numpy – Find pixel indices in a shape: Opencv and Python

Suppose I have a hollow, curved (and not necessarily convex) mask that I received from my pretreatment steps:

Hollow circle mask

I now want to try to select all the pixels that occur inside this shape and add them to the mask, as follows:

Filled circle mask

How can I do this in Python?


Code to generate the examples:

import cv2
import numpy as np
import matplotlib.pyplot as plt

# Parameters for creating the circle
COLOR_BLUE = (255, 0, 0)
IMAGE_SHAPE = (256, 256, 3)
CIRCLE_CENTER = tuple(np.array(IMAGE_SHAPE) // 2)(:-1)
CIRCLE_RADIUS = 30
LINE_THICKNESS = 5 # Change to -1 for example of filled circle

# Draw on a circle
img = np.zeros(IMAGE_SHAPE, dtype=np.uint8)
img_circle = cv2.circle(img, CIRCLE_CENTER, CIRCLE_RADIUS, COLOR_BLUE, LINE_THICKNESS)
circle_mask = img_circle(:, :, 0)

# Show the image
plt.axis("off")
plt.imshow(circle_mask)
plt.show()