)

# Get height and width of the image, define centre and radius of the disk
h = img.shape[0]  #Note shape dimension is in [h, w] format
w = img.shape[1]
xc = h//2
yc = w//2
rad = min(xc, yc)

# Get colour channels, RGB of the image. Note OpenCV stores image in BGR format
b_chan = img[:, :, 0]
g_chan = img[:, :, 1]
r_chan = img[:, :, 2]

# Create column vectors to the size of height and width of the image
x = np.arange(0, h)              #Rank '1' array
y = np.arange(0, w)              #Rank '1' array
u = np.array(x).reshape(-1, 1)   #x[np.newaxis, :] - make column vector
v = np.array(y).reshape(1, -1)   #y[:, np.newaxis] - make column vector

# Identifypixels outside the disk, ** operator works on each element of vector
mask = ((u[:]-xc)**2 + (v[:]-yc)**2) > rad**2

# Set the pixels of all 3 channels outside disk to white (255, 255, 255)
b_chan[mask] = 255
g_chan[mask] = 255
r_chan[mask] = 255

# Merge the RGB channels into a desired image
result = cv2.merge([b_chan, g_chan, r_chan])

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