I created a convex hull of a hand from an X-ray. I did this by first creating contours along the edges of the hand. I am wanting to blacken the outside region of convex hull using OpenCV in Python. How do I approach this?

The code below creates the convex hull after doing contours:

img_path = 'sample_image.png'

# Getting the threshold of the image:
image = cv2.imread(img_path)
original = image.copy() 
blank = np.zeros(image.shape[:2], dtype = np.uint8)
gray = cv2.cvtColor(image, cv2.COLOR_BGR2GRAY)
blur = cv2.GaussianBlur(gray, (5,5), 0)
thresh = cv2.threshold(blur, 140, 255, cv2.THRESH_BINARY   cv2.THRESH_OTSU)[1]

# Drawing the contours along the edges of the hand in X-ray:
contours, hierarchy = cv2.findContours(thresh, cv2.RETR_TREE, 
cv2.CHAIN_APPROX_SIMPLE)
contours = max(contours, key = lambda x: cv2.contourArea(x))
cv2.drawContours(image, [contours], -1, (255,255,0), 2)

# Drawing the hull along the digits along the edges: 
hull = cv2.convexHull(contours)
cv2.drawContours(image, [hull], -1, (0, 255, 255), 2)

Input image:

result

(the line below the dog remains since it's inside the hull created by the dog's feet)

Cropping

To crop the image to only include the hull's area, use cv2.boundingRect and slice the image array:

x, y, w, h = cv2.boundingRect(hull)
cropped = image[y:y   h, x:x   w]
cv2.imwrite(r'dog_crop.jpg', cropped)

result

Cropping

Result