1. A shadow detection method comprising: a step of obtaining a depth/disparity map and color/grayscale image from a two-lens camera or stereo camera; a step of detecting and acquiring plural foreground points; a step of projecting the acquired plural foreground points into a 3-dimensional coordinate system; a step of carrying out, in the 3-dimensional coordinate system, a clustering process with respect to the projected plural foreground points so as to divide the projected plural foreground points into one or more point clouds; a step of calculating density distribution of each of the one or more point clouds by adopting a principal component analysis algorithm so as to obtain one or more principal component values of the corresponding point cloud; and a step of determining, based on the one or more principal component values, whether the corresponding point cloud is a shadow.