[vtkusers] Picking and moving a vertex

[Meisam Aliroteh]

Hi,

I have a polydata and used a sphere source to render the polydata in
point-cloud mode (i.e only vertices are rendered, and I use spheres to show
these vertices). I want the user to be able to click on any vertex (shpere)
and move it in the 3D space (well actually it would be a motion restricted
to the current view plane). I used vtkCellPicker and know how to figure out
which vertex is picked when the user clicks on it. What I don't know is how
to figure out the motion vector when the user clicks and drags a sphere ? I
can get the old and new mouse positions from the interactor but these are 2D
coordinates.

I guess what I need is the 3D coordinates of the previous and current mouse
positions, so that when I subtract these 3D coordiantes I get a motion
vector and then I can move the picked vertex by adding this motion vector to
it. The problem is I don't know how to get the 3D coordinate of the mouse
pointer using VTK !!!

Any ideas?
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