Image Vectorisation in Digital Image Watermarking

摘要

Unlike most previous work, which used a random number of a sequence of bits or an image as watermark directly, this paper proposed a new image vectorisation method for digital image watermarking. A watermark image (image to be embedded) is firstly contourised into a sequence of contour curves by constructing a "vector" for each of the grey level values. In the contourisation process, a topology analysis method is applied for looking for local maxima, minima and saddle points to implement a topology table. It is well known that the volume of "vector" data from real image contourisation may be up to an order of magnitude greater than the raster representation. Therefore, a simplification method is adopted to analysis the great expansion of data, and to determine which contours it is necessary to preserve in a given image and which contours can be discarded in that image. With help of previously obtained the topology table, image is decomposed into a number of adjacent sub regions known as catchments basins, each of which typically surrounds of a local maximum or minimum and is defined by a contour which is referred to as a watershed or watershed boundary. Then the simplified contour points are embedded as watermark onto the cover image by using the well known spread spectrum technique. After the contour points are extracted from watermarked image, the watermark image is reconstructed by constructing the triangle mesh defined by the contour map and rendering it using conventional rendering method.