alpha_blend_color.py

def alpha(color, background="#FFFFFF"):
    def split_rgba(color):
        #return tuple of integers
        #strip hash from the beginning if exist
        if color[0] == "#":
            color  = color[1:]
        if len(color) in [3,4]:
            R = color[0] * 2
            G = color[1] * 2
            B = color[2] * 2

            if len(color) == 4:
                A = color[3] * 2
            else:
                A = "00"

        if len(color) in [6,8]:
            R = color[ :2]
            G = color[2:4]
            B = color[4:6]
            if len(color) == 8:
                A = color[6: ]
            else:
                A = "00"

        return (int("0x" + x, 16) for x in (R,G,B,A))

    def int2hex(i):
        #convert to hex and cut trailing "0x"
        value = hex(int(i))[2:]
        if len(value) == 1:
            return "0" + value
        return value

    def alpha_blend(color, background, alpha ):
        #return calculated color
        new_color = background * (0xFF-alpha)/0xFF + color * alpha/0xFF
        
        return int2hex(new_color)

    R,G,B,A = split_rgba(color)
    r,g,b,a = split_rgba(background)
    R = alpha_blend(R,r,A)
    G = alpha_blend(G,g,A)
    B = alpha_blend(B,b,A)
    return f"#{R}{G}{B}"

import unittest

class TestAplhaBlend(unittest.TestCase):

    def test_total_transparent(self):
        self.assertEqual(alpha("#12345600"), "#ffffff")
        self.assertEqual(alpha("#11111100"), "#ffffff")
        self.assertEqual(alpha("#FFFFFF00"), "#ffffff")
    def test_total_transparent_with_background(self):
        self.assertEqual(alpha("#00000000", "#000"), "#000000")
        self.assertEqual(alpha("#00000000", "#fff"), "#ffffff")
        self.assertEqual(alpha("#00000000", "#555555"), "#555555")
        self.assertEqual(alpha("#55555500", "#ffff"), "#ffffff")
    def test_total_opaque(self):
        self.assertEqual(alpha("#000000FF"), "#000000")
        self.assertEqual(alpha("#FFFFFFFF"), "#ffffff")
        self.assertEqual(alpha("#123456FF"), "#123456")
    def test_total_opaque_with_background(self):
        self.assertEqual(alpha("#654321ff", "#000000"), "#654321")
        self.assertEqual(alpha("#654321ff", "#123456"), "#654321")
        self.assertEqual(alpha("#654321ff", "#ffffff"), "#654321")
        self.assertEqual(alpha("#ffffffff", "#000000"), "#ffffff")
        self.assertEqual(alpha("#000000ff", "#000000"), "#000000")

if __name__ == '__main__':
    unittest.main()