EMPA Media Technology

Publications 2004

Assessment of RGB Encoding for Color Imaging

Ú. Steingrímsson, P. Zolliker

This work presents validation measures for the optimization of RGB color data encoding. Various RGB specifications had been promoted in the past for standard use in imaging, most of them adapted to application or equipment specific requirements. Although some, as sRGB for instance, are already widely used, none of them has been accepted yet as a universal standard. All of them are subject to limitations regarding to the spatial scope of the color space, making it impossible to satisfy any required conditions in general.

At present, a big variety of different RGB specifications for digital imaging might easily lead to color misinterpretations, because the specific RGB specifications are often missing in the image data. If either the locations of the primary colors or the gamma values are mixed up, color deficiencies are unavoidable. Since the acceptance of a global standard could help to eliminate potential confusions, attempts for the promotion of universal RGB specifications are frequently accomplished.

In the present study, the specific requirements of an RGB in respect of the coding range are examined. A concept for quantitative examination of the encodable color space extent considering the visually perceivable resolution is proposed. Different types of parameters are gathered. First, the fraction of all physically feasible surface colors contained in a certain RGB space is accounted. Next, the visibility of the quantization due to the restricted resolution of the RGB coding is measured in terms of ∆E76. Finally, some widely used RGB definitions are analyzed and assessed on the basis of this evaluation scheme.

10. Workshop Farbbildverarbeitung 2004, Koblenz, PDF

RGB Gamuts

Quality Assessment of the JPEG 2000 Compression Standard

Ú. Steingrímsson, K. Simon

Although the new JPEG 2000 data compression method has not yet truly reached the market, it has already achieved an unusually high level of attention in the industry. Since a new standard stands for major investments, JPEG 2000 has to offer convincing benefits for compensating the financial risks. In fact, compression rates of 1:200 or higher represent breakthroughs in image compression. In addition, the new data format supports a series of functions that are of major potential interest for software engineering. Ultimatively, however, the visual attributes of JPEG 2000 are of utmost importance. While the visually perceived image quality is required to be still acceptable at highest compression rates for internet imaging purpose, it is at the same time expected to satisfy the topmost requirements at lower rates, particularly for the photography and the graphic arts industry. In an extensive empirical study, the question is investigated as to whether the high expectations of JPEG 2000 in respect to visual quality are justified. The paper gives an overview of the effected tests and presents the conclusions.

CGIV 2004 -- Second European Conference on Color in Graphics, Imaging and Vision Aachen, Germany;April 2004; p. 337-342; ISBN / ISSN: 0-89208-250-X

Test Images

Three-dimensional analysis of porous BaTiO3 ceramics using FIB nanotomography

L. Holzer, F. Indutnyi, Ph. Gasser, B. Münch, M. Wegmann

Three-dimensional (3D) data represent the basis for reliable quantification of complex microstructures. Therefore, the development of high-resolution tomography techniques is of major importance for many materials science disciplines. In this paper, we present a novel serial sectioning procedure for 3D analysis using a dual-beam FIB (focused ion beam). A very narrow and reproducible spacing between the individual imaging planes is achieved by using drift correction algorithms in the automated slicing procedure. The spacing between the planes is nearly of the same magnitude as the pixel resolution on scanning electron microscopy images. Consequently, the acquired stack of images can be transformed directly into a 3D data volume with a voxel resolution of 6 x 7 x 17 nm. To demonstrate the capabilities of FIB nanotomog raphy, a BaTiO3 ceramic with a high volume fraction of fine porosity was investigated using the method as a basis for computational microstructure analysis and the results compared with conventional physical measurements. Significant differences between the particle size distributions as measured by nanotomography and laser granulometry indicate that the latter analysis is skewed by particle agglomeration/aggregation in the raw powder and by uncertainties related to calculation assumptions. Significant differences are also observed between the results from mercury intrusion porosimetry (MIP) and 3D pore space analysis. There is strong evidence that the ink-bottle effect leads to an overestimation of the frequency of small pores in MIP. FIB nanotomography thus reveals quantitative information of structural features smaller than 100 nm in size which cannot be acquired easily by other methods.

Journal of microscopy 2004 Oct;216(Pt 1):84-95.

Microscipic structure

Perceptive Quality Estimations: JPEG 2000 versus JPEG

Ú. Steingrímsson, K. Simon

The upcoming JPEG 2000 image compression standard is expected to replace the well-established JPEG format. It offers a wide range of data compressions, from lossless up to highest rates of lossy compression, in conjunction with a variety of useful new features which will presumably increase its popularity. Confident press reports suggest breakthroughs in image compression prospects. However, in the final analysis the most decisive factor in JPEG 2000’s commercial acceptance will be its visual quality attributes. Accordingly, this article presents extensive estimations of the visually perceived image quality of JPEG 2000 compressions in comparison with JPEG. The study focuses exclusively on lossy data compression and its default settings. From the viewpoint of quality assessments commonly used in graphic arts industry, we present different types of interactive quality ratings applied to large data sets for characterizing image distortions at varying compression rates. In addition, the results are compared with PSNR considerations. The objective of this study is to arrive at an estimation of the potential benefits of JPEG 2000 relative to the JPEG standard with respect to compression quality, and in particular with regard to the requirements of the graphic arts industry.

Journal of Imaging Science and Technology, nov./déc. 2003, vol.47, n°6, p.572-585

Test set

Publications 2003

Color Predicting Model for Electrophotographic Prints on Common Office Paper

S. Mourad

Digital color desktop printers are widely used in modern offices. However, the ability of the printers to reproduce input colors faithfully is limited and they need to be recalibrated frequently. This is a time-consuming and cumbersome process, which usually needs to be carried out manually.

In order to simulate the interaction of light and color prints and to facilitate the calibration process, this thesis proposes new models for digital color printers and more specifically for dry toner electrophotographic printers. The proposed mathematical models describe the main contributing physical phenomena and offer support for the closed loop control of color halftone printers.

A first model concerns the phenomenon of light scattering within paper and takes into account the fluorescence of brightened office papers. Given the spectral point transmittance of a halftone patch, the model estimates the spectral point reflectance. It is verified by spectral microscopic measurements and is able to estimate the optical dot gain in halftone prints and its effect on the color reproduction curve of common printed papers.

A second model simulates the behavior of the electrophotographic printing process. Starting from the input bitmap of a color halftone, the simulation model computes the microstructures of the toner deposition on a given printing substrate. Assuming non light scattering toners, the obtained toner relief is transformed into the spectral point transmittance required by the first model.

For an electrophotographic printer, both models together allow an accurate estimate of the spectral color response and its description as a function of given input halftoned color separation layers.

Link to EPFL, PDF

Electrostatic field

Publications 2002

A Stochastic Interpretation of Kubelka-Munk


Understanding reflection is one of the key competences in graphic arts industry. A very popular approach was given by Kubelka-Munk who derived a simple relationship between the scattering and absorption coefficients and the overall reflectance. In the course of time, the theory was extended by several authors ... for recent improvements. This paper presents an alternative approach which describes the behavior of light in matter as a random walk. In this respect Kubelka-Munk is closely correlated to both well-known stochastic theories and recent combinatorial research, in particular with catalan numbers.

2.–5. April 2002, First European Conference on Colour in Graphics, Imaging and Vision, CGIV, Poitiers, p. 468–472


Prediction of Monochrome Reflectance Spectra with an Extended Kubelka-Munk Model

S. Mourad, P. Emmel, K. Simon, R.D. Hersch

We propose a prediction model of digital printers and more specifically for electrophotographic devices. On the one hand, we propose an electrophotographic simulation model which estimates the microscopic structure of any printed toner layer based on its input halftone bitmap. Applying Bouguer-Beer-Lamberts law, the obtained spatial toner arrangement yields the spectral transmittance distribution for non-light scattering colors. On the other hand, we introduce an extension to the Kubelka-Munk (KM) model, which allows to compute the halftone reflectance spectra from the estimated transmittance spectra. The extended KM model bridges the gap between the mathematical description of the optical point spread function of common office papers and the experimental results of simple reflectance measurements. With the combination of the models, we are capable of predicting the reflectance spectra of a printed monochrome wedge with a mean estimation error of less than CIELAB DeltaE 94 = 1.

Tenth Color Imaging Conference: Color Science and Engineering Systems, Technologies, Applications Scottsdale, Arizona;November 12, 2002; p. 298-204; ISBN / ISSN: 0-89208-241-0

Extended Kubelka-Munk

Publications 2001

Extending Kubelka-Munk's Theory with Lateral Light Scattering

S. Mourad, P. Emmel, K.Simon, R.D. Hersch

Due to its simplicity, the theory of Kubelka-Munk has found a wide acceptance for modeling the optical properties of light scattering materials. However, the concept is not explicitly adapted to predict halftone prints on paper. In this respect, a recent improvement was given by Berg. Our approach is an extension of Berg's model in order to reduce the gap between the mathematical description of the paper's point spread function and the experimental results of simple reflectance measurements.

Proc. IS&T NIP17: Int'l Conf. on Digital Printing Technologies, Ft. Lauderdale, USA, 2001, 469-473

Lateral Scattering

Publications 2000

Predicting Monochrome Color Transmittance Spectra of Electrophotographic Prints

S. Mourad, P. Emmel, R.D. Hersch

We create a computer based numerical model to predict the color spectra of printed patches on dry toner electrophotographic printers. The goal of this research is to obtain a simplified model describing the input-output behavior of the printers based on the physical characteristics of the different printing process steps and the interactions between them. This leads to a better understanding of the factors that have an impact on printing quality. Furthermore, by modeling the non-linearities of the electrophotographic process, the prediction model will allow the creation of device calibration data with a minimal effort.

In order to avoid the additional optical non-linearities produced by light reflections on paper (dot-gain), we have limited the present investigation to transparency prints. In its current version, the proposed model is capable of predicting the transmittance spectra of a printed monochrome wedge down to a mean deviation less than CIELAB ∆Eab = 1.5.

The proposed simulation incorporates sub-models of the electrophotographic process [1, 2] shown in Fig. 1, such as the exposure of the photoreceptor, the generated electrostatic field, the toner’s charge and diameter distributions, as well as the transfer and the fusing steps.

IS&T's NIP16: pages 862-866.

Transfer process