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HTML

              
                <h2>Adding bezier curve eases to GSAP</h2>
<div class='foo'>
  <div class='bar' id='easeNone'>linear</div>
  <div class='bar' id='easeIn'>ease-in</div>
  <div class='bar' id='easeInOut'>ease-in-out</div>
  <div class='bar' id='easeOut'>ease-out</div>
  <div class='bar' id='ease'>ease</div>
  <div class='bar' id='customEase'>(.2, .7, .8, .3)</div>
</div>
              
            
!

CSS

              
                .foo {
  margin: 20px;
  width: 390px;
  border-left: 1px solid #000;
  border-right: 1px solid #000;
}
.bar {
  position: relative;
  width: 90px;
  height: 90px;
  background-color: #91E600;
  border-radius: 50%;
  line-height: 90px;
  text-align: center;
  white-space: pre;
}
h2 {
  margin: 20px;
}
              
            
!

JS

              
                // (tweens at bottom)


/**
 * BezierEasing - use bezier curve for transition easing function
 * is based on Firefox's nsSMILKeySpline.cpp
 * Usage:
 * var spline = BezierEasing(0.25, 0.1, 0.25, 1.0)
 * spline(x) => returns the easing value | x must be in [0, 1] range
 *
 * bezier-easing 0.4.0
 * BSD License
 * Gaëtan Renaudeau
 * https://github.com/gre/bezier-easing
 */
(function (definition) {
  if (typeof exports === "object") {
    module.exports = definition();
  }
  else if (typeof window.define === 'function' && window.define.amd) {
    window.define([], definition);
  } else {
    window.BezierEasing = definition();
  }
}(function () {

  // These values are established by empiricism with tests (tradeoff: performance VS precision)
  var NEWTON_ITERATIONS = 4;
  var NEWTON_MIN_SLOPE = 0.001;
  var SUBDIVISION_PRECISION = 0.0000001;
  var SUBDIVISION_MAX_ITERATIONS = 10;

  var kSplineTableSize = 11;
  var kSampleStepSize = 1.0 / (kSplineTableSize - 1.0);

  var float32ArraySupported = typeof Float32Array === "function";

  function BezierEasing (mX1, mY1, mX2, mY2) {
    // Validate arguments
    if (arguments.length !== 4) {
      throw new Error("BezierEasing requires 4 arguments.");
    }
    for (var i=0; i<4; ++i) {
      if (typeof arguments[i] !== "number" || isNaN(arguments[i]) || !isFinite(arguments[i])) {
        throw new Error("BezierEasing arguments should be integers.");
      } 
    }
    if (mX1 < 0 || mX1 > 1 || mX2 < 0 || mX2 > 1) {
      throw new Error("BezierEasing x values must be in [0, 1] range.");
    }

    var mSampleValues = float32ArraySupported ? new Float32Array(kSplineTableSize) : new Array(kSplineTableSize);
   
    function A (aA1, aA2) { return 1.0 - 3.0 * aA2 + 3.0 * aA1; }
    function B (aA1, aA2) { return 3.0 * aA2 - 6.0 * aA1; }
    function C (aA1)      { return 3.0 * aA1; }
   
    // Returns x(t) given t, x1, and x2, or y(t) given t, y1, and y2.
    function calcBezier (aT, aA1, aA2) {
      return ((A(aA1, aA2)*aT + B(aA1, aA2))*aT + C(aA1))*aT;
    }
   
    // Returns dx/dt given t, x1, and x2, or dy/dt given t, y1, and y2.
    function getSlope (aT, aA1, aA2) {
      return 3.0 * A(aA1, aA2)*aT*aT + 2.0 * B(aA1, aA2) * aT + C(aA1);
    }

    function newtonRaphsonIterate (aX, aGuessT) {
      for (var i = 0; i < NEWTON_ITERATIONS; ++i) {
        var currentSlope = getSlope(aGuessT, mX1, mX2);
        if (currentSlope === 0.0) return aGuessT;
        var currentX = calcBezier(aGuessT, mX1, mX2) - aX;
        aGuessT -= currentX / currentSlope;
      }
      return aGuessT;
    }

    function calcSampleValues () {
      for (var i = 0; i < kSplineTableSize; ++i) {
        mSampleValues[i] = calcBezier(i * kSampleStepSize, mX1, mX2);
      }
    }

    function binarySubdivide (aX, aA, aB) {
      var currentX, currentT, i = 0;
      do {
        currentT = aA + (aB - aA) / 2.0;
        currentX = calcBezier(currentT, mX1, mX2) - aX;
        if (currentX > 0.0) {
          aB = currentT;
        } else {
          aA = currentT;
        }
      } while (Math.abs(currentX) > SUBDIVISION_PRECISION && ++i < SUBDIVISION_MAX_ITERATIONS);
      return currentT;
    }

    function getTForX (aX) {
      var intervalStart = 0.0;
      var currentSample = 1;
      var lastSample = kSplineTableSize - 1;

      for (; currentSample != lastSample && mSampleValues[currentSample] <= aX; ++currentSample) {
        intervalStart += kSampleStepSize;
      }
      --currentSample;

      // Interpolate to provide an initial guess for t
      var dist = (aX - mSampleValues[currentSample]) / (mSampleValues[currentSample+1] - mSampleValues[currentSample]);
      var guessForT = intervalStart + dist * kSampleStepSize;

      var initialSlope = getSlope(guessForT, mX1, mX2);
      if (initialSlope >= NEWTON_MIN_SLOPE) {
        return newtonRaphsonIterate(aX, guessForT);
      } else if (initialSlope == 0.0) {
        return guessForT;
      } else {
        return binarySubdivide(aX, intervalStart, intervalStart + kSampleStepSize);
      }
    }

    if (mX1 != mY1 || mX2 != mY2)
      calcSampleValues();

    var f = function (aX) {
      if (mX1 === mY1 && mX2 === mY2) return aX; // linear
      // Because JavaScript number are imprecise, we should guarantee the extremes are right.
      if (aX === 0) return 0;
      if (aX === 1) return 1;
      return calcBezier(getTForX(aX), mY1, mY2);
    };
    var str = "BezierEasing("+[mX1, mY1, mX2, mY2]+")";
    f.toString = function () { return str; };

    return f;
  }

  // CSS mapping
  BezierEasing.css = {
    "ease":        BezierEasing(0.25, 0.1, 0.25, 1.0),
    "linear":      BezierEasing(0.00, 0.0, 1.00, 1.0),
    "ease-in":     BezierEasing(0.42, 0.0, 1.00, 1.0),
    "ease-out":    BezierEasing(0.00, 0.0, 0.58, 1.0),
    "ease-in-out": BezierEasing(0.42, 0.0, 0.58, 1.0)
  };

  return BezierEasing;

}));





// create Bezier object to match GSAP syntax
BezierCurve = {
  ease:      new Ease(BezierEasing.css["ease"]),
  easeNone:  new Ease(BezierEasing.css["linear"]),
  easeIn:    new Ease(BezierEasing.css["ease-in"]),
  easeOut:   new Ease(BezierEasing.css["ease-out"]),
  easeInOut: new Ease(BezierEasing.css["ease-in-out"])
};

var tl = new TimelineMax({ onComplete: function() {
  // rewind
  TweenLite.to(this.pause(), 1, { time: 0, onComplete: this.resume, onCompleteScope: this });
} });

tl.to("#easeNone", 4, {
      x:300, force3D:true,
      ease:BezierCurve.easeNone
    }, 1)
  .to("#easeIn", 4, {
      x:300, force3D:true,
      ease:BezierCurve.easeIn
    }, 1)
  .to("#easeInOut", 4, {
      x:300, force3D:true,
      ease:BezierCurve.easeInOut
    }, 1)
  .to("#easeOut", 4, {
      x:300, force3D:true,
      ease:BezierCurve.easeOut
    }, 1)
  .to("#ease", 4, {
      x:300, force3D:true,
      ease:BezierCurve.ease
    }, 1)
  .to("#customEase", 4, {
      x:300, force3D:true,
      ease:new Ease(BezierEasing(0.2, 0.7, 0.8, 0.3))
    }, 1)
  .set({}, {}, "+=1"); // just 1 second padding before rewind
              
            
!
999px

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