Round ViewPort... or something like this

Without viewport you could do this...

Graphics 800,600,0
image:TImage=LoadImage("max.png")
radar:TImage=LoadImage("radar.png")
viewport:TImage=CreateImage(100,100)
While Not KeyHit(key_escape)
   Cls
   DrawImage image,0,0
   DrawImage radar,0,0
   DrawRect 20,20,10,10
   Flip
Wend

where viewport image has a 0,0,0 border with a 1,1,1 inner circle.
So many possibilities.