1. A method of forming pixel-cell in a three-dimensional image display system consisting of an image display panel having a plurality of unit patterns arranged in a vertical direction and horizontal direction and an optical plate having repetitive pattern, said method comprising: determining each side of rhombus pattern having an arbitrary vertex angle as straight line having a predetermined slant; calculating a number of unit patterns in vertical direction and a number of unit patterns in horizontal direction, both corresponding to the predetermined slant; approximating the predetermined slant to stepped patterns with the number of unit patterns in vertical direction and the number of unit patterns in horizontal direction; and forming the pixel cell by use of said stepped patterns, wherein at least one integer-numbered unit pattern is arranged in the pixel cell.