US Patent 7236634 Image encoding of moving pictures

In an encoding method of moving pictures which generates a predictive picture for a current picture based on a reference picture and a motion vector, a macroblock is divided into subblocks. In each of the plurality of subblocks, an initial value of the motion vector is set and an evaluated value E on a difference between the current picture and the reference picture is calculated along a steepest descent direction to determine the minimum value. Then, the smallest evaluated value is selected among the minimum values obtained on the plurality of subblocks to determine the motion vector based on the pixel position of the smallest value.