An Approach towards Measurement of Color Shifting in Misregistration Print Defect using Euclidean and Manhattan Distance Metrics

Misregistration print defect occurs during offset printing affects the visual appearance of printed image. Registration refers to the proper superimposition of colors whereas improper alignment or shifting of colors is resulted as blurred image. Generally registration marks is used in printed sheet to detect this kind of print problem manually. In the presented study a computer vision technique is applied to detect and quantify the problem. Euclidean distance and Manhattan distance measurement method is applied for quantification of color shifting. Therefore this presented study is a novel approach in printing industry which can be a substitute of the usual human perception based method. &nbsp

An Ink Spreading Model for Dot-On-Dot Spectral Prediction

Due to increasing printing accuracies and the possibility of printing several droplets at the same pixel location, there is a renewed interest in dot-on-dot printing models. In the present contribution, we improve a dot-on-dot spectral prediction model relying on the Yule-Nielsen modified Spectral Neugebauer model by taking into account ink spreading in all ink superposition conditions. Since ink spreading is different when ink dots are printed alone, printed in superposition with one ink or printed in superposition with two inks, we create for each superposition condition an ink spreading function mapping nominal to effective dot surface coverages. When predicting the reflection spectrum of a dot-on-dot halftone patch, its known nominal surface coverage values are converted into effective coverage values by weighting the contributions from different ink spreading functions according to the corresponding ratio of colorant surface coverages. We analyze the colorimetric prediction improvement brought by our ink spreading model for dot-on-dot thermal transfer prints and for ink-jet prints. Accounting for ink spreading according to different ink superposition conditions considerably improves the prediction accuracy. In the case of ink jet prints at 120 lpi, the mean DeltaE_94 difference between predictions and measurements is reduced from 4.54 to 1.55 (accuracy improvement factor: 3). Due to the slight misregistration between the ink layers, spectral predictions accounting for ink spreading in the case of dot-on-dot screens are less accurate than corresponding predictions for classical mutually rotated screens

Modeling and Halftoning for Multichannel Printers: A Spectral Approach

Printing has been has been the major communication medium for many centuries. In the last twenty years, multichannel printing has brought new opportunities and challenges. Beside of extended colour gamut of the multichannel printer, the opportunity was presented to use a multichannel printer for ‘spectral printing’. The aim of spectral printing is typically the same as for colour printing; that is, to match input signal with printing specific ink combinations. In order to control printers so that the combination or mixture of inks results in specific colour or spectra requires a spectral reflectance printer model that estimates reflectance spectra from nominal dot coverage. The printer models have one of the key roles in accurate communication of colour to the printed media. Accordingly, this has been one of the most active research areas in printing. The research direction was toward improvement of the model accuracy, model simplicity and toward minimal resources used by the model in terms of computational power and usage of material. The contribution of the work included in the thesis is also directed toward improvement of the printer models but for the multichannel printing. The thesis is focused primarily on improving existing spectral printer models and developing a new model. In addition, the aim was to develop and implement a multichannel halftoning method which should provide with high image quality. Therefore, the research goals of the thesis were: maximal accuracy of printer models, optimal resource usage and maximal image quality of halftoning and whole spectral reproduction system. Maximal colour accuracy of a model but with the least resources used is achieved by optimizing printer model calibration process. First, estimation of the physical and optical dot gain is performed with newly proposed method and model. Second, a custom training target is estimated using the proposed new method. These two proposed methods and one proposed model were at the same time the means of optimal resource usage, both in computational time and material. The third goal was satisfied with newly proposed halftoning method for multichannel printing. This method also satisfies the goal of optimal computational time but with maintaining high image quality. When applied in spectral reproduction workflow, this halftoning reduces noise induced in an inversion of the printer model. Finally, a case study was conducted on the practical use of multichannel printers and spectral reproduction workflow. In addition to a gamut comparison in colour space, it is shown that otherwise limited reach of spectral printing could potentially be used to simulate spectra and colour of textile fabrics

Quantitative Evaluation of Misregistration Induced Color Shifts in Color Halftones

Color-to-color misregistration refers to misregistration between color separations in a printed or display image. Such misregistration in printed halftoned images can result in several image defects, a primary one being shifts in average color. The present paper examines the variation in average color for two-color halftoned images as a function of color-to-color misregistration distance. Dot-on-dot/dot-off-dot and rotated dot screen configurations are examined via simulation and supported by print measurements. The color and color shifts were calculated using a spectral Neugebauer model for the underlying simulations. As expected, dot-on-dot/dot-off-dot color shifts were very high, while rotated dots screens exhibited very little color shift under the present idealized conditions. The simulations also demonstrate that optical dot gain significantly reduces the color shifts seen in practice