Motion-compensating prediction with fractional-pel accuracy

The effect of fractional-pel accuracy on the efficiency of motion-compensating predictors is studied using various spatial prediction/interpolation filters. In model calculations, the power spectral density of the prediction error is related to the probability density function of the displacement error. Prediction can be improved both by higher accuracy of motion-compensation and by spatial Wiener filtering in the predictor. Beyond a critical accuracy, the possibility of further improving prediction by more accurate motion-compensation is small. Experiments with videophone signals and with broadcast TV signals confirm these model calculations. Sinc-interpolation, bilinear interpolation, and Wiener filtering are compared at integer-pel, 1/2-pel, 1/4-pel, and 1/8-pel accuracies. A three-state technique for reliable displacement estimation with fractional-pel accuracy is introduced. It is based on phase correlation. For motion-compensation with block size of 16 pels*16 pels, 1/4-pel accuracy appears to be sufficient for broadcast TV signals, whereas for videophone signals, 1/2-pel accuracy is desirable.>