Windows 窗口截图带透明通道

import cv2 as cv
import numpy as np


def main():
    bb = cv.imread('b.png')
    wb = cv.imread('w.png')

    assert bb.shape == wb.shape

    tb = np.zeros((bb.shape[0], bb.shape[1], 4), dtype=np.uint8)

    for y in range(bb.shape[0]):
        for x in range(bb.shape[1]):
            b1, g1, r1 = bb[y, x]
            b2, g2, r2 = wb[y, x]
            b, ab = getColoeAlpha(b1, b2)
            g, ag = getColoeAlpha(g1, g2)
            r, ar = getColoeAlpha(r1, r2)
            a = int((ab + ag + ar) / 3)
            if a > 255:
                a = 255
            tb[y, x] = [b, g, r, a]

    cv.imshow('t.png', tb)
    cv.imwrite('t.png', tb)
    cv.waitKey(0)


def getColoeAlpha(cb, cw) -> (int, int):
    """
    
    :param cb: color with black background
    :param cw: color with white background
    :return: (c1, alpha)
    """
    alpha = 255 + cb - cw
    if alpha == 255:
        return cb, 255
    c1 = cb / ((255 + cb - cw) / 255)
    if c1 != c1:  # nan
        c1 = 0
    return int(c1), alpha


if __name__ == '__main__':
    main()