Fast motion estimation discarding low-impact fractional blocks

Sub-pixel motion estimation is used in most modern video coding schemes to improve the outcomes of motion estimation. The reference frame is interpolated and motion vectors are refined with fractional components to reduce the prediction error. Due to the high complexity of these steps, sub-pixel motion estimation can be very demanding in terms of encoding time and resources. A method to reduce complexity of motion estimation schemes is proposed in this paper based on adaptive precision. A parameter is computed to geometrically characterise each block and select whether fractional refinements are likely to improve coding efficiency or not. The selection is based on an estimate of the actual impact of fractional refinements on the coding performance. The method was implemented within the H.264/AVC standard and is shown achieving considerable time savings with respect to conventional schemes, while ensuring that the performance losses are kept below acceptable limits.