gistfile1.js

window.KeySpline = function(mX1, mY1, mX2, mY2) {

    this.get = function(aX) {
        if (mX1 == mY1 && mX2 == mY2) return aX; // linear
        return CalcBezier(GetTForX(aX), mY1, mY2);
    };

    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 GetTForX(aX) {
        // Newton raphson iteration
        var aGuessT = aX;
        for (var i = 0; i < 4; ++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;
    }
};

window.KEY_SPLINES = {
    'EASE' : new window.KeySpline(0.25, 0.1, 0.25, 1.0),
    'LINEAR': new window.KeySpline(0, 0, 1, 1),
    'EASE_IN': new window.KeySpline(0.42, 0.0, 1.00, 1.0),
    'EASE_OUT': new window.KeySpline(0.00, 0.0, 0.58, 1.0),
    'EASE_IN_OUT': new window.KeySpline(0.42, 0.0, 0.58, 1.0),
    'JUMPY': new window.KeySpline(0.31, 1.51, 0.53, 0.94),
    'EASE_OUT_STRONG': new window.KeySpline(0.00, 0.5, 0.58, 1.0)
};