feat(plugin): add getLuminance

src/plugin/getLuminance/index.ts

@@ -0,0 +1,32 @@
+import type { MyColorPlugin, MyColor, MyColorFn } from "../..";
+
+interface OhColorClass extends MyColor {
+  getLuminance: () => number;
+}
+
+interface OhColorFactory extends MyColorFn {
+  (...args: unknown[]): OhColorClass;
+}
+
+const f = (x: number) => {
+  const channel = x / 255;
+  return channel <= 0.040_45
+    ? channel / 12.92
+    : Math.pow(((channel + 0.055) / 1.055), 2.4);
+}
+
+/**
+ * Returns a number (float) representing the luminance of a color.
+ */
+export const getLuminance: MyColorPlugin<OhColorFactory> = (_, c, cf) => {
+  const proto = c.prototype as OhColorClass;
+  const getRGB = proto.rgba;
+  proto.getLuminance = function() {
+    const [r, g, b] = getRGB.bind(this)();
+    return 0.2126 * f(r) + 0.7152 * f(g) + 0.0722 * f(b);
+  }
+
+  return cf as OhColorFactory;
+}
+
+export default getLuminance

src/plugin/index.ts

@@ -1,3 +1,4 @@
 export { inputHex } from "./inputHex";
 export { outputHex } from "./outputHex";
 export { inputNamed } from './inputNamed';
+export { getLuminance } from './getLuminance';

test/plugin/get-luminance.test.ts

@@ -0,0 +1,11 @@
+import { expect, it, describe } from "vitest";
+import { mycolor } from "../../src";
+import { getLuminance } from "../../src/plugin"
+
+describe("mycolor plugin: getLuminance", () => {
+  it("should get luminance", () => {
+    const mycolor2 = mycolor.extend(getLuminance);
+
+    expect(mycolor2(255, 255, 0, 1).getLuminance()).toBe(0.9278);
+  });
+})