Python OpenCV - Image overlay with transparency

Question

I am trying to make an image with transparency on top of another. Something like that:

I could not find any solution, so I decided to go by pixels and calculate the resulting color. This worked for me, but very slowly. I am new to OpenCV as well as Python.

This is my code, I came up with:

import numpy as np
import cv2

img1 = cv2.imread("img1.png", -1)
img2 = cv2.imread("img2.png", -1) # this one has transparency
h, w, depth = img2.shape

result = np.zeros((h, w, 3), np.uint8)

for i in range(h):
    for j in range(w):
        color1 = img1[i, j]
        color2 = img2[i, j]
        alpha = color2[3] / 255.0
        new_color = [ (1 - alpha) * color1[0] + alpha * color2[0],
                      (1 - alpha) * color1[1] + alpha * color2[1],
                      (1 - alpha) * color1[2] + alpha * color2[2] ]
        result[i, j] = new_color

cv2.imshow("result", result)
cv2.waitKey(0)
cv2.destroyAllWindows()

Is there any other way to do this? Faster Faster? Thank.

Answer 1

Answer:

import numpy as np
import cv2

from time import time

img1 = cv2.imread("./test_image/rgb.jpg", -1)
img2 = cv2.imread("./test_image/rgba.png", -1) # this one has transparency
h, w, c = img2.shape

img1 = cv2.resize(img1, (w, h), interpolation = cv2.INTER_CUBIC)
result = np.zeros((h, w, 3), np.uint8)

#slow
st = time()
for i in range(h):
for j in range(w):
        color1 = img1[i, j]
        color2 = img2[i, j]
        alpha = color2[3] / 255.0
        new_color = [ (1 - alpha) * color1[0] + alpha * color2[0],
                      (1 - alpha) * color1[1] + alpha * color2[1],
                      (1 - alpha) * color1[2] + alpha * color2[2] ]
        result[i, j] = new_color
end = time() - st
print(end)

#fast
st = time()
alpha = img2[:, :, 3] / 255.0
result[:, :, 0] = (1. - alpha) * img1[:, :, 0] + alpha * img2[:, :, 0]
result[:, :, 1] = (1. - alpha) * img1[:, :, 1] + alpha * img2[:, :, 1]
result[:, :, 2] = (1. - alpha) * img1[:, :, 2] + alpha * img2[:, :, 2]
end = time() - st
print(end)

cv2.imshow("result", result)
cv2.waitKey(0)
cv2.destroyAllWindows()