Hi Alain,
I've used the OpenCV blobtracker with Pd before, and I simply send blob positions and sizes over UDP using u_pdsend.c
I've attached my code and makefile... look in blobtrack.c and search for where I use the pdsend_init() and pdsend_message() functions... it's pretty simple.
Then you just use [netreceive] in Pd and parse the list as you deem necessary.
Hope this helps, -Mike
P.S. my blobtracker code is meant for use with a live camera, not with .avi files.