A comparison of fractional-pel interpolation filters in HEVC and H.264/AVC

The fractional-pel interpolation filter adopted in H.264/AVC improves motion compensation greatly. Recently, a new DCT-based fractional-pel interpolation filter is adopted in the oncoming standard HEVC. We are interested in the differences between these two types of fractional-pel interpolation filters. In this paper we describe the derivations of fractional-pel interpolation filters in HEVC and H.264/AVC in detail, and compare them on properties of frequency responses. We find that the half-pel interpolation filters in HEVC and H.264/AVC are very similar, but the low-pass properties of quarter-pel interpolation filters in HEVC are much better than those in H.264/AVC. Experimental results validate this phenomenon, the fractional-pel interpolation in H.264/AVC tends to increase BD-rates by more than 10% compared with that in HEVC, and this performance loss mainly comes from quarter-pel interpolation filters. On the other hand, the complexity of fractional-pel interpolation filtering in HEVC is greatly increased than that in H.264/AVC.