[vtkusers] Does VTK support render cropping?

[Marc Cotran]

Hello fellow vtkusers,

I have an application wherein a vtkVolume is displayed in the render 
window. This volume is large (200meg) and as such it takes a long time 
to render ( ~5 seconds). I have a little sphere that picks a point on/in 
the volume and moves to that point as the user moves the cursor.

The problem I am facing is that every time the cursor moves, a Render() 
is called on the render window to get the sphere to render in its new 
position. Since I have this huge volume in the render window, the 
Render() takes so long that the application is very uninteractive.

I have tried using LODs... This works, but the volume image quality is 
too low during interaction for my purposes.

I have considered somehow using the backing store combined with 
renderers in different layers of the render window... But this just 
seems like the *wrong* thing to do.

I am wondering if there is a way in VTK to specify a subregion of a 
vtkViewport (vtkRenderer) to render. I would like to, for instance, 
render only the region around the little sphere each time I move the cursor.

I know that cropping windows are fairly common in computer graphics. 
Does anyone know if VTK offers this type of functionality?

Any help/ideas would be greatly appreciated!

Thanks

Marc