1. A method comprising: receiving an image captured by a depth camera; converting the image, in a depth camera coordinate system, to an image in a real space coordinate system based on an angle and a position of the depth camera relative to at least one subject of the image; detecting at least one subject from the image in the real space coordinate system; calculating a distance from the depth camera to the at least one subject based on the image in the real space coordinate system; and determining whether the at least one subject is a person using criteria that varies depending on the respective distance from the depth camera to the at least one subject.