translation(jp): merge contrast color calculation

src/components/page/theming/_utils/color.ts

@@ -133,6 +133,28 @@ const rgbToYIQ = ({ r, g, b }: RGB): number => {
   return (r * 299 + g * 587 + b * 114) / 1000;
 };
 
+const RED = 0.2126;
+const GREEN = 0.7152;
+const BLUE = 0.0722;
+const GAMMA = 2.4;
+
+const luminance = ({ r, g, b }: RGB) => {
+  const a = [r, g, b].map((v) => {
+    v /= 255;
+    return v <= 0.03928 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, GAMMA);
+  });
+  return a[0] * RED + a[1] * GREEN + a[2] * BLUE;
+};
+
+// Original source: https://stackoverflow.com/a/9733420
+const contrast = (rgb1: RGB, rgb2: RGB) => {
+  const lum1 = luminance(rgb1);
+  const lum2 = luminance(rgb2);
+  const brightest = Math.max(lum1, lum2);
+  const darkest = Math.min(lum1, lum2);
+  return (brightest + 0.05) / (darkest + 0.05);
+};
+
 export class Color {
   readonly hex: string;
   readonly hsl: HSL;
@@ -176,8 +198,11 @@ export class Color {
     return /(^#[0-9a-fA-F]+)/.test(value.trim());
   }
 
-  contrast(threshold = 128): Color {
-    return new Color(this.yiq >= threshold ? '#000' : '#fff');
+  contrast(): Color {
+    const blackContrastRatio = contrast(this.rgb, { r: 0, g: 0, b: 0 });
+    const whiteContrastRatio = contrast(this.rgb, { r: 255, g: 255, b: 255 });
+
+    return new Color(blackContrastRatio >= whiteContrastRatio ? '#000' : '#fff');
   }
 
   mix(from: string | RGB | HSL | Color, amount = 0.5): Color {