Added SmootherStep interpolation function.

Author: hexus

src/math/interpolation/LinearInterpolation.js

@@ -11,7 +11,7 @@ var Linear = require('../Linear');
  *
  * @function Phaser.Math.Interpolation.Linear
  * @since 3.0.0
- * @see https://en.wikipedia.org/wiki/Linear_interpolation
+ * @see {@link https://en.wikipedia.org/wiki/Linear_interpolation}
  *
  * @param {number[]} v - The input array of values to interpolate between.
  * @param {!number} k - The percentage of interpolation, between 0 and 1.

src/math/interpolation/SmoothStepInterpolation.js

@@ -11,7 +11,7 @@ var SmoothStep = require('../SmoothStep');
  *
  * @function Phaser.Math.Interpolation.SmoothStep
  * @since 3.9.0
- * @see https://en.wikipedia.org/wiki/Smoothstep
+ * @see {@link https://en.wikipedia.org/wiki/Smoothstep}
  *
  * @param {number} x - The input value.
  * @param {number} min - The minimum value, also known as the 'left edge', assumed smaller than the 'right edge'.

src/math/interpolation/SmootherStepInterpolation.js

@@ -0,0 +1,29 @@
+/**
+ * @author       Richard Davey <rich@photonstorm.com>
+ * @copyright    2018 Photon Storm Ltd.
+ * @license      {@link https://github.com/photonstorm/phaser/blob/master/license.txt|MIT License}
+ */
+
+var SmootherStep = require('../SmootherStep');
+
+/**
+ * A Smoother Step interpolation method.
+ *
+ * @function Phaser.Math.Interpolation.SmootherStep
+ * @since 3.9.0
+ * @see {@link https://en.wikipedia.org/wiki/Smoothstep#Variations}
+ *
+ * @param {number} x - The input value.
+ * @param {number} min - The minimum value, also known as the 'left edge', assumed smaller than the 'right edge'.
+ * @param {number} max - The maximum value, also known as the 'right edge', assumed greater than the 'left edge'.
+ *
+ * @return {number} The interpolated value.
+ */
+var SmootherStepInterpolation = function (x, min, max)
+{
+    var t = SmootherStep(x, min, max);
+
+    return (max - min) * t + min;
+};
+
+module.exports = SmootherStepInterpolation;

src/math/interpolation/index.js

@@ -15,6 +15,7 @@ module.exports = {
     CubicBezier: require('./CubicBezierInterpolation'),
     Linear: require('./LinearInterpolation'),
     QuadraticBezier: require('./QuadraticBezierInterpolation'),
-    SmoothStep: require('./SmoothStepInterpolation')
+    SmoothStep: require('./SmoothStepInterpolation'),
+    SmootherStep: require('./SmootherStepInterpolation')
 
 };