Object-based analysis-synthesis coding of image sequences at very low bit rates

An object-based analysis-synthesis image sequence coder for transmission bit rates between 8 and 16 kbit/s is presented. Each moving object is described by three sets of parameters defining its shape, motion, and colour. Coding is based on the source model of flexible 2D objects which move translationally in the image plane, as it has been used in an implementation for a 64 kbit/s ISDN videophone by Hotter (1990). In order to cut down the bit rate from 64 kbit/s to 8 kbit/s, QCIF image resolution instead of CIF resolution is applied. Image analysis and coding of object parameters have been adapted to the reduced resolution and to the changed parameter statistics, respectively. In addition to Hotter´s coder, predictive coding is used for encoding polygons and splines to improve the coding efficiency of shapes. Vector quantization is applied instead of DCT for coding the luminance and chrominance parameters of the object textures. Uncovered background regions are encoded by applying adaptive prediction from either the neighbouring static background or a special background memory. Experimental results, obtained by computer simulations, confirm that the presented coding algorithm is a promising scheme for the application at extremely low transmission bit rates. This is shown by comparing the picture qualities obtained with the presented algorithm and a block-based hybrid-DCT scheme corresponding to H.261/RM8 at 11 kbit/s