In most secret image sharing schemes, the secrecy level of each pixel is the same. However, in real applications, different pixels may require different secrecy levels. For example, in a patient's medical image, the region with the patient's name must be highly protected. To solve this problem, Wang and Lin proposed a scheme that allows a dealer to divide an image into multiple regions, each of which is associated with a certain level of secrecy. In their scheme, an extra pixel-security-level map (PSLM) is required to record the secrecy level of each pixel, and the shadow size of the PSLM is approximately 20 % of that of the secret image. It is unreasonable to have such a large overhead. To reduce the shadow size of the PSLM, they restricted each region to be a rectangle. Furthermore, any two participants can reconstruct the PSLM, enabling the content of regions with high secrecy levels to be compromised easily. To resolve these disadvantages, this paper proposes a new encoding scheme for PSLMs. The location of each region is protected according to its secrecy level, thus improving the security of the image. Moreover, the shape of each region is unrestricted, and the shadow size of the PSLM is reduced significantly.
- Secret image sharing; Shadow size; Pixel-security-level map; Predictive differential quantizing