sv.Detections has exactly N of each member, except for tracker_id.

[LinasKo]

Search before asking

• I have searched the Supervision issues and found no similar bug report.

Bug

Found in: #1044, #1076.

This is a sub-issue of #943 and should be solved before starting that one.

sv.Detections has a list of numpy arrays as its members: xyxy, mask, confidence, class_id, trackier_id When N detections are made, all of these except tracker_id will have a shape (N, ...).

For example: When there are no tracks, update_with_detections will return detections, with detections.tracker_id set to [].

BUT, a few lines above, it only returns those detections where tracker_id was not -1. Many places in the codebase, e.g. the TraceAnnotator expect len(tracker_id) to match len(detections). There's many zips in examples, zipping with tracker_id.

So there's 2 paradigms:

1. Make the tracker ID length always match the length of other members by having a special value such as -1, already used in some places.
2. Make tracker_id different than other members of sv.Detections, keep the current implementation, but root out incorrect usage.

Are you willing to submit a PR?

• Yes I'd like to help by submitting a PR!

[LinasKo]

I'm heavily in favour of 1. The steps for this would be:

• Find, learn of all places where tracker_id is used.
• Modify the core of ByteTrack - if there is a detection but there's no tracker - instead of returning nothing, return -1. Make sure the length of tracker_id matches the length of detections being returned.
• Think a bit how this affects case where we deliberately hide uncertain trackers - this would be introduced as part of add minimum_consecutive_frames to bytetrack #1076.
• Propagate the changes in the core of ByteTrack, to the uses by various components - annotators, examples
• Some tests would be good to have.

This would be a draft PR until #1076 is merged. This PR new should remove the Detections.empty call from it.

[LinasKo]

Example: line_zone.py:trigger() checks if iterating over tracker_id array produces None

Related: Test case in #1206 checks it too.

[LinasKo]

Example: smoother.py:update_with_detections() checks if iterating over tracker_id array produces None

[LinasKo]

I was wrong about the -1 case - the only issues that exist are unnecessary None checks.

When the related PR is merged, this can be closed.