[vtkusers] Converting 2d coordinates to 3d

[jon n]

Hi,

I'm attempting to set things up so the user can draw a box in 2d indicating 
a crop they want to perform on a 3d object. If i just approach it straight 
forward then
the perspective is taken into account so the rectangular section which is 
cropped is always smaller than the box they created in 2d. Is there any way 
to convert the 2d coordinates of a box into the 3d coordinates which are 
being drawn there? I suppose the operation would be like doing a backwards 
pick.

Currently I'm using a very slow technique involving drawing a ray from the 
mouse location down to where the crop would actually be performed.

Thanks for any help

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