Multiresolution framework for backward motion compensation

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Abstract
Hierarchical decomposition of images and their relationship with motion fields continues to be a hotly pursued topic, and the role of backward motion information in coding is beginning to capture the interest of the video coding community. This paper simultaneously addresses some of the fundamental issues in multi-resolution and backward motion systems. From a coding viewpoint, a multi-resolution motion hierarchy should be coupled with an estimation system that deals with a maximally subsampled wavelet decomposition of the frames, to avoid redundancy of representation. Through a frequency domain argument, we expose the difficulties associated with such an approach, and in fact show that a band-to-band motion compensated estimation in a wavelet domain is not possible. This analysis leads to an alternative approach for estimation of detail bands. The resulting estimation errors were coded through a zerotree quantizer. We circumvented the causality problem associated with determination of zerotree information in a recursive coder through using a suitable substitute zerotree. Simulations show that a prototype coder of this type is very competitive, with a performance similar to forward (block-based) coders. The results show that the commonly held belief by many, that backward (pel-based, or pel-recursive) coding algorithms are inherently inferior to block-based methods, is not true, and will hopefully spawn interest and open a debate on the role of backward motion information in video coding.

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