mask.py

import numpy as np
import cv2

def mask_it(img, points):
    height, width, depth = img.shape
    mask = np.zeros( (height, width), np.uint8)
    cv2.fillPoly(mask, pts = np.int32([points]), color=(255,255,255))

    return cv2.bitwise_and(img,img, mask=mask)


#
# LOAD IMAGE
#
import urllib.request
req = urllib.request.urlopen('https://docs.opencv.org/2.4-beta/_static/opencv-logo-white.png')
arr = np.asarray(bytearray(req.read()), dtype=np.uint8)
img = cv2.imdecode(arr, -1) # 'Load it as it is'

#
# DRAW RECTANGLE
#
height, width, depth = img.shape
top = height / 2 - 150
left = width / 2 - 200
bottom = height / 2 - 50
right = width / 2 + 200

contours = np.array( [
    [left, top],
    [left, bottom],
    [right, bottom],
    [right, top] ] )
masked = mask_it(img, contours)

#
# DISPLAY
#
cv2.imshow("original", img)
cv2.imshow("masked", masked)

cv2.waitKey()