Hi,
1. How to detect a ROI in video continuously?
This is not directly possible in Vision Builder AI (VBAI). You will need LabVIEW and the Vision Development Module to create a VI which extrats the individual frames of your avi file.
In VBAI you can run this VI using the "Run LabVIEW VI" Step and get a single frame to inspect with every call of the VI.
2. Can we interface usb webcam to vision builder?
Yes. Since VBAI 2009 you can interface USB cameras through the DirectShow Interface.
3. How to detect circles in specified area in an video?
VBAI provides a Step called "Find Circular Edge" which does this.
4. Can we use mathscript in vision builder?
Again not directly, but you can call a LabVIEW VI which performs the MathScript.
Overall I would rather suggest to use LabVIEW and the Vision Development Module for task instead of VBAI.
Christian