1. An edge detection method comprising: classifying a plurality of candidate directions of an image region into a first direction set and at least one second direction set; computing a gradient for each candidate direction of the first direction set to obtain a first gradient set; determining if the first gradient set satisfies the conditions that one edge direction exists in the first direction set; and if the first gradient set does not satisfy the conditions, determining that none of the plurality of candidate directions is the edge direction of the image region.