[vtkusers] pick point in 3D volume

[Peter Spring]

Dear all,
     i have a question regarding point picking in a 3D volume.

here what i want to do is... i have a set of CT data in DICOM format.
the data is read into program by vtkDICOMImageReader. and then i did
some iso-surface rending based on the data.

after that i want plot some points on the surface of the data (in 3D space).

what methods should i use? vtkSplineWidget or vtkPoints?
i want to left click my mouse and plot the points.

moreover, after i plot the points, how can i get to know the
coordinates(x,y,z) of the points?

and is it possible to save the points coordinates in some format (text
files maybe)and load those points again for next time?

i hope someone out there can enlighten me.

thanks in advance.

Yours

Peter