Projection methods in motion estimation and compensation

The standard approach to exploiting motion elds in block-based hybrid video coding schemes is motion compensation of the current frame using the motion eld, followed by DCT coding of the residue. In this paper we argue that this separation of approximation methods (non-linear prediction followed by transform coding) is unfortunate in low bit-rate applications. In low bit-rate applications, only a limited set of DCT coeecients is retained. Expressing the end result of motion estimation/compensation followed by separate residue coding in terms of basic linear algebra, we nd a sub-optimal linear approximation scheme. In this paper we replace motion compensation and separate residue coding by (optimal) orthogonal projection. We show that for low bit-rate applications, the orthogonal projection method performs better than conventional methods. In one extreme case, it is better to use projection with only 15 DCT coeecients retained, than to use separate residue coding with all 64 DCT coeecients retained. A critical ingredient in the proposed scheme is the choice of an orthogonal basis in the vector spaces involved. In the current scheme, these bases are determined locally, implying a high computational complexity. Possible directions for the reduction of this computational complexity are discussed.