I would like to display my segmentations as 2D outlines in the 3D view port. I have the NIfTI segmentations, but currently, I can only show the outlines in the 2D views (red, yellow, green). In the 3D viewport, I can only display the segmentations as solid 3D objects, not as outlines.
If your NifTI segmentation can be identified as a vtkMRMLSegmentationNode, you may use this module to generate the outline as a Model visible in the first 3D view.
You can convert the segmentation to labelmap and then display the labelmap as coloring or contour in the slice that is visible also in 3D.
This need has not come up often enough to justify implementing it for segmentations. Usually people showing the segmentation in 3D as a 3D object (often with some transparency), not just the intersection. Can you tell a bit about your workflow and why do you need to display the segmentation just as a contour?