Highlighting: a gesturing communication tool for real-time collaborative systems

Highlighting text is a common feature in most single-user editors. It provides users with a mechanism of communication and collaboration between the author and readers by emphasizing some important text. This feature is also necessary and even more valuable in multi-user collaborative editors. However, it is non-trivial to extend this feature from the single-user environment to the multi-user collaborative environment because of the following challenges: 1) the need to differentiate highlights by different users, 2) the need to tackle inconsistency problems caused by concurrent operations, and 3) the need to provide a flexible undo facility with the capability of undoing any highlighting operation at any time. In this paper we systematically address these issues and offer our solutions, which have been implemented in the REDUCE (real-time distributed unconstrained collaborative editing) system.