Group undo framework and algorithms in real-time collaborative image editing systems

The ability to undo operations is an indispensable feature of single user editing systems, but supporting group undo in real-time collaborative editing systems is still a difficult problem. In this paper, we propose an undo framework and algorithms to achieve group undo in image-based collaborative graphics editing systems. The basic idea is to interpret an undo command as a concurrent inverse operation by means of image operation transformation algorithm, so that an operation is always undoable under its current context. Through exploiting relations among operations, space cost for operation preservation is greatly reduced. The global undo, local undo and selective undo mode are supported in our solution. The undo algorithms are also applicable in single-user applications. The algorithms are implemented in CoDesign-a multi-level collaborative graphics designing system, which aims at supporting both object-based and image-based collaborative pattern design.