Optical information processing can perform a myriad of signal processing operations. It is primarily due to the complex amplitude processing capability. Optical information processors are capable of performing one- or two-dimensional operations using simple linear operators. However, it would not happen without the support of coherent property of illumination. In this paper we shall discuss the inherent optical complex transformation, by which information data processing can be exploited. Fourier and spatial domain processing, composite filtering, 3-D pattern recognition, larger capacity photo-refractive processing optical neural computing and others will be stressed.