1. A method for generating a just noticeable depth difference (JNDD) model of a three-dimensional (3D) display, the method comprising: controlling a real size of an object to maintain a perceived size of the object for a plurality of depth testing levels, wherein the controlling of the real size of the object comprises adjusting the real size of the object so as to be inversely proportional to a depth of the object for the plurality of depth testing levels; measuring a JNDD of the 3D display for the plurality of depth testing levels by increasing or decreasing the real size of the object; and generating a JNDD model of the 3D display based on the measurement result.