Integrating selective undo of feature-based modeling operations for real-time collaborative CAD systems

In collaborative editing systems, selective undo is a fundamental function that allows dispersed users to undo any executed operation in the history. Most selective undo mechanisms have been applied in collaborative editing systems with simple objects (plain text or image). However, how to provide a selective undo mechanism for collaborative editing systems with complex objects (feature-based CAD model) has been a technical challenge. This paper proposes a novel CRDT (Conflict-free Replicated Data Type)-based selective undo mechanism for feature-based Co-CAD (Collaborative CAD) systems. Firstly, a selective undo framework is presented to integrate (do/undo) operations. Secondly, three constraint relations of (do/undo) operations are defined and feature-based conflict detection conditions are proposed to detect the three constraint relations. Thirdly, a CRDT-based conflict resolution approach is proposed to integrate (do/undo) operations. Fourthly, collaborative case studies illustrate how the selective undo mechanism works and achieves consistent results. Finally, the complexities in theory are analyzed and the formal proofs are provided to verify the correctness. (C) 2019 Elsevier B.V. All rights reserved.