Why deal with new (unconfirmed) tracklets by only using box IoU?

[Johnqczhang]

Towards-Realtime-MOT/tracker/multitracker.py

Lines 269 to 310 in 44907e1

	''' Step 3: Second association, with IOU'''
	detections = [detections[i] for i in u_detection]
	# detections is now a list of the unmatched detections
	r_tracked_stracks = [] # This is container for stracks which were tracked till the
	# previous frame but no detection was found for it in the current frame
	for i in u_track:
	if strack_pool[i].state == TrackState.Tracked:
	r_tracked_stracks.append(strack_pool[i])
	dists = matching.iou_distance(r_tracked_stracks, detections)
	matches, u_track, u_detection = matching.linear_assignment(dists, thresh=0.5)
	# matches is the list of detections which matched with corresponding tracks by IOU distance method
	for itracked, idet in matches:
	track = r_tracked_stracks[itracked]
	det = detections[idet]
	if track.state == TrackState.Tracked:
	track.update(det, self.frame_id)
	activated_starcks.append(track)
	else:
	track.re_activate(det, self.frame_id, new_id=False)
	refind_stracks.append(track)
	# Same process done for some unmatched detections, but now considering IOU_distance as measure
	for it in u_track:
	track = r_tracked_stracks[it]
	if not track.state == TrackState.Lost:
	track.mark_lost()
	lost_stracks.append(track)
	# If no detections are obtained for tracks (u_track), the tracks are added to lost_tracks list and are marked lost
	'''Deal with unconfirmed tracks, usually tracks with only one beginning frame'''
	detections = [detections[i] for i in u_detection]
	dists = matching.iou_distance(unconfirmed, detections)
	matches, u_unconfirmed, u_detection = matching.linear_assignment(dists, thresh=0.7)
	for itracked, idet in matches:
	unconfirmed[itracked].update(detections[idet], self.frame_id)
	activated_starcks.append(unconfirmed[itracked])
	# The tracks which are yet not matched
	for it in u_unconfirmed:
	track = unconfirmed[it]
	track.mark_removed()
	removed_stracks.append(track)

Why not use appearance affinity to associate unmatched detections in the current frame with unconfirmed tracklets which were created as new tracks in the last frame?

Why use different thresholds of box IoU in Step 3 and Step 4?