1. A method for expressing a motion object, the method comprising: obtaining a stereo image in which the motion object has been captured, the stereo image including a depth image; extracting a key point from the motion object in the stereo image; determining, based on statistical information relating to three-dimensional motion of pixels within a first predetermined region surrounding the key point, a dominant direction of the key point; determining, based on the dominant direction of the key point, motion vectors of pixels within a second predetermined region surrounding the key point to obtain rotation invariant motion vectors; and extracting, based on the determined motion vectors of the pixels within the second predetermined region surrounding the key point, a feature describing the key point.