color metric

cie94.py

# https://gist.github.com/fikr4n/368f2f2070e0f9a15fb4

from math import sqrt


def _square(x):
    return x * x


def rgb(x):
    """Convert #[AA]RRGGBB color in integer or string to (r,g,b) tuple

    Alpha (AA) component is simply ignored.

    rgb(0xff0000ff)
    >>> (0, 0, 255)
    rgb('#ff0000')
    >>> (255, 0, 0)
    """

    if isinstance(x, str) and x[0] == '#':
        x = int(x[1:], 16)
    return ((x >> 16) & 0xff, (x >> 8) & 0xff, (x) & 0xff)


def cie94(L1_a1_b1, L2_a2_b2):
    """Calculate color difference by using CIE94 formulae

    See http://en.wikipedia.org/wiki/Color_difference or
    http://www.brucelindbloom.com/index.html?Eqn_DeltaE_CIE94.html.

    cie94(rgb2lab((255, 255, 255)), rgb2lab((0, 0, 0)))
    >>> 58.0
    cie94(rgb2lab(rgb(0xff0000)), rgb2lab(rgb('#ff0000')))
    >>> 0.0
    """

    L1, a1, b1 = L1_a1_b1
    L2, a2, b2 = L2_a2_b2

    C1 = sqrt(_square(a1) + _square(b1))
    C2 = sqrt(_square(a2) + _square(b2))
    delta_L = L1 - L2
    delta_C = C1 - C2
    delta_a = a1 - a2
    delta_b = b1 - b2
    delta_H_square = _square(delta_a) + _square(delta_b) - _square(delta_C)
    return sqrt(_square(delta_L) + _square(delta_C) / _square(1.0 + 0.045 * C1) +
                delta_H_square / _square(1.0 + 0.015 * C1))


def rgb2lab(R_G_B):
    """Convert RGB colorspace to Lab

    Adapted from http://www.easyrgb.com/index.php?X=MATH.
    """

    R, G, B = R_G_B

    # Convert RGB to XYZ

    var_R = R / 255.0        # R from 0 to 255
    var_G = G / 255.0        # G from 0 to 255
    var_B = B / 255.0        # B from 0 to 255

    if var_R > 0.04045:
        var_R = ((var_R + 0.055) / 1.055) ** 2.4
    else:
        var_R = var_R / 12.92
    if var_G > 0.04045:
        var_G = ((var_G + 0.055) / 1.055) ** 2.4
    else:
        var_G = var_G / 12.92
    if var_B > 0.04045:
        var_B = ((var_B + 0.055) / 1.055) ** 2.4
    else:
        var_B = var_B / 12.92

    var_R = var_R * 100.0
    var_G = var_G * 100.0
    var_B = var_B * 100.0

    # Observer. = 2°, Illuminant = D65
    X = var_R * 0.4124 + var_G * 0.3576 + var_B * 0.1805
    Y = var_R * 0.2126 + var_G * 0.7152 + var_B * 0.0722
    Z = var_R * 0.0193 + var_G * 0.1192 + var_B * 0.9505

    # Convert XYZ to L*a*b*

    var_X = X / 95.047         # ref_X =  95.047   Observer= 2°, Illuminant= D65
    var_Y = Y / 100.000        # ref_Y = 100.000
    var_Z = Z / 108.883        # ref_Z = 108.883

    if var_X > 0.008856:
        var_X = var_X ** (1.0/3.0)
    else:
        var_X = (7.787 * var_X) + (16.0 / 116.0)
    if var_Y > 0.008856:
        var_Y = var_Y ** (1.0/3.0)
    else:
        var_Y = (7.787 * var_Y) + (16.0 / 116.0)
    if var_Z > 0.008856:
        var_Z = var_Z ** (1.0/3.0)
    else:
        var_Z = (7.787 * var_Z) + (16.0 / 116.0)

    CIE_L = (116.0 * var_Y) - 16.0
    CIE_a = 500.0 * (var_X - var_Y)
    CIE_b = 200.0 * (var_Y - var_Z)
    return (CIE_L, CIE_a, CIE_b)