1. A motion vector estimator for reducing film judder, which measures a motion vector of a block of an image comprised of blocks of pixels arranged in a matrix form, a block at an i-th row and j-th column of the image being indicated by (i, j) where i, j are integers, the estimator comprising: a motion vector selection device having a plurality of motion vector selectors to receive a plurality of motion vectors respectively, each motion vector selector generating N candidate vectors based on its motion vector received, and selecting the N candidate vectors of a motion vector selector to output based on a motion estimation stage signal, where N is a positive integer, in which the motion vector selection device comprises: a first motion vector selector for receiving the output motion vector of the block (i, j) of the last image and the output motion vectors of the blocks (i−1, j−1), (i−1, j), (i, j−1) of the present image and generating a first set of N candidate vectors, wherein the first motion vector selector comprises a calculation and selection device, a first adder and a second adder, the calculation and selection device calculating differences between the output motion vector of the block (i, j−1) of the present image and a predetermined motion vector, the output motion vector of the block (i, j) of the last image, the output motion vector of the block (i−1, j−1) of the present image, and the output motion vector of the block (i−1, j) of the present image to accordingly select one with the greatest difference as a first candidate vector for output, the calculation and selection device directly outputting the output motion vector of the block (i, j−1) of the present image to be a second candidate vector, the first adder adding a first update vector to the second candidate vector to generate a third candidate vector, and the second adder adding a second update vector to the second candidate vector to generate a fourth candidate vector; a second motion vector selector for receiving the output motion vectors of the block (i, j−1), (i−1, j+1), (i+1, j+1) of the present image and generating a second set of N candidate vectors; and a multiplexer connected to the first and the second motion vector selectors for selecting the first set of N candidate vectors or the second set of N candidate vectors based on the motion estimation stage signal as the N candidate vectors for output; N block matchmakers connected to the motion vector selection device for performing a block matching on the N candidate vectors to generate N matching costs respectively; and a motion vector determinator connected to the N block matchmakers and the motion vector selection device for determining an output motion vector of the block based on the N matching costs and feeding the output motion vector back to the motion vector selection device.