Computer-aided design of dither signals for binary display of images

The hybrid screen--improving the breed

The hybrid screen is a halftoning method that generates stochastic dispersed dot textures in highlights and periodic clustered dot textures in midtones. Each tone level is sequentially designed from highlight to midtone by applying an iterative halftoning algorithm such as direct binary search (DBS). By allowing random seeding followed by swap-only DBS in a predefined core region within each microcell, each level can be designed while satisfying the stacking constraint and guaranteeing a smooth transition between levels. This paper describes a complete design process for the hybrid screen, introduces a number of enhancements to the original hybrid screen, and evaluates their impact on print quality. These enhancements include a multilevel screen design based on either extending a bilevel screen or directly generating a multilevel screen on a high resolution grid, and extending the hybrid screen design procedure to color by jointly optimizing the color screens using color DBS. For the multilevel screen, we show that the best choice for the core size critically depends on the bit depth and screen frequency. For the color hybrid screen, we demonstrate a significant improvement in the highlights over halftones generated by independently designed screens.

Optimized halftoning using dot diffusion and methods for inverse halftoning

Unlike the error diffusion method, the dot diffusion method for digital halftoning has the advantage of pixel-level parallelism. However, the image quality offered by error diffusion is still regarded as superior to most of the other known methods. We show how the dot diffusion method can be improved by optimization of the so-called class matrix. By taking the human visual characteristics into account we show that such optimization consistently results in images comparable to error diffusion, without sacrificing the pixel-level parallelism. Adaptive dot diffusion is also introduced and then a mathematical description of dot diffusion is derived. Furthermore, inverse halftoning of dot diffused images is discussed and two methods are proposed. The first one uses projection onto convex sets (POCS) and the second one uses wavelets. Of these methods, the wavelet method does not make use of the knowledge of the class matrix. Embedded multiresolution dot diffusion is also discussed, which is useful for rendering at different resolutions and transmitting images progressively.

Look-up-table based halftoning algorithm

Screening is a low complexity halftoning algorithm that has been widely used in many applications. However, screen design requires that the stacking property be obeyed. This constraint limits the texture quality at each gray level. We present a look-up-table based halftoning algorithm for which the stacking constraint is not necessarily satisfied; but the binary patterns for individual levels are still correlated. The binary patterns are designed level by level using the direct binary search method. The algorithm improves halftone image quality compared with screening.

Binary display of images when spot size exceeds step size

Many binary digital devices for display or printing of images are capable of placing spots of size S at an array of addressable points spaced R apart where S < R. A new representation is described for display or printing of continuous-tone images with such devices for which S/R < 2. The presentation takes into account spot overlap to assure correct tone reproduction, minimize pattern visibility, and maximize detail rendition. The algorithm is a modification of ordered dither that utilizes in one direction the subspot size addressibility of the device to increase the number of available levels of tone per unit area and enhance the good visual properties of the dither matrix. While not implementable by thresholding, the algorithm may be implemented by a nearly point-to-point spatially periodic mapping from continuous-tone image-to-device input. Fourier analysis provides a theoretical basis for the representation. Experimental work is presented that confirms the results of the analysis.