Rate-distortion optimization of hierarchical displacement fields

The displacement vector field for motion-compensated coding of image sequences is represented and coded using a hierarchy of displacement vector fields over successively finer sampling grids. An efficient method based on the generalized Breiman-Friedman-Olshen-Stone (BFOS) tree-pruning algorithm is used to find a variable-depth hierarchy that satisfies a given bit rate or distortion constraint. The variable-depth hierarchy corresponds to using a nonuniformly sampled displacement field for motion-compensation. The proposed scheme is compared with conventional fixed-block-size block-matching motion-compensation. The motion information bit rate or the motion-compensation residual energy of the conventional scheme serves as the constraint. Simulation results show that the proposed scheme reduces the residual energy by between 0.6-1.5 dB, or the bit rate by 19-54%, while using only 40-100% of the computational complexity of the conventional scheme. The prediction image sequence produced by the hierarchical scheme offers much better viewing quality.