Render a black subwindow

[llqqff]

Now I want to change the vertical field of view by blacking out the pixels I don't want. I am using the following code:
import viz
viz.go()

viz.add('panorama.ive')

#Add a new window.
subWindow = viz.addWindow()
#Set the size and position of the window.
subWindow.setSize(1,.1845)
subWindow.setPosition(0,1)
subWindow.clearcolor(viz.BLACK)

But the subwindow still shows part of the scene of the virtual environment. I don't know why and how to render the subwindow black. Thanks.

Qiufeng