Optimizing the capacity of distortion-freewatermarking on palette images

In a palette image file, each color is pointed by at least one palette entry and each pixel is represented with one of the color pointers. Since many palette images use a portion of palette entries only, this paper presents a distortion-free watermarking on palette images by exploiting the unused entries. It allocates x palette entries to one color such that each pixel of the color is able to embed log₂ x bits. In order to achieve high embedding capacity, we formulate the relationship among embedding capacity, color occurrence, and unused palette entries. By solving the formula and allocating the palette entries in a sub-optimal manner, the present scheme provides a roughly optimal capacity. Our experiment demonstrates that the present scheme is of high watermarking capacity.