Object-based coding through multigrid representation

This paper presents a very low bit-rate coding algorithm based on image split in order to represent it through an adaptive multigrid supported by a binary tree structure. Independently of its tree representation, the picture is segmented via a watershed procedure and several criteria are combined to automatically extract interesting areas of the image. This object information is not transmitted but used to reduce picture complexity, and therefore the bit-rate, while keeping a good subjective quality. This is achieved by a merge procedure which homogenizes values of the tree subblocks belonging to a same non-interesting object. This treatment affects both intra- and inter-images. For intra-images, the resulting tree structure is entropy coded while its leaves are encoded through a DPCM procedure followed by a multi- huffman coder. For inter-images, a motion field is adaptated by an adaptative block matching algorithm which is a kind of BMA for which blocksize is chosen in order to reach a sufficient level of confidence. Residues, essential to correct motion compensation artifacts, are sent through local intra-trees or, if the bit-rate allows it, through DCT blocks, allowing to reach an arbitrary level of quality. During the reconstruction step, an object oriented approach combined with the use of overlapping functions allows to reduce block artifacts while keeping sharp edges.