/* 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 ( '' + c + "" ); }) .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); }