If you are using Java I'll assume you are suing the SmartDashboard, then you can make a extension which extends the WPICameraExtension, and then you can overide the process image method- using rawImage.drawPolygon(I think there a more methods you can use tlike drawLine or drawRectangle) and then return the new rawImage.
This way you will have whatever shape you pick on the camera Image and the video running behind it
