These
preconditions include information about what has to
change to trigger an interaction, and where in 3D space the change has to occur to be noticed. The first part – what
has to change – is described by an object parameter type.
To trigger an event object parameter that is modified, have
to be of specified object parameter type. The second part,
which describes where in 3D space the change has to
occur, is a specification of a fragment of 3D space, called
3D interaction space. To satisfy location precondition, the
event has to occur inside the specified 3D interaction
space.