  Information security is a critical issue in modern society and image
watermarking can effectively prevent unauthorized information access. Optical
image watermarking techniques generally have advantages of parallel high-speed
processing and multi-dimensional capabilities compared with digital approaches.
This paper provides a comprehensive review on the research works related to
optical image hiding and watermarking techniques conducted in the past decade.
The past research works are focused on two major aspects, various optical
systems for image hiding and the methods for embedding optical system output
into a host image. A summary of the state-of-the-art works is made from these
two perspectives.
