Modify luminance calculation and extract related functions into single files (#24586)

Close #24508

Main changes:
As discussed in the issue

1. Change luminance calculation function to use [Relative
Luminance](https://www.w3.org/WAI/GL/wiki/Relative_luminance)
2. Move the luminance related functions into color.go/color.js
3. Add tests for both the files (Not sure if test cases are too many
now)

Before (tests included by `UseLightTextOnBackground` are labels started
with `##`):
https://try.gitea.io/HesterG/testrepo/labels

After:
<img width="1307" alt="Screen Shot 2023-05-08 at 13 37 55"
src="https://user-images.githubusercontent.com/17645053/236742562-fdfc3a4d-2fab-466b-9613-96f2bf96b4bc.png">
<img width="1289" alt="Screen Shot 2023-05-08 at 13 38 06"
src="https://user-images.githubusercontent.com/17645053/236742570-022db68e-cec0-43bb-888a-fc54f5332cc3.png">
<img width="1299" alt="Screen Shot 2023-05-08 at 13 38 20"
src="https://user-images.githubusercontent.com/17645053/236742572-9af1de45-fb7f-460b-828d-ba25fae20f51.png">

---------

Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

web_src/js/components/ContextPopup.vue

@@ -26,7 +26,7 @@
 <script>
 import $ from 'jquery';
 import {SvgIcon} from '../svg.js';
-import {useLightTextOnBackground} from '../utils.js';
+import {useLightTextOnBackground, hexToRGBColor} from '../utils/color.js';
 
 const {appSubUrl, i18n} = window.config;
 
@@ -77,7 +77,8 @@ export default {
     labels() {
       return this.issue.labels.map((label) => {
         let textColor;
-        if (useLightTextOnBackground(label.color)) {
+        const [r, g, b] = hexToRGBColor(label.color);
+        if (useLightTextOnBackground(r, g, b)) {
           textColor = '#eeeeee';
         } else {
           textColor = '#111111';

web_src/js/features/repo-projects.js

@@ -1,5 +1,5 @@
 import $ from 'jquery';
-import {useLightTextOnBackground} from '../utils.js';
+import {useLightTextOnBackground, hexToRGBColor} from '../utils/color.js';
 
 const {csrfToken} = window.config;
 
@@ -190,7 +190,8 @@ export function initRepoProject() {
 }
 
 function setLabelColor(label, color) {
-  if (useLightTextOnBackground(color)) {
+  const [r, g, b] = hexToRGBColor(color);
+  if (useLightTextOnBackground(r, g, b)) {
     label.removeClass('dark-label').addClass('light-label');
   } else {
     label.removeClass('light-label').addClass('dark-label');

web_src/js/utils.js

@@ -135,17 +135,3 @@ export function toAbsoluteUrl(url) {
   return `${window.location.origin}${url}`;
 }
 
-// determine if light or dark text color should be used on a given background color
-// NOTE: see models/issue_label.go for similar implementation
-export function useLightTextOnBackground(backgroundColor) {
-  if (backgroundColor[0] === '#') {
-    backgroundColor = backgroundColor.substring(1);
-  }
-  // Perceived brightness from: https://www.w3.org/TR/AERT/#color-contrast
-  // In the future WCAG 3 APCA may be a better solution.
-  const r = parseInt(backgroundColor.substring(0, 2), 16);
-  const g = parseInt(backgroundColor.substring(2, 4), 16);
-  const b = parseInt(backgroundColor.substring(4, 6), 16);
-  const brightness = (0.299 * r + 0.587 * g + 0.114 * b) / 255;
-  return brightness < 0.35;
-}

web_src/js/utils/color.js

@@ -0,0 +1,42 @@
+// Check similar implementation in modules/util/color.go and keep synchronization
+// Return R, G, B values defined in reletive luminance
+function getLuminanceRGB(channel) {
+  const sRGB = channel / 255;
+  return (sRGB <= 0.03928) ? sRGB / 12.92 : ((sRGB + 0.055) / 1.055) ** 2.4;
+}
+
+// Reference from: https://www.w3.org/WAI/GL/wiki/Relative_luminance
+function getLuminance(r, g, b) {
+  const R = getLuminanceRGB(r);
+  const G = getLuminanceRGB(g);
+  const B = getLuminanceRGB(b);
+  return 0.2126 * R + 0.7152 * G + 0.0722 * B;
+}
+
+// Get color as RGB values in 0..255 range from the hex color string (with or without #)
+export function hexToRGBColor(backgroundColorStr) {
+  let backgroundColor = backgroundColorStr;
+  if (backgroundColorStr[0] === '#') {
+    backgroundColor = backgroundColorStr.substring(1);
+  }
+  // only support transfer of rgb, rgba, rrggbb and rrggbbaa
+  // if not in these formats, use default values 0, 0, 0
+  if (![3, 4, 6, 8].includes(backgroundColor.length)) {
+    return [0, 0, 0];
+  }
+  if ([3, 4].includes(backgroundColor.length)) {
+    const [r, g, b] = backgroundColor;
+    backgroundColor = `${r}${r}${g}${g}${b}${b}`;
+  }
+  const r = parseInt(backgroundColor.substring(0, 2), 16);
+  const g = parseInt(backgroundColor.substring(2, 4), 16);
+  const b = parseInt(backgroundColor.substring(4, 6), 16);
+  return [r, g, b];
+}
+
+// Reference from: https://firsching.ch/github_labels.html
+// In the future WCAG 3 APCA may be a better solution.
+// Check if text should use light color based on RGB of background
+export function useLightTextOnBackground(r, g, b) {
+  return getLuminance(r, g, b) < 0.453;
+}

web_src/js/utils/color.test.js

@@ -0,0 +1,34 @@
+import {test, expect} from 'vitest';
+import {hexToRGBColor, useLightTextOnBackground} from './color.js';
+
+test('hexToRGBColor', () => {
+  expect(hexToRGBColor('2b8685')).toEqual([43, 134, 133]);
+  expect(hexToRGBColor('1e1')).toEqual([17, 238, 17]);
+  expect(hexToRGBColor('#1e1')).toEqual([17, 238, 17]);
+  expect(hexToRGBColor('1e16')).toEqual([17, 238, 17]);
+  expect(hexToRGBColor('3bb6b3')).toEqual([59, 182, 179]);
+  expect(hexToRGBColor('#3bb6b399')).toEqual([59, 182, 179]);
+  expect(hexToRGBColor('#0')).toEqual([0, 0, 0]);
+  expect(hexToRGBColor('#00000')).toEqual([0, 0, 0]);
+  expect(hexToRGBColor('#1234567')).toEqual([0, 0, 0]);
+});
+
+test('useLightTextOnBackground', () => {
+  expect(useLightTextOnBackground(215, 58, 74)).toBe(true);
+  expect(useLightTextOnBackground(0, 117, 202)).toBe(true);
+  expect(useLightTextOnBackground(207, 211, 215)).toBe(false);
+  expect(useLightTextOnBackground(162, 238, 239)).toBe(false);
+  expect(useLightTextOnBackground(112, 87, 255)).toBe(true);
+  expect(useLightTextOnBackground(0, 134, 114)).toBe(true);
+  expect(useLightTextOnBackground(228, 230, 105)).toBe(false);
+  expect(useLightTextOnBackground(216, 118, 227)).toBe(true);
+  expect(useLightTextOnBackground(255, 255, 255)).toBe(false);
+  expect(useLightTextOnBackground(43, 134, 133)).toBe(true);
+  expect(useLightTextOnBackground(43, 135, 134)).toBe(true);
+  expect(useLightTextOnBackground(44, 135, 134)).toBe(true);
+  expect(useLightTextOnBackground(59, 182, 179)).toBe(true);
+  expect(useLightTextOnBackground(124, 114, 104)).toBe(true);
+  expect(useLightTextOnBackground(126, 113, 108)).toBe(true);
+  expect(useLightTextOnBackground(129, 112, 109)).toBe(true);
+  expect(useLightTextOnBackground(128, 112, 112)).toBe(true);
+});