1. A method for expressing a motion object, the method comprising: obtaining a stereo image of the motion object, the stereo image including a depth image; extracting a key point from the motion object in the stereo image; determining a dominant direction of the key point, based on statistical information associated with three-dimensional motion of pixels within a first region surrounding the key point wherein determining the dominant direction includes, obtaining a two-dimensional histogram having a first dimension and a second dimension, the first dimension being a depth of motion vectors of the pixels and the second dimension being directions of components of the motion vectors of the pixels in planes perpendicular to the depth, and determining the dominant direction of the key point based on a direction relating to a peak value in the two-dimensional histogram; determining, based on the dominant direction of the key point, motion vectors of pixels within a second region surrounding the key point to obtain rotation invariant motion vectors; and generating, based on the determined motion vectors of the pixels within the second region surrounding the key point, a feature associated with the key point; and generating a classification signal based on the generated feature, the classification signal including information indicating a classification of the motion object.