If we try to explain what we see as an object, we could conclude that we always make a choice between an infinite series of real three-dimensional objects having the same flat retinal projection. From this infinity, our perception selects usually only one "natural" (most probable or, sometimes, simplest as possible) interpretation. In the case of a drawing we have an obverse process: attempt to "reconstruct" a 3D-object from its projection(s). Let us just notice that all different lines from the following illustration have the same projection.