US Patent 7852936 Motion vector prediction in bi-directionally predicted interlaced field-coded pictures

Forward motion vectors are predicted by an encoder/decoder using previously reconstructed (or estimated) forward motion vectors from a forward motion vector buffer, and backward motion vectors are predicted using previously reconstructed (or estimated) backward motion vectors from a backward motion vector buffer. The resulting motion vectors are added to the corresponding buffer. Holes in motion vector buffers can be filled in with estimated motion vector values. For example, for interlaced B-fields, to choose between different polarity motion vectors (e.g., “same polarity” or “opposite polarity”) for hole-filling, an encoder/decoder selects a dominant polarity field motion vector. The distance between anchors and current frames is computed using various syntax elements, and the computed distance is used for scaling reference field motion vectors.