pthom

import numpy as np
import cv2

def overlay_alpha_image_lazy(background_rgb, overlay_rgba, alpha):
    # cf https://en.wikipedia.org/wiki/Alpha_compositing#Alpha_blending
    # If the destination background is opaque, then
    #   out_rgb = overlay_rgb * overlay_alpha + background_rgb * (1 - overlay_alpha)
    overlay_alpha = overlay_rgba[: , : , 3].astype(np.float) / 255. * alpha
    overlay_alpha_3 = np.dstack((overlay_alpha, overlay_alpha, overlay_alpha))
    overlay_rgb = overlay_rgba[: , : , : 3].astype(np.float)