I have a sequence of images (video frames). I need to identify the objects as they move (and also some leave the region of interest or new ones appear in the region of interest). By using bwlabel the object to the left of the region of interest is always object 1, but i want to maintain the numbering of the objects as they appear in time (and move out of the region of interest). When one or more objects move out of the region of interest I still want the other objects to have the labeling as if that object was still there. In addition the objects change size throught time. For better clarity I'm attaching a sketch of what i want to achive.

Does anybody have any tips/sugesstions or knows about a function that does this (i was not able to find any)?