Achieving group undo in bitmap-based collaborative editing systems

The ability to undo operations is an indispensable feature of editing systems, but supporting group undo in real-time collaborative editing systems is still a difficult problem. We propose an undo framework and algorithms to achieve group undo in bitmap-based collaborative editing systems (BCES). The basic idea is to interpret an undo command as a concurrent inverse operation by means of bitmap operational transformation, so that an operation is always undoable regardless of its undo context. Through exploiting relations among operations, space cost is reduced. The algorithms are implemented in CoDesign - a prototype system of collaborative graphics editing.