A computer-implemented method for transforming encoded video information using a video decoder, the method comprising: receiving encoded video information in a bitstream; and reconstructing a picture using the encoded video information, wherein the picture is a current interlaced forward-predicted field, including: decoding, from the encoded video information in the bitstream, a variable length code that jointly represents differential motion vector information and a motion vector predictor selection for selecting dominant polarity or non-dominant polarity for a motion vector predictor to be used for a motion vector, wherein the variable length code jointly represents a first code length index for a horizontal differential motion vector, a second code length index for a vertical differential motion vector, and the motion vector predictor selection for a block or macroblock of the current interlaced forward-predicted field, and the dominant polarity is same when neighbor motion vectors predominantly reference a reference field having the same polarity as the current interlaced forward-predicted field, and otherwise the dominant polarity is opposite; determining a motion vector predictor for the motion vector for the reference field to be used for the motion vector; and reconstructing the motion vector from the differential motion vector information and the motion vector predictor. receiving encoded video information in a bitstream; and reconstructing a picture using the encoded video information, wherein the picture is a current interlaced forward-predicted field, including: decoding, from the encoded video information in the bitstream, a variable length code that jointly represents differential motion vector information and a motion vector predictor selection for selecting dominant polarity or non-dominant polarity for a motion vector predictor to be used for a motion vector, wherein the variable length code jointly represents a first code length index for a horizontal differential motion vector, a second code length index for a vertical differential motion vector, and the motion vector predictor selection for a block or macroblock of the current interlaced forward-predicted field, and the dominant polarity is same when neighbor motion vectors predominantly reference a reference field having the same polarity as the current interlaced forward-predicted field, and otherwise the dominant polarity is opposite; determining a motion vector predictor for the motion vector for the reference field to be used for the motion vector; and reconstructing the motion vector from the differential motion vector information and the motion vector predictor. decoding, from the encoded video information in the bitstream, a variable length code that jointly represents differential motion vector information and a motion vector predictor selection for selecting dominant polarity or non-dominant polarity for a motion vector predictor to be used for a motion vector, wherein the variable length code jointly represents a first code length index for a horizontal differential motion vector, a second code length index for a vertical differential motion vector, and the motion vector predictor selection for a block or macroblock of the current interlaced forward-predicted field, and the dominant polarity is same when neighbor motion vectors predominantly reference a reference field having the same polarity as the current interlaced forward-predicted field, and otherwise the dominant polarity is opposite; determining a motion vector predictor for the motion vector for the reference field to be used for the motion vector; and reconstructing the motion vector from the differential motion vector information and the motion vector predictor.