docstr: Start a new track
docstr_tok: ["Start","a","new","track"]
code: def activate(self, kalman_filter, frame_id):
      """Start a new track"""
      self.kalman_filter = kalman_filter
      # update track id for the object class
      self.track_id = self.next_id(self.cls_id)
      self.mean, self.covariance = self.kalman_filter.initiate(
          self.tlwh_to_xyah(self._tlwh)
      )
  
      self.track_len = 0
      self.state = TrackState.Tracked  # set flag 'tracked'
  
      if frame_id == 1:  # to record the first frame's detection result
          self.is_activated = True
  
      self.frame_id = frame_id
      self.start_frame = frame_id
  
imports: import numpy
  from collections import defaultdict
  from collections import deque, OrderedDict
  from ..matching import jde_matching
label_desc: Use image node_id 7 for calling the STrack obj's underlying member method code with example usage: obj.activate(kalman_filter, frame_id) without return types