Error diffusion is a popular technique widely used in desktop printers, especially inkjet printers. However, since the conventional error diffusion is computed in raster order, it produces worm artifacts in the highlights and shadows. In addition, as a serial algorithm, it limits the efficiency and flexibility of hardware implementations. To address these two issues, we propose a novel serpentine based error diffusion algorithm that uses tone dependent error weights and thresholds. We also propose an expanded error weight location matrix to improve the halftone quality in the extreme tones. With this new algorithm, we achieve better halftones comparing to the original tone dependent fast error diffusion, especially in the quarter tones.

Halftoning is a key stage of any printing image processing pipeline. With colorant-channel approaches, a key challenge for matrix-based halftoning is the co-optimization of the matrices used for individual colorants, which becomes increasingly complex and over-constrained as the number of colorants increases. Both choices of screen angles (in clustered-dot cases) or structures, and control over how individual matrices relate to each other and result in over-versus side-by-side printing of the colorants, impose challenging restrictions. The solution presented in this paper relies on the benefits of a halftone area Neugebauer separation pipeline, where local Neugebauer Primary use is specified at each pixel and where halftoning can be performed using a single matrix, regardless of the number of colorants. The provably complete plane dependence of the resulting halftones will be presented among the solution's benefits.

The goal of this paper is to investigate the effect of choosing a certain halftone screen on image graininess development and to establish the metrics for image graininess in high-end digital printing technologies. With the obtained knowledge, our model helps us choose the optimal periodicity matrices for designing regular or irregular clustered dot halftones. The main advantage of the proposed model lies in predicting the graininess ratios based solely on the periodicity matrix. We conduct an in-depth Fourier analysis, which exposes to us all the details regarding the spectrum of a desired halftone screen. Hence, at this stage of the design process, there is no need to generate the halftone screens themselves.

Periodic clustered-dot screens are widely used for electrophotographic printers due to their print stability. However, moiré is a ubiquitous problem that arises in color printing due to the beating together of the clustered-dot, periodic halftone patterns that are used to represent different colorants. The traditional solution in the graphic arts and printing industry is to rotate identical square screens to angles that are maximally separated from each other. However, the effectiveness of this approach is limited when printing with more than four colorants, i.e., N -color printing, where N > 4 . Moreover, accurately achieving the angles that have maximum angular separation requires a very high-resolution plate writer, as is used in commercial offset printing. Commercially available high-end digital printers cannot achieve this resolution. In this paper, we propose a systematic way to design color screen sets for periodic, clustered-dot screens that offer more explicit control of the moiré properties of the resulting screens when used in color printing. We develop a principled approach for the moiré-free screen design that is called lattice-based screen design. The basic concept behind our approach is the creation of the screen set on a 2D lattice in the frequency domain, and then picking each fundamental frequency vector of the individual colorant planes in the created spectral lattice according to the desired properties. The lattice-based screen design offers more flexibility in designing N -color screen sets with different halftone geometries, and all of them are guaranteed to be all-orders moiré-free. We demonstrate the efficacy of our proposed method by introducing several new screen designs, and a comparison with published screen designs.

A general two-dimensional coding method is presented that allows recovery of data based on only a cropped portion of the code, and without knowledge of the carrier image. A description of both an encoding and recovery system is provided. Our solution involves repeating a payload with a fixed number of bits, assigning one bit to every symbol in the image - whether that symbol is data carrying or non-data carrying - with the goal of guaranteeing recovery of all the bits in the payload. Because the technique is applied to images, for aesthetic reasons we do not use fiducials, and do not employ any end-of-payload symbols. The beginning of the payload is determined by a phase code that is interleaved between groups of payload rows. The recovery system finds the phase row by evaluating candidate rows, and ranks confidence based on the sample variance. The target application is data-bearing clustered-dot halftones, so special consideration is given to the resulting checkerboard subsampling. This particular application is examined via exhaustive simulations to quantify the likelihood of unrecoverable bits and bit redundancy as a function of offset, crop window size, and phase code spacing.

This paper presents a novel method to recover planar projections (homographies) from perspectively distorted images of clustered-dot halftones. The method uses the variation of the local affine deformation of the halftone (estimated using a Fourier method) to recover the additional projective components. Evidence provided from simulated distortion and mobile capture shows that the approach provides an effective first step in the problem of using mobile device to recover stenographic information embedded in unknown halftone images, as pixel-accurate alignment is needed to recover the data.

Dot gain is a classic problem in digital printing that causes printed halftones and text to appear darker than desired. For printing of text, we propose a method to preprocess the image sent to the printer in order to compensate for dot gain. It is based on an accurate model that predicts the printed absorptance for given local neighborhood in the digital image, a cost function to penalize lack of fidelity to the desired target text image, and the use of direct binary search (DBS) to minimize the cost.

Multilevel halftoning algorithms are becoming increasingly important as the capabilities of image output devices improve. The traditional approach to multilevel halftoning is hampered by the appearance of contouring in the vicinity of native tones of the output device. To overcome this limitation, we propose a novel framework based on maintaining a consistent periodic, clustered-dot halftone texture across the tone scale. We develop metrics for granularity and structure dissimilarity, and show how these can be used to guide the manner in which the halftone texture evolves from native tone to native tone, across the tone scale. Experimental results confirm the benefits of our new approach.

In a print production system, the ability to match a printed document with its original electronic form enables services that improve robustness of the production process, such as content tracking mechanisms, or highly targeted quality assurance checks. One approach to this problem is to use overt markings that are later removed. This work, however, adopts a method that uses no markings, relying instead on fast image matching criteria. Though general image matching can be a difficult problem, constraints associated with the production environment allow construction of a simple, efficient solution to the matching problem. A reduced-reference quality assessment algorithm is presented that is used to match every document in a collection of printed and scanned documents with its original electronic form, based on down-sampling, equalization, quantization and per-pixel comparison. Using a 1085 image test set, a 100% matching rate is achievable by representing each image with only 5 bits-per-pixel at 8 dots-per-inch. The proposed method also compares favorably with other current reduced-reference quality assessment algorithms in terms of speed, storage and achieved matching rate.

In this paper, we closely scrutinize the spatial and spectral properties of aperiodic halftoning schemes on rectangular and hexagonal sampling grids. Traditionally, hexagonal sampling grids have been shunned due to their inability to preserve the high-frequency components of blue-noise dither patterns at gray-levels near one-half, but as will be shown, only through the introduction of diagonal correlations between dots can even rectangular sampling grids preserve these frequencies. And by allowing the sampling grid to constrain the placement of dots, a particular algorithm may introduce visual artifacts just as disturbing as excess energy below the principal frequency. If, instead, the algorithm maintains radial symmetry by introducing a minimum degree of clustering, then that algorithm can maintain its grid defiance illusion fundamental to the spirit of the blue-noise model. As such, this paper shows that hexagonal grids are preferrable because they can support gray-levels near one-half with less required clustering of minority pixels and a higher principal frequency. Along with a thorough Fourier analysis of blue-noise dither patterns on both rectangular and hexagonal sampling grids, this paper also demonstrates the construction of a blue-noise dither array for hexagonal grids.

"Red-eye" is a phenomenon that causes the eyes of flash photography subjects to appear unnaturally reddish in color. Though commercial solutions exist for red-eye correction, all of them require some measure of user intervention. A method is presented to automatically detect and correct redeye in digital images. First, faces are detected with a cascade of multi-scale classifiers. The red-eye pixels are then located with several refining masks computed over the facial region. The masks are created by thresholding per-pixel metrics, designed to detect red-eye artifacts. Once the redeye pixels have been found, the redness is attenuated with a tapered color desaturation. A detector implemented with this system corrected 95% of the red-eye artifacts in 200 tested images.

The importance of enlarging and reducing two-level images such as graphical and documentary matter by digital means continues to grow as more such images are digitally represented. A nonlinear scaling scheme is devised which exploits the simplicity of this binary nature, treating images logically instead of arithmetically; a convolution-like effect is achieved without a single addition or multiplication! This method yields high-fidelity digital scaling and meets the objectives of being fast, conducive to hardware realization, and void of special pre-encoding requirements.