A simple drawing app made on OpenCV on Python3

opencv_drawing app.py

import numpy as np
import cv2

# Global Variables
canvas = np.ones([500,500,3],'uint8')*255
radius = 3
color = (0,255,0)
pressed = False

# click callback
def click(event,x,y,flags,param):
    global canvas,pressed
    if event == cv2.EVENT_LBUTTONDOWN:
       pressed = True
       cv2.circle(canvas,(x,y),radius,color,-1)
    if event == cv2.EVENT_MOUSEMOVE and pressed == True:
       cv2.circle(canvas,(x,y),radius,color,-1)
    if event == cv2.EVENT_LBUTTONUP:
       pressed = False

# Window initialization and callback assignmment
cv2.namedWindow("canvas")
cv2.setMouseCallback("canvas",click)

# Forever draw loop
while True:
    cv2.imshow("canvas",canvas)
    # key capture every 1 ms
    ch = cv2.waitKey(1)
    if ch & 0xFF == ord('q'):
        break
    # Change colors
    elif ch & 0xFF == ord('b'):
        color = (255,0,0)
    elif ch & 0xFF == ord('g'):
        color = (0,255,0)
    elif ch & 0xFF == ord('r'):
        color = (0,0,255)

cv2.destroyAllWindows()