将透明背景变为白色背景pythoncv2

import cv2
def alpha2white_opencv2(img):
    sp=img.shape
    width=sp[0]
    height=sp[1]
    for yh in range(height):
        for xw in range(width):
            color_d=img[xw,yh]
            if(color_d[3]==0):
                img[xw,yh]=[255,255,255,255]
    return img