1. A method comprising: deriving N motion vector predictors for a first block that has N motion vectors corresponding to N lists of reference frames and a current frame, including constructing one of the N motion vector predictors when a second block that neighbors the first block and is used for prediction has at least one invalid motion vector, where N is an integer greater than 1, wherein constructing one of the N motion vector predictors in a first list of the N lists is performed by applying a function to another of the other N motion vector predictors in a second list of the N lists different than the first list and signed temporal distances from the current frame to the reference frames associated with each motion vector in the N motion vectors of the first block; generating N differential motion vectors based on the N motion vectors and N motion vector predictors; and encoding the N differential motion vectors.