[vtkusers] Converting from X, Y screen to X, Y world

[Greg Scott]

Hello,

I have implemented a simple interactive free-form deformation system using
vtkPicker. Points on a mesh can be picked and dragged with the mouse.
Currently, however, the points only move correctly when the viewing
direction is along the positive Z axis, i.e. when X and Y in the world
really correspond with the X and Y screen directions (left,right and up,down
the window).

When viewing the mesh from any other side, the movement (as you can imagine)
is no longer intuitive, as the X and Y axis of the world are no longer in
correspondance with the screen X and Y directions.

My question is this: how can I convert the X and Y screen movements to world
coordinate space movements, such that the points move as they should? What
filter should do this easily for me?

Thank you for your help everyone.

Greg