We focus on two aspects of interacting in a shared virtual world: actions initiated by a participant and actions observed by a participant. In both cases, we work with solely those cases where unbounded network lag can cause extremely undesirable effects, such as a frozen world. For example, navigating through a scene, while initiated by a participant, is generally not a source of unbounded network lag, as navigation is typically handled locally. Thus, we make no attempt to mitigate the effects of lag on navigation, though there is lag present and our effects could be reformulated perhaps to address that lag.
Typical actions initiated by a participant include grabbing an object and moving it through a scene. Grabbing a shared object is a source of network-based lag, as a shared object must be controlled by a mutually exclusive lock. Acquiring such a lock requires a network round trip (unless the lock coincidentally resides on the same machine as the participant who wishes to take the lock - a situation which is generally not true). A user interface that does not allow the user to manipulate an object until a lock is granted could have disturbing pauses. If the pauses were long enough, they would prevent the user from feeling in control of the object. Conversely, if the UI lets the user manipulate the object before the lock is granted, the user will lose control once the system realizes that the user does not in fact have permission to modify the object.
Some have suggested that social conventions can suffice in place of a lock, such as a convention that users not grab objects that others are already holding. However, in a collaborative environment these social conventions often break down. For example, in the midst of exciting collaboration, participants may lose track of social graces (as when two people in a physical room get excited about a project or get into a heated argument).
The actions initiated by other participants are also a source of lag, since a remote participant's actions necessarily require the potentially high-lag network transmission of information. An avatar might appear to move slowly or not at all, simply due to poor network performance. Objects being controlled by other participants suffer from similar problems.