Arithmetic coding of a lossless contour based representation of label images

We propose a new method for the encoding of label images (also known as segmentation maps or alpha planes) that are often used to identify object location in region-based image and video coders. The method is contour-based and lossless with a contour model composed of two parts: a contour graph describing the topology of the contour network and a directional chain code to deal with the geometric part of the label image (internal contour points). The graph-based description of the topology is designed to minimize the cost of encoding the nodes, while the directional chain codes are compressed by arithmetic coding. The approach is flexible since separating the contour network into topological and geometrical parts allows the use of other lossless or lossy methods to encode the geometric part without changing the graph representation. The proposed method has been compared with an arithmetic encoder used in MPEG-4.