1. A method, comprising: obtaining a frame of depth data from a depth sensor, the depth sensor mounted to provide a top view of a scene; discerning foreground objects from background objects from within the frame of depth data; for a given foreground object, calculating expected relative head size, in pixels of the frame of depth data, of the given foreground object at a depth of the given foreground object, the expected relative head size calculated using a pre-determined head size, a width in pixel units of the frame of depth data, an angle the depth sensor covers, and a depth of the given foreground object determined using the frame of depth data obtained; and determining if the given foreground object matches a single-scale reference model of a target object, the single-scale reference model of the target object determined based upon the expected relative head size at the depth of the foreground object, wherein the expected relative head size is calculated according to the following equation: R = rM 2 * X ⁢ ⁢ tan ⁡ ( α 2 ) where R is the expected relative head size in pixels, r is an average head radius, α is the angle the depth sensor covers, X is a distance from the depth sensor to a head center, and M is an image width.