The cause of the paradox of impossible objects is a shifting of the optical fixed point of the picture. This can be seen easily by the figure below. Let us assume a plane S intersecting the tribar. If we cover the part of the object being below the plane and sketch the intersections with S, we get the information that bar 1 is nearer to us than bar 2. If we cover the parts above the plane, we get the opposite result: We are dealing with an impossible object.
