You can access the 'reference' geometry of a sliced geometry or a plane geometry. The reference geometry is the 'original' geometry that has been 'resliced' along a given direction (aka. 'view direction') to make the given sliced geometry and plane geometries.

The reslicing is done by mitk::SliceNavigationController::Update().

The slice navigation controller also provides access to the 'original' geometry, just it is referred to as 'input' geometry in the function names.

For the sake of simplification and consistency, I propose to rename these functions and use the term 'reference geometry' everywhere.