element-web/src/utils/colour.ts

/*
Copyright 2019 Michael Telatynski <7t3chguy@gmail.com>

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

export function textToHtmlRainbow(str: string): string {
    const frequency = (2 * Math.PI) / str.length;

    return Array.from(str)
        .map((c, i) => {
            if (c === " ") {
                return c;
            }
            const [a, b] = generateAB(i * frequency, 1);
            const [red, green, blue] = labToRGB(75, a, b);
            return (
                '<font color="#' +
                red.toString(16).padStart(2, "0") +
                green.toString(16).padStart(2, "0") +
                blue.toString(16).padStart(2, "0") +
                '">' +
                c +
                "</font>"
            );
        })
        .join("");
}

function generateAB(hue: number, chroma: number): [number, number] {
    const a = chroma * 127 * Math.cos(hue);
    const b = chroma * 127 * Math.sin(hue);

    return [a, b];
}

function labToRGB(l: number, a: number, b: number): [number, number, number] {
    // https://en.wikipedia.org/wiki/CIELAB_color_space#Reverse_transformation
    // https://en.wikipedia.org/wiki/SRGB#The_forward_transformation_(CIE_XYZ_to_sRGB)

    // Convert CIELAB to CIEXYZ (D65)
    let y = (l + 16) / 116;
    const x = adjustXYZ(y + a / 500) * 0.9505;
    const z = adjustXYZ(y - b / 200) * 1.089;

    y = adjustXYZ(y);

    // Linear transformation from CIEXYZ to RGB
    const red = 3.24096994 * x - 1.53738318 * y - 0.49861076 * z;
    const green = -0.96924364 * x + 1.8759675 * y + 0.04155506 * z;
    const blue = 0.05563008 * x - 0.20397696 * y + 1.05697151 * z;

    return [adjustRGB(red), adjustRGB(green), adjustRGB(blue)];
}

function adjustXYZ(v: number): number {
    if (v > 0.2069) {
        return Math.pow(v, 3);
    }
    return 0.1284 * v - 0.01771;
}

function gammaCorrection(v: number): number {
    // Non-linear transformation to sRGB
    if (v <= 0.0031308) {
        return 12.92 * v;
    }
    return 1.055 * Math.pow(v, 1 / 2.4) - 0.055;
}

function adjustRGB(v: number): number {
    const corrected = gammaCorrection(v);

    // Limits number between 0 and 1
    const limited = Math.min(Math.max(corrected, 0), 1);

    return Math.round(limited * 255);
}