SoQtConstrainedViewer(3) on Linux
The text 5dd6fa6981bc, as Debian 8 (Jessie) shipped it.
The SoQtConstrainedViewer class is the superclass for viewers with constrains on the viewpoint. This abstract viewer class adds new methods to keep the camera in an upright position according to some arbitrary up-direction. The up-direction can be decided by either the application programmer (by using SoQtConstrainedViewer::setUpDirection()) or the end-user, through interaction mechanisms defined by the subclasses.