1. A method of detecting a user input, the method comprising: obtaining a depth of a first region in a depth image; obtaining a depth of a second region in the depth image, wherein the second region surrounds the first region; comparing the depth of the first region to the depth of the second region; and classifying, by a processor, at least one point included in the first region as an interfacing region in response that the depth of the first region is less than the depth of the second region by at least a threshold.