code: function getVelocityData(ev, delta_time, delta_x, delta_y) {
      var cur = this.current
        , velocityEv = cur.lastVelocityEvent
        , velocity = cur.velocity;
  
      // calculate velocity every x ms
      if (velocityEv && ev.timeStamp - velocityEv.timeStamp > Hammer.UPDATE_VELOCITY_INTERVAL) {
        velocity = Utils.getVelocity(ev.timeStamp - velocityEv.timeStamp,
                                     ev.center.clientX - velocityEv.center.clientX,
                                    ev.center.clientY - velocityEv.center.clientY);
        cur.lastVelocityEvent = ev;
      }
      else if(!cur.velocity) {
        velocity = Utils.getVelocity(delta_time, delta_x, delta_y);
        cur.lastVelocityEvent = ev;
      }
  
      cur.velocity = velocity;
  
      ev.velocityX = velocity.x;
      ev.velocityY = velocity.y;
    }
docs: calculate velocity
  @param   {Object}  ev
  @param   {Number}  delta_time
  @param   {Number}  delta_x
  @param   {Number}  delta_y