[vtkusers] HOWTO: pick points on 2D Plane & show points?

[kent williams]

I've been looking through the mailing list archive and VTK examples
for some time and not come up with any good way to what I need to do,
so I'm asking you-all.

We want to be able to pick points on a 2D view plane, display some
sort of geometry (a circle?) where a point has been placed, and be
able to move points around after the fact.

In other words the use case would be this.

1. User clicks 'add point' button to go into point-choosing mode.
2. User clicks in 2D view, and the point clicked is marked with some
geometry -- e.g. a white circle.
3. After a point has been placed, the user can pick and drag the point.

Actually, what would be great would be for there to be a widget that
acts like the vtkContourWidget handles.  You click, it drops a white
dot, and once the Contour is closed, you can pick and drag the points.

Suggestions?