A method for designing a three-dimensional, 3D, display apparatus including a display panel and an optical layer, providing a 3D image using an optical system including a mirror and/or a lens, the method comprising: determining parameters comprising a pitch between adjacent pairs of pixels of the display panel, an optical viewing distance, and a gap between the display panel and the optical layer; determining a pitch between adjacent pairs of optical elements of the optical layer based on the determined parameters; wherein a relationship between a pitch between adjacent pairs of the optical elements of the optical layer, a number of viewpoints represented by a single optical element of the optical layer, a pitch between adjacent pairs of the pixels of the display panel, an optimum viewing distance and a gap between the display panel and the optical layer is expressible based on the following equation: P opt NP pix = 1 + d gap d view , when the optical layer is provided in front of the display panel, or P opt NP pix = 1 − d gap d view , when the optical layer is provided behind the display panel, wherein P opt denotes the pitch between the adjacent pairs of optical elements, N denotes the number of pixels emitting rays to a single optical element of the optical layer, P pix denotes the pitch between the adjacent pairs of the pixels, d view denotes the optimum viewing distance, and d gap denotes the gap between the display panel and the optical layer; wherein the optimum viewing distance corresponds to a distance between the display panel and an optical image acquired by applying an optical transformation of the optical system to a predetermined viewpoint of a user. determining parameters comprising a pitch between adjacent pairs of pixels of the display panel, an optical viewing distance, and a gap between the display panel and the optical layer; determining a pitch between adjacent pairs of optical elements of the optical layer based on the determined parameters; wherein a relationship between a pitch between adjacent pairs of the optical elements of the optical layer, a number of viewpoints represented by a single optical element of the optical layer, a pitch between adjacent pairs of the pixels of the display panel, an optimum viewing distance and a gap between the display panel and the optical layer is expressible based on the following equation: P opt NP pix = 1 + d gap d view , when the optical layer is provided in front of the display panel, or P opt NP pix = 1 − d gap d view , when the optical layer is provided behind the display panel, P opt NP pix = 1 + d gap d view , when the optical layer is provided in front of the display panel, or P opt NP pix = 1 − d gap d view , when the optical layer is provided behind the display panel, wherein P opt denotes the pitch between the adjacent pairs of optical elements, N denotes the number of pixels emitting rays to a single optical element of the optical layer, P pix denotes the pitch between the adjacent pairs of the pixels, d view denotes the optimum viewing distance, and d gap denotes the gap between the display panel and the optical layer; wherein the optimum viewing distance corresponds to a distance between the display panel and an optical image acquired by applying an optical transformation of the optical system to a predetermined viewpoint of a user. <EOS>