Digital halftoning and the physical reconstruction function

Originally presented as author's thesis (Ph. D.--Massachusetts Institute of Technology), 1986.Bibliography: p. 397-405."This work has been supported by the Digital Equipement Corporation."by Robert A. Ulichney

Digital Color Imaging

This paper surveys current technology and research in the area of digital color imaging. In order to establish the background and lay down terminology, fundamental concepts of color perception and measurement are first presented us-ing vector-space notation and terminology. Present-day color recording and reproduction systems are reviewed along with the common mathematical models used for representing these devices. Algorithms for processing color images for display and communication are surveyed, and a forecast of research trends is attempted. An extensive bibliography is provided

Prediction of screener-induced moire in digital halftone pattern generation

In the graphic arts, objectionable moire patterns are often observed on films or printed products due to the interaction of various periodic structures of halftone images. A particular type of moire pattern that results from digital halftoning at arbitrary angles and frequencies using a virtual screen function has been studied. A computer program was developed that produces uniform digital halftone patterns using a virtual screen approach and that calculates the corresponding amplitude spectra. It was found that aliasing due to the sampling of the virtual screen causes low frequency components in the amplitude spectrum. Moire patterns with fundamental vector frequencies equal to those of the strong aliased components were observed in halftone images reconstructed on a film recorder. Moire was also observed at frequencies not represented or under represented in the amplitude spectrum. It is shown that this moire effect is due to the additive beating of two or more higher frequency components that differ by the frequency of the observed moire. It is suggested that the non-linearities of the film recording process amplify this effect . The effects on the resulting moire patterns of varying the halftone parameters of dot size, dot shape, screen angle, and screen frequency were examined. In general, the amplitude spectra are complex, indicating many overlapping patterns. Screener induced moire was found to behave in a nearly identical manner to that induced by digital scanning of an existing halftone

Neural network for optimization of binary computer-generated hologram with printing model

Hopfield neural nets are used to optimize point-oriented binary computer-generated holograms (CGHs). It can be considered as a parallel and iterative \u27halftoning\u27 process in the spatial frequency domain. The results are comparable to other iterative methods but require shorter computation times. In this process, the generation of the CGH by FFT, binarization, and IFFT is viewed as a black box with inputs and outputs consisting of 512 arrays containing an object of size 64 . The neural-network optimization feeds back the Fourier transform of the reconstruction error to update the neuron states, which correspond to the samples of the continuous hologram. To reduce the error of the reconstruction, the input is allowed to deviate from the original array in different specified ways. For example, a previously reported approach using Projection Onto Constraint Sets (POCS) varied only the region of the input array outside the object, while we allow the entire array to be modified, thus providing more freedom in the optimization. The method may be applied either to magnitude-only or phase-only holograms. A modification of the parallel updating function is also reported. Different optimization options are compared. Use of a practical printing model requires optimization under assumed constraints to test the convergence properties of the algorithm

Digital halftoning using fibonacci-like sequence pertubation and using vision-models in different color spaces

A disadvantage in error diffusion is that it creates objectionable texture patterns at certain gray levels. An approach, threshold perturbation by Fibonacci-like sequences, was studied. This process is simpler than applying a vision model and it also decreases the visible patterns in error diffusion. Vector error diffusion guarantees minimum color distance in binarization provided that a uniform color space is used. Four color spaces were studied in this research. It was found that vector error diffusion in two linear color spaces made no reduction in the quality of halftoning compared with that in CIEL*a*b* or CIEL*u*v* color spaces. A luminance vision MTF and a chroma vision MTF were used in model-based error diffusion to further improve the halftone image quality