Existing digital watermarking techniques are systematically reviewed in this paper, with a special focus on rotation, scaling, and translation (RST) invariant image watermarking algorithms. The RST invariant-based approach has recently demonstrated its effectiveness in performance and robustness against various geometrical transformations and image degradations.

The paper comprehensively covers existing algorithms and provides a detailed performance analysis. The authors did a good job in structuring and organizing the content, which made the paper very readable.

My only concern is with Section 2, “Fundamental Theories and Techniques,” which seems too long (17 pages). It basically reviews very fundamental image processing knowledge that could be easily found in any image processing textbook; I assume that readers interested in this paper already have such knowledge.

Also, it would have been nice if the authors had included a section at the end to comment on future work and possible research directions in this area (according to their findings and experiences in this domain). Overall, this is a very good and comprehensive survey paper—I highly recommend it.