from dataclasses import dataclass

@dataclass
class Color:
    red: float
    green: float
    blue: float
    alpha: float

    def __mul__(self, o) -> "Color":
        return Color(self.red * o, self.green * o,
                     self.blue * o, self.alpha * o)

    def __truediv__(self, o) -> "Color":
        return Color(self.red / o, self.green / o,
                     self.blue / o, self.alpha / o)

    def __add__(self, o) -> "Color":
        return Color(self.red + o.red, self.green + o.green,
                     self.blue + o.blue, self.alpha + o.alpha)

def composite1(C_fg: Color, a_fg: float, C_bg: Color):
    return C_fg * a_fg + C_bg * (1.0 - a_fg)

def composite2(C_fg: Color, a_fg: float, C_bg: Color, a_bg: float = 1):
    a_final = a_fg + a_bg * (1.0 - a_fg)
    return (C_fg * a_fg + C_bg * a_bg * (1.0 - a_fg)) / a_final

C_bg = Color(0.0, 0.0, 0.0, 1.0)