Beam vector in SlicerRT defined by two points: isocenter and source-axis distance and beam transformation. You can set required isocenter position in “External Beam Planning” module.
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Beam transformation can be changed by clicking the “Edit” button on beam node
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I would actually like to get the vector from the 3D view only, without setting up the beam
There is no such feature (not yet) in DRR calculation in SlicerRT. May be later.