Ján Morovic

Accurate 3D image colour histogram transformation

A method for transforming an images 3D colour histogram to make it accurately match a predetermined target state is described here. This method involves colour quantisation, clustering and the EMD histogram difference metric to provide a transformation LUT between original and target histograms.

A fast, non-iterative and exact histogram matching algorithm

A fast, non-iterative algorithm is presented for transforming an image so as to give it exactly a given target histogram. This is achieved for any original and target histogram combinations and examples of results are given in addition to a comparison with applying the earth movers distance (EMD) method to this task.

Evaluating Gamut Mapping Algorithms for Universal Applicability

The aim of this article is to present the evaluation of gamut mapping algorithms (GMAs) in a series of three experiments intended to serve as the basis for developing solutions that are accurate and universally applicable. An evolutionary gamut mapping development strategy is used, in which five test images are reproduced between a CRT and printed media obtained using different GMAs. Initially, a number of previously published algorithms were chosen and psychophysically evaluated, whereby an important characteristic of this evaluation was the separate evaluation for individual colour regions within test images. New algorithms were then developed on this experimental basis, subsequently evaluated, and the process was repeated once more. In this series of experiments, the new GCUSP algorithm, which consists of a chroma-dependent lightness compression followed by a compression towards the lightness of the reproduction cusp on the lightness axis, gave the most accurate and stable performance overall. The results of these experiments were also useful for improving the understanding of some gamut mapping factors—in particular gamut difference between media.

Large color gamut displays with diffraction gratings

The ability to display a broad variety of colors has great benefits not only in the context of entertainment but also as a means to streamline design in prototyping and manufacturing processes. Displays that use RGB filters or backlights cannot span all colors that occur in nature. To improve the accuracy of color reproduction, there have been attempts to include additional color primaries in displays. Existing solutions, however, have an impact on cost, scalability, and spatial resolution and are predominantly applicable to projection systems. We propose an approach based on combining diffraction grating extractors and the HANS imaging pipeline initially developed for printing. This combination offers unprecedented potential to attain large color gamuts with the same backlights commercially used today.

Gamuts of input and output color imaging media

The color gamuts of color imaging media are important parameters in the reproduction of color images between them and their assumed magnitudes directly influence the degree to which colors are modified. In spite of this, the determination of gamut boundaries is often done in a way that ignores some basic implications that follow from the definition of color gamuts. This is partly due to the fact that some of these implications are not understood and partly due to the fact that if they are understood their magnitude is underestimated. Hence, the approach that is taken in this paper is to first discuss the theoretical implications of what color gamuts are and subsequently to illustrate them by experimental means..

Direct Pattern Control Halftoning of Neugebauer Primaries

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.

Evaluation of colour fidelity for reproductions of fine art paintings

This paper describes the results of experiments carried out at the National Gallery in London, comparing original paintings with printed colour reproductions made by processing digital images captured by a high resolution digital camera. A pairwise comparison method was used for three paintings under several different illumination conditions with a panel of observers. The results indicate that the optimum choice of colour gamut mapping procedure depends on the pictorial content of the painting.

Computational Print Control

Printing may seem like a dinosaur among today’s imaging technologies, since its roots stretch back to Becquerel’s work that lead to the first color photographs and the first mechanical color reproduction at the end of the nineteenth century. Ten years ago, we then made the fundamental discovery of a new print control domain, where instead of choices about colorant amounts that are akin to the effect of color filters used since the beginning of color printing, print can be specified by the probabilities of colorant combinations, the Neugebauer Primaries. This has led to the ability to print patterns that were previously inaccessible and consequently, by using large-scale computational optimization, to delivering more color gamut, greater ink use efficiency and greater sharpness and detail in print, while using the same materials and printing system as before. This keynote will present the basic principles of the HANS print control paradigm, review the highlights of results obtained using it to date and indicate its potential future developments.

Color characterization of cine film

This paper describes the characterization of cine film, by identifying the relationship between the Status A density values of positive print film and the XYZ values of conventional colorimetry. Several approaches are tried including least-squares modeling, tetrahedral interpolation, and distance weighted interpolation. The distance weighted technique has been improved by the use of the Mahalanobis distance metric in order to perform the interpolation, and this is presented as an innovation.

3D histograms in color image reproduction

Previous work on color reproduction has shown that all existing solutions perform in ways that are image dependent. A series of experiments has therefore been carried out to systematically study the influence of a range of image characteristics on color reproduction and it was previously shown that neither image gamuts nor one-dimensional color histograms play an important role. The aim of the present paper then is to investigate whether the 3D histogram of images colors is an image characteristic that significantly influences the performance of gamut mapping algorithms (GMAs). As this is done with the help of sets of artificial images, where each member of the set has different content but the same 3D color histogram, their use in studying color reproduction is also discussed in this paper. The results of a psychophysical experiment evaluating the influence of 3D color histograms on color reproduction are presented and analyzed in detail. These results show clearly that a significant proportion (approximately 2/3 to 3/4) of the variation in GMA performance caused by differences in image characteristics is due to 3D color histograms.