RGB color hex to lightness

color.ts

// based on https://stackoverflow.com/a/56678483/65387

type RGB = [r: number, g: number, b: number]

const UNK = 255 / 2

/**
 * @param hex RGB hex string like "#CCCFDB"
 * @returns RGB tuple in [0-255]
 */
function hexToRgb(hex: string): RGB {
    const h = String(hex).replace(/^#/, '')
    if(h.length === 3) {
        return [
            parseRadix(h[0] + h[0], 16, UNK),
            parseRadix(h[1] + h[1], 16, UNK),
            parseRadix(h[2] + h[2], 16, UNK),
        ]
    }
    if(h.length === 6) {
        return [
            parseRadix(h.slice(0, 2), 16, UNK),
            parseRadix(h.slice(2, 4), 16, UNK),
            parseRadix(h.slice(4, 6), 16, UNK),
        ]
    }
    return [UNK, UNK, UNK]
}

function sRGBtoLin(colorChannel: number) {
    // Send this function a decimal sRGB gamma encoded color value
    // between 0.0 and 1.0, and it returns a linearized value.

    if(colorChannel <= 0.04045) {
        return colorChannel / 12.92
    }

    return Math.pow((colorChannel + 0.055) / 1.055, 2.4)
}

/**
 * @param r Red, [0-1]
 * @param g Green, [0-1]
 * @param b Blue, [0-1]
 * @returns Luminance, [0-1]
 */
function rgbToY(r: number, g: number, b: number) {
    return 0.2126 * sRGBtoLin(r) + 0.7152 * sRGBtoLin(g) + 0.0722 * sRGBtoLin(b)
}

/**
 * Luminance to perceived lightness.
 *
 * @param Y Luminance, [0-1]
 */
function YtoLstar(Y: number) {
    // Send this function a luminance value between 0.0 and 1.0,
    // and it returns L* which is "perceptual lightness"

    if(Y <= (216 / 24389)) {  // The CIE standard states 0.008856 but 216/24389 is the intent for 0.008856451679036
        return Y * (24389 / 27)  // The CIE standard states 903.3, but 24389/27 is the intent, making 903.296296296296296
    }

    return Math.pow(Y, (1 / 3)) * 116 - 16
}

/**
 * Calculate perceived lightness from RGB hex string.
 *
 * @param rgb RGB hex string like "#CCCFDB"
 * @returns Lightness value, [0-100].
 */
export function rgbHexToLightness(rgb: string) {
    const [r, g, b] = hexToRgb(rgb)
    return YtoLstar(rgbToY(r / 255, g / 255, b / 255))
}