Patent ID: 7852936

Claim:
A method of decoding encoded video using a video decoder, the method comprising: with the video decoder: receiving encoded data in a bit stream; and decoding an interlaced B-field using the encoded data, including, for a macroblock in the interlaced B-field, the macroblock having a reconstructed first motion vector in a first prediction direction and using motion compensation only in the first prediction direction: storing the reconstructed first motion vector in a first motion vector buffer for the first prediction direction for use in subsequent motion vector prediction; calculating a motion vector predictor for a second prediction direction opposite the first prediction direction, wherein the motion vector predictor is based at least in part on one or more motion vector predictor candidates from a second motion vector buffer for the second prediction direction, wherein the motion vector predictor is a predicted value of a fill-in second motion vector in the second prediction direction, and wherein the calculating the motion vector predictor for the second prediction direction comprises, based at least in part on polarity information for the one or more motion vector predictor candidates from the second motion vector buffer, selecting between: using, as the motion vector predictor, a same polarity motion vector predictor that references a first reference field of the interlaced B-field in the second prediction direction, the first reference field having same polarity as the interlaced B-field, or using, as the motion vector predictor, an opposite polarity motion vector predictor that references a second reference field of the interlaced B-field in the second prediction direction, the second reference field having opposite polarity as the interlaced B-field; wherein the selecting includes counting how many of the one or more motion vector predictor candidates from the second motion vector buffer are opposite polarity motion vectors and/or counting how many of the one or more motion vector predictor candidates from the second motion vector buffer are same polarity motion vectors; and storing the motion vector predictor as the fill-in second motion vector in the second motion vector buffer for use in subsequent motion vector prediction.