1. An edge detection method comprising: computing a gradient for each candidate direction of a first direction set of an image region to obtain a first gradient set, the first direction set comprising a vertical direction, a horizontal direction, and at least two candidate directions between the vertical direction and the horizontal direction; calculating a first gradient difference between two predetermined candidate directions; determining if a second candidate direction corresponding to a maximum gradient in the first gradient set is substantially perpendicular to a first candidate direction corresponding to a minimum gradient in the first gradient set; and determining if an edge exists in the image region according to whether the first gradient difference is greater than a first threshold and whether the second candidate direction is substantially perpendicular to the first candidate direction.