PROBLEM TO BE SOLVED: To provide techniques and tools for encoding and decoding predicted images, in interlaced video. SOLUTION: For example, a video encoder or decoder computes a motion vector predictor for a motion vector for a portion of an interlaced P-field (e.g., a block or a macroblock), including selecting between using a same polarity or opposite polarity motion vector predictor for the portion. The encoder/decoder processes the motion vector based at least in part on the motion vector predictor computed for the motion vector. The process can comprise computing a motion vector differential between the motion vector and the motion vector predictor during encoding and reconstructing the motion vector from a motion vector differential and the motion vector predictor during decoding. The selecting can be based at least in part on a count of opposite polarity motion vectors for a neighborhood around the portion and/or a count of same polarity motion vectors for the neighborhood. COPYRIGHT: (C)2009,JPO&INPIT