import cv2
import numpy as np
 
img = cv2.imread("/home/shuai/Desktop/lena.jpg")
 
b_channel, g_channel, r_channel = cv2.split(img)
 
alpha_channel = np.ones(b_channel.shape, dtype=b_channel.dtype) * 255
# 最小值为0
alpha_channel[:, :int(b_channel.shape[0] / 2)] = 100
 
img_BGRA = cv2.merge((b_channel, g_channel, r_channel, alpha_channel))
 
cv2.imwrite("lena.png", img_BGRA)

原图

透明图 （100）

透明图（0）