src/graphic/shape/BezierCurve.js
function (t) {
        var p = someVectorAt(this.shape, t, true);
        return vec2.normalize(p, p);
    }
function (t) {
        var p = someVectorAt(this.shape, t, true);
        return vec2.normalize(p, p);
    }
function ( t ) { var p = someVectorAt ( this . shape , t , true ) ; return vec2 . normalize ( p , p ) ; }
Get tangent at percent
@param  {number} t
@return {Array.<number>}
Get tangent at percent