Region-based coding scheme for the transmission of video sequences via channels of varying very low bit rate
Résumé
Region-based coding methods can provide a solution of maximal quality for the transmission of video sequences through channels of low bit-rate due to their 'scene- adaptive' nature. Here, the visual quality of frames predicted by motion compensation on nearly semantic homogeneous regions is better than for the conventional block-based and quad-tree based methods. If the error signal is not encoded, the most bit-consuming component is the description of the regions borders and of the topology of the segmentation map. A planar graph with polygonal arcs is used to represent the geometrical form and relations of regions in each frame. A method allowing to adapt the segmentation description to the variable available bit-rate is proposed based on rate-distortion theory and constrained optimization. The method uses the concept of description layers. The 'basic' layer contains only the triplet nodes of the graph and the vertices of the highest contrast and curvature. The 'maximal enhancement' layer contains all the nodes and polygonal vertices of the segmentation graph. The choice of 'optimal' layer for each polygonal arc is done independently, minimizing Lagrangian cost function. The last one combines rate and distortion measures. The entropy estimates for the encoding of all vertices of a given arc are taken as the rate measure. The distortion measure is the total sum of squared DFD in the area delimited by the basic layer and maximal enhancement layer for a given arc. The experiments on the 420-625 CCIR videoconference sequence showed a 30 percent decrease of the bit-rate for an unnoticeable loss in quality.