fix: preserve alpha when interpolating in oklch

.changeset/oklch-mix-alpha.md

@@ -0,0 +1,5 @@
+---
+'chroma-js': patch
+---
+
+fix: preserve alpha when interpolating in oklch

src/interpolator/_hsx.js

@@ -1,4 +1,5 @@
 import Color from '../Color.js';
+import { reverse3 } from '../utils/index.js';
 
 export default (col1, col2, f, m) => {
     let xyz0, xyz1;
@@ -19,8 +20,8 @@ export default (col1, col2, f, m) => {
         xyz0 = col1.hcl();
         xyz1 = col2.hcl();
     } else if (m === 'oklch') {
-        xyz0 = col1.oklch().reverse();
-        xyz1 = col2.oklch().reverse();
+        xyz0 = reverse3(col1.oklch());
+        xyz1 = reverse3(col2.oklch());
     }
 
     let hue0, hue1, sat0, sat1, lbv0, lbv1;

test/mix.test.js

@@ -100,4 +100,13 @@ describe('Some tests for chroma.color()', () => {
         expect(result.hex()).toBe('#34343480');
         expect(result.css()).toBe('rgb(52 52 52 / 0.5)');
     });
+
+    it('mix transparent color in oklch', () => {
+        // alpha on an input must not change the interpolated L, C and H,
+        // only the alpha channel. So the rgb part has to match the opaque mix.
+        const opaque = chroma.mix('#ff0000', '#0000ff', 0.5, 'oklch');
+        const transparent = chroma.mix('#ff000080', '#0000ff', 0.5, 'oklch');
+        expect(transparent.hex('rgb')).toBe(opaque.hex('rgb'));
+        expect(transparent.hex()).toBe('#ba00c2bf');
+    });
 });