We address the problem of the efficient encoding of object boundaries. This problem is becoming increasingly important in applications such as content-based storage and retrieval, studio and television postproduction, and mobile multimedia applications. The MPEG-4 visual standard will allow the transmission of arbitrarily shaped video objects. The techniques developed for shape coding within the MPEG-4 standardization effort are described and compared first. A framework for the representation of shapes using their contours is presented next. Such representations are achieved using curves of various orders, and they are optimal in the rate-distortion sense. Finally, conclusions are drawn.

A major problem in object oriented video coding is the efficient encoding of the shape information of arbitrarily shaped objects. Efficient shape coding schemes are also needed in encoding the shape information of video object planes (VOP) in the MPEG-4 standard. In this paper, we present an efficient method for the lossy encoding of object shapes which are given as 8-connect chain codes (Meier et al., 1997). We approximate the object shape by a second order B-spline curve and consider the problem of finding the curve with the lowest bit rate for a given distortion. The presented scheme is optimal, efficient and offers complete control over the trade-off between bit-rate and distortion. We present results with the proposed scheme using objects shapes of different sizes.