Method and system for detecting a change of ratio of occlusion of a tracked object

A computer implemented method of deciding to refrain from using re-identification in tracking an object in image frames of a video sequence, the method comprising:

• determining, for each of a plurality of image frames of the video sequence, a bounding box or a mask of the tracked object;
• calculating, for each pair of successive image frames, an intersection over union, IoU, of a first bounding box in a first image frame of the pair of successive image frames and a second bounding box in a second image frame of the pair of successive image frames or of a first mask in the first image frame of the pair of successive image frames and a second mask in the second image frame of the pair of successive image frames;
• calculating, for a further pair of successive image frames subsequent to the plurality of pairs of successive image frames in the plurality of image fames, a further intersection over union, IoU, of a first bounding box in a first image frame of the further pair of successive image frames and a second bounding box in a second image frame of the further pair of successive image frames or of a first mask in the first image frame of the further pair of successive image frames and a second mask in the second image frame of the further pair of successive image frames;
• and
• on condition that the further IoU differs from the calculated IoUs of the plurality of pairs of successive image frames by more than a threshold amount, deciding to refrain from using re-identification in tracking the object in the further pair of successive image frames.

A device comprising circuitry configure to execute:

• A determining function configured to determine, for each of a plurality of image frames of a video sequence, a bounding box or a mask of the tracked object;
• a first calculating function configured to calculate, for each pair of successive image frames, an intersection over union, IoU, of a first bounding box in a first image frame of the pair of successive image frames and a second bounding box in a second image frame of the pair of successive image frames or of a first mask in the first image frame of the pair of successive image frames and a second mask in the second image frame of the pair of successive image frames;
• a second calculating function configured to calculate, for a further pair of successive image frames subsequent to the plurality of pairs of successive image frames in the plurality of image fames, a further intersection over union, IoU, of a first bounding box in a first image frame of the further pair of successive image frames and a second bounding box in a second image frame of the further pair of successive image frames or of a first mask in the first image frame of the further pair of successive image frames and a second mask in the second image frame of the further pair of successive image frames;
• and
• a detecting function configured to, on condition that the further IoU differs from the calculated IoUs of the plurality of pairs of successive image frames by more than a threshold amount, detect that a ratio of occlusion of the tracked object has changed.