Locally Adaptive Non-Separable Interpolation Filter for H.264/AVC

In order to reduce the bit rate of video signals, current coding standards apply hybrid coding with motion-compensated prediction and transform coding of the prediction error. In former publications, it was shown that aliasing components contained in an image signal, as well as inaccuracies in description of motion, are limiting the prediction efficiency obtained by motion compensation. For H.264/AVC, we showed that the analytical calculation of an optimal interpolation filter at given constraints (6-tap, frame-based) is possible, resulting in total coding improvements of up to 0.9 dB for HDTV sequences and up to 0.5 dB for CIF sequences and main profile. However, applying an adaptive interpolation filter to the entire image enables only an average prediction improvement. Further improvements are possible if the filter is also adapted to local image characteristics. Thus, an optimal adaptive interpolation filter is calculated only for the macroblocks, which prediction is distorted by aliasing or by inaccurately estimated motion. This enables further improvements of up to 0.2 dB, compared to globally adaptive filter or up to 0.6 dB, compared to the standard H.264/AVC, for CIF sequences.