[vtkusers] Region of interest

[Glenn Pierce]

Hi I wonder if anyone could offer some advice ?

I have a GTK app (http://homepage.ntlworld.com/glenn.pierce)
that integrates a vtkRenderWindow. I can currently display various
images and can perform
a couple of statistical functions on th images, such as generating a
histogram. 

However I need to be able to perform some statistical functions on
certain regions of interest.
I need to be able to select a region using something similar to a
freehand section such as scissor tool
in the Gimp and then generate statistics on just that region.

Can anyone suggest the best way to achieve this and at what level (ie
vtk, Gtk) or know of any examples ?

Thanks very much

Glenn