[vtkusers] background color

[Gopalakrishnan, Gowri]

How can you set two different background color in the same renderwindow?. 
 ex: viewport (0.0,0.0,0.5,0.5) - back ground colour white
and for the rest of the screen viewport(0.5,0.5,1.0,1.0) some other color. 

Is it possible, I know you can use two different renderer and get this
effect, but it has the disadvatage
that when you are in one renderer it ignores the other.

Thanks
Gowri