colorblend.py

#from https://stackoverflow.com/questions/726549/algorithm-for-additive-color-mixing-for-rgb-values/726578

rgb_scale = 255
cmyk_scale = 100

def rgb_to_cmyk(r, g, b):
    if (r == 0) and (g == 0) and (b == 0):
        # black
        return 0, 0, 0, cmyk_scale

    # rgb [0,255] -> cmy [0,1]
    c = 1 - r / float(rgb_scale)
    m = 1 - g / float(rgb_scale)
    y = 1 - b / float(rgb_scale)

    # extract out k [0,1]
    min_cmy = min(c, m, y)
    c = (c - min_cmy)
    m = (m - min_cmy)
    y = (y - min_cmy)
    k = min_cmy

    # rescale to the range [0,cmyk_scale]
    return c * cmyk_scale, m * cmyk_scale, y * cmyk_scale, k * cmyk_scale

def cmyk_to_rgb(c, m, y, k):
    """
    """
    r = rgb_scale*(1.0 - (c + k) / float(cmyk_scale))
    g = rgb_scale*(1.0 - (m + k) / float(cmyk_scale))
    b = rgb_scale*(1.0 - (y + k) / float(cmyk_scale))
    return int(r), int(g), int(b)

def ink_add_for_rgb(list_of_colors):
    """input: list of rgb, opacity (r,g,b,o) colors to be added, o acts as weights.
    output (r,g,b)
    """
    C = 0
    M = 0
    Y = 0
    K = 0

    for (r, g, b, o) in list_of_colors:
        c, m, y, k = rgb_to_cmyk(r, g, b)
        C += o * c
        M += o * m
        Y += o * y
        K += o * k

    return cmyk_to_rgb(C, M, Y, K)