1. A video coding system comprising: a prediction module configured to perform multi-reference picture motion estimation, the prediction module including a first motion estimation module configured to perform full-pixel motion estimation for a current block in a macroblock using at least one group of reference pictures; a reference picture selection module configured to select, for the current block, reference pictures from the at least one group of reference pictures based on a cost computed for each reference picture in the at least one group, wherein the cost is based on full-pixel motion estimation; a second motion estimation module configured to perform sub-pixel motion estimation for the current block using the reference pictures selected by the reference picture selection module; and a coding module configured to generate a coded current macroblock for the current picture using the sub-pixel motion estimation.