SoQtConstrainedViewer(3) on Linux
The text 7f919c631170, as Debian 9 (Stretch) shipped it. The page shows this text now.
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.