Convert an RGBA color over a background RGB color to RGB

rgba_on.py

import sys

if __name__ == '__main__':
    r = int(sys.argv[1])
    g = int(sys.argv[2])
    b = int(sys.argv[3])
    a = float(sys.argv[4])

    if len(sys.argv) > 5:
        bg_rgb = sys.argv[5]
    else:
        bg_rgb = 'ffffff'

    bg_r = int(bg_rgb[0:2], 16)
    bg_g = int(bg_rgb[2:4], 16)
    bg_b = int(bg_rgb[4:6], 16)

    red = int(r*a + bg_r*(1-a))
    green = int(g*a + bg_g*(1-a))
    blue = int(b*a + bg_b*(1-a))

    print ("rgba(%s, %s, %s, %s) on #%s --> rgb(%s, %s, %s) or #%x%x%x " % (
        r, g, b, a, bg_rgb, red, green, blue, red, green, blue))

$ python ~/rgba_on.py 0 136 255 0.2 ffffff
rgba(0, 136, 255, 0.2) on #ffffff --> rgb(204, 231, 255) or #cce7ff