Predictive shape coding using generic polygon approximation

We introduce an efficient predictive binary shape coding method that consists of (1) global motion estimation, (2) local motion estimation, (3) matched segment coding, and (4) residual segment coding. Global and local motion estimation use contour pel matching and knowledge of previously reconstructed contours. After motion compensation, we code the one-dimensional reference contour indices of the matched contour positions. The final step codes the mismatched contour segments using residual coding. We use a maximum shape distortion tolerance parameter (d_max), which is zero for lossless coding, for both motion estimation and residual coding. We apply the new shape coding method to MPEG-4 binary mask test sequences in QCIF and SIF formats for a wide range of d_max values. The key contribution of our method is in lossy shape coding in which the average coding gain is more than 100% over generalized differential chain coding. The proposed scheme can be applied to MPEG-4 compliant shape coding and to efficient shape representation for future MPEG-7 applications