1. A video pre-processing method for motion estimation, comprising: receiving a first image in a video, the first image consisting of a plurality of first blocks, each of which corresponding to one of a plurality of first pixel matrices; for each of the first blocks, calculating a similarity descriptor of the first block, the similarity descriptor of the first block configured to indicate whether a plurality of pixel values of at least one pair of adjacent pixels in the corresponding first pixel matrix are identical; calculating a similarity descriptor of a second block of a second image in the video, the second block corresponding to a reference block among the first blocks and to a second pixel matrix, the similarity descriptor of the second block configured to indicate whether a plurality of pixel values of at least one pair of adjacent pixels in the second pixel matrix are identical; reaching a verdict on whether the similarity descriptor of the second block and the similarity descriptor of the reference block are identical; and selectively performing motion estimation on the second block based on the verdict; wherein there are m lines along a first direction and n lines along a second direction in each of the first pixel matrices or in the second pixel matrix, m being a positive integer not less than 1, n being a positive integer greater than 1, and calculating the similarity descriptor of each of the first blocks or the second block comprises: obtaining a first truth value by determining whether a plurality of pixel values of all the pixels on the first of the n lines are identical; and obtaining a second truth value associated with the i-th of the n lines by determining whether the i-th and the (i−1)-th of the n lines have identical characteristics, i being a positive integer greater than 1 and not greater than n; wherein the similarity descriptor of each of the first blocks or the second block comprises the first truth value, the second truth value, and a pixel value of an anchor pixel on the first of the n lines.