What is parameter 'bound'

[NickiXD]

Hi,
I was wondering for what the parameter 'bound' exactly is and why you store x and y of the flow separately?
The i3d I use needs flows in the array size for example (224, 224, 2), are the frames merged later?
Sounds like most of you have the same intention.
I would be very happy if someone could help me out of this gap :)

Thanks for help in advance