1. An image processing method comprising: calculating a partial distance between a pixel of interest in an image and each of a plurality of reference pixels; sequentially calculating a total distance between the pixel of interest and each of the plurality of the reference pixels based on the partial distance; determining a shortest total distance among the total distances that have been already calculated, in the sequential calculation of the total distance; and categorizing the pixel of interest based on the reference pixel corresponding to the shortest total distance, wherein, if the partial distance between the pixel of interest and a specific one of the reference pixels to be calculated is equal to or greater than the shortest total distance in the sequential calculation of the total distance, the calculation of the total distance between the pixel of interest and the specific one of the reference pixels to be calculated is omitted.