The algorithmic architecture and performance of a video coder are described that switches between the model-based and H.261 modes, depending on image content. The switching criterion incorporates measures of both picture quality and bit rate. The model-based mode uses generalized cylindrical models and a 3D matching technique to estimate motion. The coder was tested with four different image sequences in CBR and VBR operational modes: the performance was generally superior, and in some cases significantly superior, to that of a free-running H.261 coder operating on the same sequences. The switched architecture represents a possible route for the integration of model-based coding with H.261 to give improved performance at very low bit rates.
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