Motion estimation by scene adaptive block matching and illumination correction

For low bit rate coding of moving video motion information has to be exploited. Motion estimation algorithms to be used in hybrid coding schemes with motion compensated prediction, e.g. three-step block matching, are chosen with regard to computational requirements and/or estimation accuracy. But all algorithms, once employed, remain fixed throughout coder operation and therefore lack any means of adaptivity to the scene. The computational complexity of a block matching motion estimation scheme is directly proportional to the number of search positions. Every search position requires the calculation of the error criterion. Employing codebook generation techniques, as known from vector quantization, the selection and total number of search positions can be optimized by a vedorbook design. The vectorbook may be tailored to the computational requirements. It can be switched adaptively during coder operation (on-the-fly), in order to trade matching accuracy against computational complexity. The vector-book also may be altered adaptively to substitute rarely needed search positions by those, which are better suited for coding of the actual scene. Due to the error criterion which is based on averaged differences ,the reliability of the displacement vector field is strongly affected by changes in the overall illumination of a scene. Upon a change in mean brightness, usually blocks are moved in order to fit the average brightness, to some degree regardless of the image structures inside the block. By an illumination correction interlaced with motion estimation, block matching always works on mean-removed blocks.