From bf98e89a44f8696e8d8be809b6372428b9fb4079 Mon Sep 17 00:00:00 2001
From: kurkle <jukka.kurkela@gmail.com>
Date: Tue, 29 Mar 2022 17:32:01 +0300
Subject: [PATCH] Add interpolate method

---
 .editorconfig | 14 ++++++++++++++
 src/color.js  | 52 +++++++++++++++++++++++++++++++++------------------
 src/rgb.js    | 14 ++++++++++++++
 src/srgb.js   | 26 ++++++++++++++++++++++++++
 test/index.js |  8 ++++++++
 5 files changed, 96 insertions(+), 18 deletions(-)
 create mode 100644 .editorconfig
 create mode 100644 src/srgb.js

diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..063c99a
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,14 @@
+# https://editorconfig.org
+root = true
+
+[*]
+indent_style = space
+indent_size = 2
+end_of_line = lf
+charset = utf-8
+trim_trailing_whitespace = true
+insert_final_newline = true
+
+[*.html]
+indent_style = tab
+indent_size = 4
diff --git a/src/color.js b/src/color.js
index bb16799..cb7cedd 100644
--- a/src/color.js
+++ b/src/color.js
@@ -3,11 +3,12 @@
  * @module index
  */
 
+import {b2n, n2b, round} from './byte';
 import {hexParse, hexString} from './hex';
-import {rgbParse, rgbString} from './rgb';
-import {hueParse, hsl2rgb, rgb2hsl, rotate, hslString} from './hue';
+import {hsl2rgb, hslString, hueParse, rgb2hsl, rotate} from './hue';
 import {nameParse} from './names';
-import {b2n, n2b, round} from './byte';
+import {rgbMix, rgbParse, rgbString} from './rgb';
+import {interpolate} from './srgb';
 
 /**
  * @typedef {import('./index.js').RGBA} RGBA
@@ -151,29 +152,31 @@ export default class Color {
    * Mix another color to this color.
    * @param {Color} color - Color to mix in
    * @param {number} weight - 0..1
+   * @returns {Color}
    */
   mix(color, weight) {
-    const me = this;
     if (color) {
-      const c1 = me.rgb;
-      const c2 = color.rgb;
-      let w2; // using instead of undefined in the next line
-      const p = weight === w2 ? 0.5 : weight;
-      const w = 2 * p - 1;
-      const a = c1.a - c2.a;
-      const w1 = ((w * a === -1 ? w : (w + a) / (1 + w * a)) + 1) / 2.0;
-      w2 = 1 - w1;
-      c1.r = 0xFF & w1 * c1.r + w2 * c2.r + 0.5;
-      c1.g = 0xFF & w1 * c1.g + w2 * c2.g + 0.5;
-      c1.b = 0xFF & w1 * c1.b + w2 * c2.b + 0.5;
-      c1.a = p * c1.a + (1 - p) * c2.a;
-      me.rgb = c1;
+      rgbMix(this._rgb, color._rgb, weight);
+    }
+    return this;
+  }
+
+  /**
+   * Interpolate a color value between this and `color`
+   * @param {Color} color
+   * @param {number} t - 0..1
+   * @returns {Color}
+   */
+  interpolate(color, t) {
+    if (color) {
+      this._rgb = interpolate(this._rgb, color._rgb, t);
     }
-    return me;
+    return this;
   }
 
   /**
    * Clone
+   * @returns {Color}
    */
   clone() {
     return new Color(this.rgb);
@@ -182,6 +185,7 @@ export default class Color {
   /**
    * Set aplha
    * @param {number} a - the alpha [0..1]
+   * @returns {Color}
    */
   alpha(a) {
     this._rgb.a = n2b(a);
@@ -191,6 +195,7 @@ export default class Color {
   /**
    * Make clearer
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   clearer(ratio) {
     const rgb = this._rgb;
@@ -200,6 +205,7 @@ export default class Color {
 
   /**
    * Convert to grayscale
+   * @returns {Color}
    */
   greyscale() {
     const rgb = this._rgb;
@@ -212,6 +218,7 @@ export default class Color {
   /**
    * Opaquer
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   opaquer(ratio) {
     const rgb = this._rgb;
@@ -219,6 +226,10 @@ export default class Color {
     return this;
   }
 
+  /**
+   * Negates the rgb value
+   * @returns {Color}
+   */
   negate() {
     const v = this._rgb;
     v.r = 255 - v.r;
@@ -230,6 +241,7 @@ export default class Color {
   /**
    * Lighten
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   lighten(ratio) {
     modHSL(this._rgb, 2, ratio);
@@ -239,6 +251,7 @@ export default class Color {
   /**
    * Darken
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   darken(ratio) {
     modHSL(this._rgb, 2, -ratio);
@@ -248,6 +261,7 @@ export default class Color {
   /**
    * Saturate
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   saturate(ratio) {
     modHSL(this._rgb, 1, ratio);
@@ -257,6 +271,7 @@ export default class Color {
   /**
    * Desaturate
    * @param {number} ratio - ratio [0..1]
+   * @returns {Color}
    */
   desaturate(ratio) {
     modHSL(this._rgb, 1, -ratio);
@@ -266,6 +281,7 @@ export default class Color {
   /**
    * Rotate
    * @param {number} deg - degrees to rotate
+   * @returns {Color}
    */
   rotate(deg) {
     rotate(this._rgb, deg);
diff --git a/src/rgb.js b/src/rgb.js
index 3561740..d8f25e6 100644
--- a/src/rgb.js
+++ b/src/rgb.js
@@ -60,3 +60,17 @@ export function rgbString(v) {
       : `rgb(${v.r}, ${v.g}, ${v.b})`
   );
 }
+
+/**
+ * Mix rgb2 to rgb1 by percent ratio (in place).
+ * @param {RGBA} rgb1 - the color (also the return value)
+ * @param {RGBA} rgb2 - the mixed color
+ * @param {number} t - 0..1 (default 0.5)
+ */
+export function rgbMix(rgb1, rgb2, t = 0.5) {
+  t = 1 - t;
+  rgb1.r = 0xFF & rgb1.r + t * (rgb2.r - rgb1.r) + 0.5;
+  rgb1.g = 0xFF & rgb1.g + t * (rgb2.g - rgb1.g) + 0.5;
+  rgb1.b = 0xFF & rgb1.b + t * (rgb2.b - rgb1.b) + 0.5;
+  rgb1.a = rgb1.a + t * (rgb2.a - rgb1.a);
+}
diff --git a/src/srgb.js b/src/srgb.js
new file mode 100644
index 0000000..de50c18
--- /dev/null
+++ b/src/srgb.js
@@ -0,0 +1,26 @@
+import {b2n, n2b} from './byte';
+
+/**
+ * @typedef {import('./index.js').RGBA} RGBA
+ */
+
+const to = v => v <= 0.0031308 ? v * 12.92 : Math.pow(v, 1.0 / 2.4) * 1.055 - 0.055;
+const from = v => v <= 0.04045 ? v / 12.92 : Math.pow((v + 0.055) / 1.055, 2.4);
+
+/**
+ * @param {RGBA} rgb1 from color
+ * @param {RGBA} rgb2 to color
+ * @param {number} t 0..1
+ * @returns {RGBA} interpolaced
+ */
+export function interpolate(rgb1, rgb2, t) {
+  const r = from(b2n(rgb1.r));
+  const g = from(b2n(rgb1.g));
+  const b = from(b2n(rgb1.b));
+  return {
+    r: n2b(to(r + t * (from(b2n(rgb2.r)) - r))),
+    g: n2b(to(g + t * (from(b2n(rgb2.g)) - g))),
+    b: n2b(to(b + t * (from(b2n(rgb2.b)) - b))),
+    a: rgb1.a + t * (rgb2.a - rgb1.a)
+  };
+}
diff --git a/test/index.js b/test/index.js
index 46964ef..9d72264 100644
--- a/test/index.js
+++ b/test/index.js
@@ -92,3 +92,11 @@ assert.strictEqual(new Color('rgba(10, 10, 10, 0.8)').clone().rgbString(), 'rgba
 
 assert.strictEqual(new Color('invalid-color-value').lighten(0.1).hslString(), undefined);
 assert.strictEqual(new Color('invalid-color-value').mix(undefined, 0.5).rgbString(), undefined);
+
+assert.strictEqual(new Color('red').interpolate(new Color('green'), 0).rgbString(), 'rgb(255, 0, 0)');
+assert.strictEqual(new Color('red').interpolate(new Color('green'), 0.25).rgbString(), 'rgb(225, 65, 0)');
+assert.strictEqual(new Color('red').interpolate(new Color('green'), 0.5).rgbString(), 'rgb(188, 92, 0)');
+assert.strictEqual(new Color('red').interpolate(new Color('green'), 0.75).rgbString(), 'rgb(137, 112, 0)');
+assert.strictEqual(new Color('red').interpolate(new Color('green'), 1).rgbString(), 'rgb(0, 128, 0)');
+
+assert.strictEqual(new Color('#fefa').interpolate(new Color('#ced5'), 0.6).hexString(), '#E2EDEC77');