How to change the opacity of boxes (cv2.rectangle)?

Question

I draw some rectangles in OpenCV and put text in them. My general approach looks like this:

# Draw rectangle    p1(x,y)    p2(x,y)    Student name box
cv2.rectangle(frame, (500, 650), (800, 700), (42, 219, 151), cv2.FILLED )
font = cv2.FONT_HERSHEY_DUPLEX
cv2.putText(frame, name, (510, 685), font, 1.0, (255, 255, 255), 1

Everything works so far. The only thing is, that the opacity in all boxes is at 100 %. My question is: How can I change the opacity?

The final result should look like this:

Answer 1

I would like to add a small optimization to the @HansHirse answer, Instead of creating the canvas for whole image, we can crop the rectangle first from the src image and then later swap it with the cv2.addWeighted result as:

import cv2
import numpy as np

img = cv2.imread("lena.png")

# First we crop the sub-rect from the image
x, y, w, h = 100, 100, 200, 100
sub_img = img[y:y+h, x:x+w]
white_rect = np.ones(sub_img.shape, dtype=np.uint8) * 255

res = cv2.addWeighted(sub_img, 0.5, white_rect, 0.5, 1.0)

# Putting the image back to its position
img[y:y+h, x:x+w] = res