I. Introduction
Recently, a number of researchers are involved in development of natural computing systems which employ physical, chemical and biological phenomena as the direct media for manifesting the computation [1]–[3]. Chiefly, studies handling the lights as key components of computation attracted the scientists and engineers since these studies are potentially applicable to the signal processing through optical interconnections between electronic devices [4], [5]. Among optical computing protocols, the optical Fourier transformation is one of successful examples of the analog photo-computing [6]. On the other hand, digital computing using photonics (digital photo-computing) has also been proposed. For an instance, the optical parallel logic gates proposed by Tanida and Ichioka [7]–[9] allow spontaneous and parallel computing by simple spatial coding with lights by overlaying a pair of images A and B positioned between the light sources and screen. The resultant images displayed on the screen gives an array of solutions to the given sets of parallel operations.