Incremental IDF

According to today's discussion, we will use tf-idf model to rank the relevance values of images. However, since our tags are incremental, the idf values are changed over time. I found some related works on this issue.
FYI.

Anthony Tomasic, Hector GarciaMolina, and Kurt Shoens, "Incremental Updates of Inverted Lists for Text Document Retrieval," In Proceedings of the 1994 ACM SIGMOD Conference, pages 289--300, New York, USA, 1994.

Sergey Melnik, Sriram Raghavan, Beverly Yang, and Hector Garcia-Molina, "Building a Distributed Full-Text Index for theWeb," In 10th World Wide Web, pages 396--406, 2001

L. Lim, M. Wang, S. Padmanabhan, J. S. Vitter, and R. Agarwal, "Dynamic Maintenance of Web Indexes Using Landmarks," In Proceedings of the 12th International World Wide Web Conference, pages 102--111, May 2003.