[vtkusers] Interactively select segment of cylindrical volume

[Elvis Stansvik]

Hi all,

I have a setup where I have a long cylindrical volume centered around the Z
axis, and the camera focal point is locked to the cylinder center line with
the camera up vector always pointed in the positive Z direction ("up").
It's sort of like you're flying around the outside of a tower, with your
feet always towards the ground. The camera uses parallel projection.

What I need now is to make a tool where the user can drag with the mouse in
the viewport to select a segment of the cylinder he/she is seeing on the
screen. The feature should work like this:

* User drags from one point in the view port to another.

* During dragging, a semi-transparent rectangular overlay is shown, the
overlay should be the full width of the viewport, but should start and end
in the vertical direction at the start and end point of the area being
selected.

* Upon releasing the mouse, I need to get access to the Z coordinates of
the press/release (start/end) points, projected onto the Z axis. I need
this to use to show the selected cylindrical segment in another view (at
higher resolution, so extracted from another set of images).

Any tips/advice on how to go about this using VTK is much appreciated.

Thanks in advance,
Elvis
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