Abstract
When we observe a three-dimensional (3D) object such as a table, its contour along the gaze line is dramatically shortened on the retinal image. This phenomenon is called perspective foreshortening. The visual system compensates for the foreshortening, at least partly, in order to achieve valid object shape perception in 3D space. This compensation seems automatic and sometimes causes visual illusion. In the Shepard illusion, for example, the two parallelograms identical in shape are seen as two different shapes because of the compensation. Here I report a novel illusion of this kind. The outline of a trapezium, without internal details, does not induce apparent depth perception. When two spindle shapes are drawn inside, the figure is seen as a foreshortened rectangle with surface texture. The spindles are seen as two foreshortened disks, which indicates that they provide an apparent context for perspective foreshortening. The length of the side line of the trapezium with spindles is perceived to be longer in comparison to the length of the side line of an identical trapezium without spindles. Surface texture affects shape perception. The former figure is similar to a two-circle mahjong tile. Two experiments that included a magnitude estimation procedure and a method of adjustment were conducted to confirm the illusion. The contrast between the two figures confirmed the illusion. The illusion was found either in realistic perspective line-drawings or in schematized figures. The illusory effect was not fully accounted for by the Oppel-Kundt illusion or the illusory perception of Helmholtz's square.