var tween = { linear: function (t, b, c, d) { return c * t / d + b; }, easeIn: function (t, b, c, d) { return c * ( t /= d ) * t + b; }, strongEaseIn: function (t, b, c, d) { return c * ( t /= d ) * t * t * t * t + b; }, strongEaseOut: function (t, b