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Blue-noise dither halftoning methods have been found to produce images with pleasing visual characteristics. Results similar to those generated with error-diffusion algorithms can be obtained using an image processing algorithm that is computationally much simpler to implement. This paper reviews and compares the various techniques that have been used to design blue-noise dither matrices. In particular, a series of visual cost function based methods, a several techniques that involve designing the dither matrices by analyzing the spatial dot distribution are discussed. Ways to extend the basic blue-noise dither techniques to multilevel and color output devices are also described, including recent advances in the design of jointly optimized color blue-noise dither matrices.

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An investigation into methods for achromatizing the coupling angle characteristics of waveguide input/output couplers is described. The basic approach involves correcting the inherent angular dispersion of conventional waveguide couplers with a diffraction grating. Two configurations are analyzed in detail: a hybrid prism/grating coupler and a double grating coupler. Expressions are derived for values of the grating parameters that produce achromatic coupling. A method is also presented to predict the achromatic wavelength range and maximize it with the available degrees of freedom. For a coupling angle tolerance of 0.005 degrees , it is found that with double grating couplers achromatic wavelength ranges of the order of 10 nm can be obtained, and that with prism/grating couplers this range can be as large as 200 nm.

Multilevel halftoning (multitoning) is an extension of bitonal halftoning, in which the appearance of intermediate tones is created by the spatial modulation of more than two tones, i.e., black, white, and one or more shades of gray. In this paper, the conventional multitoning approach and a previously proposed approach, both using stochastic screen dithering, are investigated. A human visual model is employed to measure the perceived halftone error for both algorithms. The performance of each algorithm at gray levels near the printer's intermediate output levels is compared. Based on this study, a new overmodulation algorithm is proposed. The multitone output is mean preserving with respect to the input and the new algorithm requires little additional computation. It will be shown that, with this simple overmodulation scheme, we will be able to manipulate the dot patterns around the intermediate output levels to achieve desired halftone patterns. Implementation issues related to optimal output level selection and inkjet-printing simulation for this new scheme will also be reported.