1. An image processing method for splitting image data into a plurality of clusters, the image processing method comprising: a first distance calculating step of calculating a first distance that is the distance between a feature vector of a processing object pixel and a representative feature vector of a cluster to which a nearby pixel of the processing object pixel belongs; a first allocating step of allocating the processing object pixel to the cluster to which the nearby pixel belongs when the first distance is less than or equal to a first threshold; a second distance calculating step of respectively calculating, when there are defined clusters and the first distance is greater than the first threshold, distances between the feature vector of the processing object pixel and representative feature vectors of the respective defined clusters, and calculating a second distance that is a minimum distance among the respective calculated distances; and a second allocating step of allocating the processing object pixel to a cluster corresponding to the second distance when the second distance is less than or equal to the second threshold, and newly defining a cluster to which the processing object pixel is to be allocated when there are no defined clusters or when the second distance is greater than the second threshold and allocating the processing object pixel to the newly defined cluster.