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Høgskolen i Gjøviks rapportserie, 2004 nr. 9

Proceedings fromGjøvik Color Imaging Symposium 2004

Jon Y. Hardeberg and Peter Nussbaum (editors)Department of Computer Science and Media Technology

Gjøvik University College

Gjøvik 2004ISSN 0806-3176

Contents

Introduction 3

Nussbaum: Introduction to Color Management Workflow 4

Holmquist: The KDI Project 5

Bando: Predicting Visual Image Degradation 6

Nussbaum: Quality as defined by the Customer 7

Antoine: Objective print quality assessment of complex images 8

MacDonald: Research challenges for the graphic arts industry 9

Preus: The primary sisters in the age of digitalism 10

Brenden: Improving print-quality by using FM screening 11

Nurmi: RGB Workflow and standardization 13

Hardeberg: Introduction to Multispectral Color Imaging 14

Connah: Multispectral imaging: How many sensors do we need? 15

Alsam: Recovering Natural Reflectances With Convexity 16

Marin: Usage of a liquid crystal tunable filter for multispectral imagery 18

Gerhardt: Spectral Inkjet Reproduction 19

Bakke: Multispectral Gamut Mapping 20

Lenz: Fast color-based browsing of large image databases 21

Westland: Characterization of color cameras 23

Hardeberg: Challenges for projection display tiling 24

Strand: Color Image Quality in Projection Displays 26

MacDonald: Projection displays and colour appearance 27

MacDonald: Digitization of stained glass windows - the VITRA project 28

Schmitt: Crisatel Multispectral Imaging System 29

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Introduction

For the second consecutive year Gjøvik University College and The Norwegian ColorResearch Laboratory organized an international symposium on color imaging. GjøvikColor Imaging Symposium 2004 took place November 4 and 5, 2004, at Gjøvik UniversityCollege in Gjøvik, Norway.

The first day of the conference focused on color and quality in paper based media, andwas targeted mainly at the graphic arts industry. Invited speakers talked about varioussubjects such as color managed workflow, image quality, and FM halftoning. Amongthe speakers we found Professor Lindsay MacDonald from London College of Commu-nications, Leif Preus from Preus Foto, and Morgan Brenden from Hjemmet MortensenTrykkeri.

The second day was more oriented towards color imaging research, and you couldhear speakers from Norway, Sweden, France, and England present their research in areassuch as multispectral image capture, spectral reproduction, content based search in imagedatabases, art imaging, and projection display.

During the symposium we also carried out the official opening of the new ColorLaboratory, and colorful demonstrations were given by Color Lab students and staff.

For more information about the conference, including copies of the visuals for selectedpresentations, please refer to http://www.colorlab.no.

Dr. Jon Y. Hardeberg, Conference Chair

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Introduction to Color Management WorkflowPeter Nussbaum

The Norwegian Color Research LaboratoryDept of Computer Science and Media Technology

Gjøvik University College (Norway)

Although a big numbers of papers and publications can be found on the topic colormanagement and already various publishers have implemented color management, it hasbeen concluded that the subject still is a very complex issue.

This paper aims to provide an introduction to color management workflow. In par-ticular it looks at the needs for color management, the main variables, which causesdifferent colors on different devices and describes the process of making device profiles.The relationship between device specific color models and device independent color mod-els are fundamental for the understanding of color management. The basic componentsof color management, such as the profile connection space, device profiles and render-ing intents are described. Considering a color space conversion the color managementsystem performs a series of steps according to the applied device profiles. Neverthelessthe distinction between device calibration versus device characterization are an essentialpart of the study. Consequently the process of generating profiles for input and out-put devices will be explained. Furthermore, the paper provides suggestions for buildingcolor-managed workflow that suits the production requirements.

Although there are various factors involved, which affect the color appearance onreproduction mediums the performance of the device profiles are determined. Thereforethe accuracy of a color from an input, to display, to printed image in a color managementsystem depends primarily on the quality of the profiles involved. Finally the studydescribes tests for the colorimetric accuracy of camera, display and printer profiles.

Biography

Peter Nussbaum obtained his MSc in imaging science from the Colour & Imaging Insti-tute, University of Derby, GB in 2002. The MSc thesis had to investigate the factors andtheir impact affecting the appearance of print. Currently he is in process to enrol as aPhD part time student at the Oslo University but located at Gjøvik University College inthe field of colour science. The area of study will be “Colour Image Quality Assessment.”

He is also a lecturer at Gjøvik University College within the Department of ComputerScience and Media Technology where he is teaching digital image reproduction. Moreoverhe is a member of the Norwegian Colour Research Laboratory.

Before joining Gjøvik University College in September 2000, Peter Nussbaum wasan Application Engineer for Colour Management and consultant for GretagMacbeth,Switzerland. His professional memberships include IS&T, TAGA and IFRA Colour Man-agement Working Group.

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Quality-Assured Content Management: The KDI ProjectKnut Holmquist

The Norwegian Computing Center, Oslo (Norway)

The KDI project acts as a hub for innovation and development among companies that areworking with digital content and wants to secure quality of their processes and contentmanagement independent of media and technology. The project involves businesses thatare representing a broad range of the content value chain: From content owners toadvertising, prepress, newspapers and printing companies.

Based on the challenges of converging medias the KDI project has developed a frame-work, a methodology and suggested actions to assure the quality of content managementthroughout the content value chain. The main focus is on workflow and colour manage-ment.

The project has two primary objectives:

• to develop theoretical and technological artifacts which target the domain of Qualityassured Content Management (KDI)

• to facilitate, spread and leverage knowledge about this new core competence, inorder to support and strengthen Norwegian media industries.

The project has developed a theoretical framework for Quality assured Content Man-agement based on the CIP4 JDF standard. Methodologies by which to apply and validatethe framework against specific media / content management processes are being devel-oped and are being studied in pilot installations.

The project is also engages in the production, maintenance and active disseminationof KDI knowledge, resources and information, e.g. by participating in this conference.

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Predicting Visible Image Degradationby Colour Image Difference Formulae

Eriko Bando, Jon Y. Hardeberg, David Connah, and Ivar FarupThe Norwegian Color Research Laboratory

Dept of Computer Science and Media TechnologyGjøvik University College (Norway)

Digital imagery has become one of the major image reproduction methods, and ac-cording to the diversity of imaging methods, there is a strong need to quantify howreproduced images have been changed by the reproduction process and how much ofthese changes are perceived by the human eye.

A traditional colour difference equation, the CIELAB ∆E∗ab colour difference for-

mula, is still the most widely used as a colour difference metric in the graphic artsindustry, although it was designed to derive colour difference for a single pair of colourpatches. Therefore, several digital image distortion metrics, which are designed to takeinto account the human visual system, have been researched and developed over recentyears. We carried out a CRT monitor based psychophysical experiment to investigatethe quality of three colour image difference metrics, the ∆E∗

ab equation, the iCAM andthe S-CIELAB metrics. Six original images were reproduced through six gamut mappingalgorithms for the observer experiment. The result indicates that the colour image dif-ference calculated by each metric does not directly relate to perceived image differencebut, it also means that there are potential improvements for iCAM and S-CIELAB.

Further research will be carried out the subjects of image difference and image qualitymetric, perceptual difference in different media, and background effect of perceptualimage difference.

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Case study:Color and Print Quality as Defined by the Customer

Peter Nussbaum, Jon Y. Hardeberg, Sven Erik SkarsbøThe Norwegian Color Research Laboratory


Potential costumers within the digital printer market have various demands consider-ing their requirements with regards to desired color and print quality. This paper aims toinvestigate the color and print quality according to predefined quality factors to determinethe appropriate printing equipment. In particular the study describes the methods andresults of a research project conducted by researchers at the Norwegian Color ResearchLaboratory for the Norwegian Government Administration Service (NGAS). The objec-tive of the project was to develop methods and procedures to perform test prints fromdigital printers including the evaluation and assessment of the color and print qualityaccording to predefined quality factors.

Considering the methods to assess the digital color and print quality in this study ithas been proposed a number of quality factors. According to the presented list of appro-priate factors the costumer of this project, NGAS, has determined the most significantones and divided them into 3 assessment categories, namely “visual logo assessments”,“general print quality assessments” and finally the “copy quality assessments” Then NGASnationally announced a printer quality contest and printer manufactures were invited toparticipate in the test.

Although the Norwegian Color Research Laboratory has proposed further potentialquality factors, which could have been applied to this study to evaluate the color and printquality (e.g. visual image quality, print temporal stability, colorimetric reproduction,uniformity and addressability) NGAS as the customer of the project has determinedthe most relevant factors. Compared to a number of other studies, whose aim is thecomparison of color and print quality of digital presses concentrating in the field ofrendering complex images, the main focus in this work is printer evaluation accordingto reproduction requirements of the Norwegian ministries logo (National Coat-of-Arms).The results of the study indicate the performance of various digital printers in termsof their obtained print quality. Although other aspects, such as the commercial andthe technical point of view, might be considered for the acquisition of the appropriateequipment, the results of the printer evaluation will be used by NGAS to determine thefinal purchase decisions.

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Objective print quality assessment of complex imagesChristine Antoine, Ola Diserud, Olav Solheim, Elisabeth Berli

Paper and Fibre Research Institute AS (PFI), Trondheim (Norway)

Part 1: Print Quality Assessment by Means of the Fast Fourier Transform

The printing of an image is a complex process involving numerous parameters. A goodquality print, evaluated either subjectively or objectively, is usually the final goal of theprinting process. As subjective evaluation of images is a difficult and time-consumingtask, often done by considering complex (real) images, we propose a model that objec-tively evaluates print quality in images. First, the printed images are acquired by ahigh-resolution digital camera. Print quality is then evaluated from the power spectrumof the images, obtained in the Fourier domain. In the model, we can choose to includea reference image (’original’) and / or a contrast sensitivity function (CSF) that modelsthe sensitivity of the human eye. For multicolour images, we have included traditionalcolour difference indices as additional quality parameters.

Part 2: A Model for Perceptual Print Quality in Complex Black and WhiteImages

We have in the preceding paper presented a new optical method that objectively evaluatesprint quality in complex images (Antoine et al., 2004). Our approach is based on analysisof high-resolution digital images acquired from commercially printed newsprints. Theseobjective measurements are tested for their ability to differentiate between samples withalmost equal print quality (papers from the same market segment). Their precision arefound to be satisfactory, so also their ability to capture the most important elements ofsubjective print quality.

Based on these optical measurements, we have developed a multivariate model for theevaluation of perceptual print quality. The model is constructed from three variables, alldescribing image features from the high frequency part of the spectrum, i.e. features withwavelengths less than 1 mm. These variables can be linked to detail reproduction andedge sharpness, half tone dot characteristics and the regularity of the dot localization.With this model, we can predict how a panel of expert judges will rank samples from abenchmarking study focusing on print quality in black and white images on newsprint.

Biography

Christine Antoine graduated in 1993 at the Ecole Française de Papeterie et des IndustriesGraphiques, Grenoble, France as a pulp and paper engineer. From 94, she has beenworking in different Pulp and Paper research institutes around the world (Paprican,Canada, 94-95; Papro, New-Zealand, 97-98 ; STFI, Sweden, 99, PFI, Norway, 00-04)She has been working mainly with paper printability and print quality and is usingcontinuously image analysis as a tool to develop procedure to quantify different paper orprint characteristics.

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Research challenges for the graphic arts industryLindsay MacDonald

London College of Communication, University of the Arts London (UK)

The past 20 years has seen very rapid evolution in the graphic arts industry fromthe expensive proprietary systems of the 1980s to the heterogeneous standard-platformsystems of today. Features have been lower skill levels for operators, closer integrationof the production process with customer systems, and repurposing of content acrossmultiple media.

Key trends evident at the recent DRUPA exhibition in Düsseldorf (May 2004) were:

1. Large-format inkjet printers abounded, in all shapes and sizes, serving the demandfrom advertisers for large posters. There is clearly an enormous demand and po-tential for innovation in this market area. Spectra were even printing directly ontocookies!

2. UV curing inks are now supported by many presses, in which UV stages havebeen incorporated, giving improved productivity. Sun Chemical, for example, wereshowing a new process called WetFlex-a liquid-ink system designed to use electronbeam curable inks in flexo presses for packaging.

3. JDF is being widely touted and adopted as a standard for interoperability. The JobDefinition Format (JDF) specification provides a common syntax and agreementfor how to automate workflows, preserve job data and instructions throughout aprint job’s lifecycle, and for messaging to, and between, individual devices on theshop floor. As an example, Agfa were trying to demonstrate that the future is heretoday and that it has these characteristics:

Open: vendor to vendor operability

Collaborative: online communications between production partners and clients

Connected: data exchange with MIS to connect production with business processes

Expanded: end to end tracking from creator to delivery, across locations

Unlimited: add functionality as your needs change

4. Making money seemed to be the primary sales feature many of the integratedpress systems. No doubt this has always been the case, but my impression wasthat there was more emphasis than ever before on this subject. The key industrytrends are toward shorter setup times, quicker response to customers, less waste,closer integration with the client database for personalised document printing andproduction of mailing labels.

Of particular interest, and in some ways typifying the challenges for workflow andprocess integration generally, is the question of colour management. The InternationalColor Consortium has recently released Version 4 of its Profile Specification, and hasestablished Working Groups in the areas of Digital Photography and Digital Cinema. Itis clear that the traditional ink-on-paper processes of the graphic arts can no longer beregarded as a self-contained world, but must be treated in the broader context of digitalmedia communication. This puts research into a new light.

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The primary sisters in the age of digitalism –Glimpses of direct colour processes based on interference

(Lippmann) and diffraction (Wood)Leif Preus

Preus Foto AS, Horten (Norway)

Leif Preus starts his lecture with an astonishing demonstration of the worlds veryfirst colour slide show ( 17.th of May 1861) were the physicist James Clerc Maxwell,based on the Young/Helmholtz theory of the mans colour vision, wished to prove thetheory. The lecturer pays visits to Newton, Seebeck, Goethe, du Hauron, Vogel, Miethe,Ives, Lumiere and other philosophers, scientists, photographers etc. to describe the longand hard way from the very first b/w-photograph made possible in 1839 up to what thelecturer call "our age of digitalism".

The lecture deals with the ways we are receiving, seeing and interpreting colours withour senses. Except man and apes all other mammal has no colour vision at all. Evennot the bull can see any colour in the red scarf.

Two sophisticated ways of making colour photographs in the so called direct way, thatmeans colour photos without using any type of dyes, will be discussed in a special section:The Lippmann method based on the interference of light, and the Wood diffractionphotographs. Both gives us interesting knowledge concerning theories of light and colour.None of them were successful, but Gabriel Lippmann received the Nobel price for hisresearch . The lecture will finish with an experiment that shows how we can see coloursthat are not there....

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Improving print-quality by using FM screeningMorgan Brenden

Hjemmet Mortensen Trykkeri AS, Oslo (Norway)

There are two main reasons why Hjemmet Mortensen Trykkeri chose to change fromconventional screening to frequent modulated screening technology.

Firstly, and the most important one is the increasing quality of the printed product.HMT sees as its task to improve quality on the behalf of theirs costumers. HMT’scostumers haven’t forced this decision, but since there is an intense competition on thepublishing marked, this is one step in the right directions for HMT’s costumers to givetheirs readers a high quality product.

Secondly, there is a saving potential of cost connected to color consumption. Theargument of this allegation can be explained by the size of the dot printed onto thepaper.

Improving Print Quality

There are mainly 5 factors that get improved by using FM screening.

Object oriented moiré. By using FM screening, the pictures produced on the paperaren’t that sensitive to screen angles, as if using of conventional screening. Thisresults in less object oriented moiré caused by the printer, but if there is moiré inoriginal picture file, will this appear on the print.

Greater details. There is no question that details in contrast areas are greater withFM screening.

"Cleaner impression". Red snow and reddish metallic lacquer on cars are problemareas in web offset printing. This problem isn’t eliminated, but it is less commonwith FM screening.

"Printing by numbers". This is a vision in the color management work at HMT.Why? Since there are about 24 skilled workers in the production, each and oneof them have their own opinion what print quality is. This may not be poor forthe quality issue, but this can cause different color scheme, for instance betweensignatures.

Extended color gamut. Example speaks for it self.

Cost savings

Theoretically there is potential to save a great deal in make ready waste by using FMscreening. In the case at HMT this is already down at a minimum, with help of QTICCS. The CCS gets the machine into correct density, before e.g. the folding unit hasbeen set into a correct position. Therefore the make ready waste doesn’t represent anygreat deal of cost saving in the case of HMT.

The area that matters is cost savings in color consumption. Since the dot is drasticallysmaller with FM- then conventional screening, there is a great potential to use less colorin long run, which will be reflected by lower expenses to the color vendor.

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AM FM

Equal and correct density in AM and FM screening.

Equal, but to high density in AM and FM screening.

Figure 1: Extract from Creo Inc. sales material, which shows the different betweenconventional screening and FM screening.

FM AM

Figure 2: Extract from Creo Inc. sales material, which shows the different betweenconventional screening and FM screening.

Figure 3: E.g. conventional screening 50% dot (left), FM screening 50% dot (right)

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RGB Workflow and standardizationOlli Nurmi

VTT Information Technology, Media, Espoo (Finland)

In RGB-workflow the image is processed as long as possible in RGB-format andthe conversion to CMYK is done at latest possible stage. In newspapers workflowsmajority of editorial pictures can be processed automatically in RGB format and theCMYK-conversion is so far done at image workstations. All industrial processed needstandardisation to enable the process to be effective and the quality to be predictable.

Nowadays newspapers receive advertisement material in digital form. In many coun-tries there are special companies that sell media space for a group of newspapers. News-papers receive the same digital advertisement material and the customer expects theprinted result to be as similar as possible in different newspapers.

Newspapers are trying to reach this goal by applying the ISO 12647-3 standard.A system developed for Finnish newspapers to control and benchmark the quality ispresented in this paper. Grey balance and special test targets are printed with theadvertisements and measured data is send to the central database for reporting.

The pesentation will describe some general priciples in RGB-workflow and describeshow the print standardisation can be done. The Finnish system for Kärkimedia newspa-pers will be described as well as the reseach work behind closed loop density control innewspaper printing.

Biography

Group manager Olli Nurmi has worked in several positions in the Media research area.His research group research and develop methods and systems, which ensure the qualityand productivity of the media processes. The research deals with colour imaging systems,colour management and media logistics. One of the aims is to ensure the true colourrepresentation in different media platforms in various user environments

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Introduction to Multispectral Color ImagingJon Y. Hardeberg



Conventional color imaging science and technology is based on the paradigm that threevariables are sufficient to characterize a color. However, in particular, due to the effectof metamerism, three color channels are often insufficient for high quality imaging e.g.,for museums and digital archives. In this short talk the basic concepts of multispectralcolor imaging are presented.

We start with an introduction to 3-color imaging and limitations to the current sys-tems. Metamerism in image capture and reproduction systems are explained. Severalpractical systems for multispectral image capture and delivery will then be described,along with their strengths and weaknesses.

We give a brief outline current research topics in this field, in particular those wework with in our project “Multispectral Color Imaging” funded by the Research Councilof Norway.

Biography

Jon Y. Hardeberg received his sivilingeniør (M.Sc.) degree in signal processing from theNorwegian Institute of Technology in Trondheim, Norway in 1995. He received his Ph.D.in signal and image processing from Ecole Nationale Supérieure des Télécommunicationsin Paris, France in 1999, with a dissertation on color image acquisition and reproduction,using both colorimetric and multispectral approaches. He then worked for 2.5 years asa color scientist in Bellevue, Washington, USA, designing, implementing, and evaluatingcolor imaging system solutions for multifunction peripherals and other imaging devicesand systems. He is currently Associate Professor with Gjøvik University College inNorway, where he is teaching and researching in the field of color imaging science. Heis the Norwegian representative to CIE Division 8, and member of IS&T, SPIE, ISCC,and IARIGAI. He maintains a website with information about color and imaging athttp://color.hardeberg.com.

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Multispectral imaging: How many sensors do we need?David Connah, Ali Alsam, Jon Y. HardebergThe Norwegian Color Research Laboratory


The surface reflectance functions of natural and man made surfaces are invariablysmooth. It is desirable to exploit this smoothness in a multispectral imaging system byusing as few sensors as possible to capture and reconstruct the data. In this paper weinvestigate the minimum number of sensors to use, whilst also minimising reconstructionerror. We do this by deriving different numbers of optimised sensors, constructed bytransforming the characteristic vectors of the data, and simulating reflectance recoverywith these sensors in the presence of noise. We find an upper limit to the number ofoptimised sensors one should use, above which the noise prevents decreases in error. Fora set of Munsell reflectances, captured under educated levels of noise, we find that thislimit occurs at approximately 9 sensors.

Biography

David Connah obtained a BSc in Biology in 1997 and an MSc in Machine Perceptionand Neurocomputing in 1998, both from the University of Keele, England. In 2004 hecompleted his PhD in Colour Science at the Colour and Imaging Institute, Universityof Derby, and since then has worked as a researcher in the Norwegian Color ResearchLab at Gjøvik University College. His primary research topic is theoretical and practicalaspects of multispectral imaging.

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Recovering Natural Reflectances With ConvexityAli Alsam and David Connah



Mathematically, the camera rgb triplet results from projecting the spectral data from a 31dimensional space onto the sensor’s 3-d space. As a result of this projection from a highdimension space onto a much lower space, it is well understood that the inverse procedure,i.e. solving for the reflectance from camera outputs and sensors is an ill-defined problem.Said differently, there is an infinite number of spectrally different surfaces which are,mathematically, feasible solutions. This is always true unless the colour signal is definedin the sensor’s three dimensional space a condition which in general is not satisfied.Surfaces which integrate to the same camera response are said to be metameric (Stiles+Drew) to each other.

Morovic and Finlayson (Morovic) proved that the space enclosing the infinite setof surfaces which are metamers to each other under a given illuminant is closed andconvex. For a given rgb triplet, sensor set and illuminant spectral power distributionSPD, the algorithm developed in (Morovic 2002) returns a set of metamers, which ex-actly satisfy the constraints imposed on the solution space. Thus, the goodness of theestimated reflectance depends, directly, on the constraints used to define the solutionspace. Traditionally, the reflectance spectra have been assumed to populate a linearspace. Moreover, it is widely accepted that this space can be defined using a limitednumber of basis functions, normally taken to be the first few principal vectors describingthe data (Maloney). The assumption of linearity is extensively used in the reflectancerecovery literature (Wandell + Maloney 86, Morovic + Finlayson).

Constraining an unknown reflectance to be a linear combination of the first few prin-cipal vectors is known to result in estimates which match our knowledge of natural re-flectances more closely. For example, it is well understood that constraining the recoveredreflectance to be a linear sum of the first 3-8 basis vectors results in an estimate which issmooth and has a small norm. These characteristics are known to be in agreement withthose of naturally occurring reflectances. Unfortunately, by itself the assumption of lin-earity does not guarantee that the recovered reflectance is itself natural, for example it iscommon that a reflectance estimated with this assumption would include negative values,which violates the natural properties of reflectances. To further improve the estimate,additional constraints such as non-negativity are normally imposed on the solution space(Adding constraints results in a smaller feasible solution space, i.e. fewer metamers).

It is possible to enforce naturalness by imposing a set of linear constraint on thesolution space, for example Morovic and Finlayson constrained the domain in which theweights of the recovered reflectance are allowed to lie. In (Haneishi) the authors arguedthat the smallest possible feasible solution space is the one defined by the convexhull of thereflectances. Unfortunately, due to the high computational complexity of the convexhullcomputation in higher dimension the authors resorted to computing the convexhull in alow dimensional weight space.

In this paper we prove that the convexhull enclosing the spectral data is sensitiveto the number of dimensions used. Further, we introduce a computational method tosolve for a small number, 10-20, of bases vectors which enclose the data as tightly as

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possible. As a second step we use the bases to define the feasible space of metamers.Constraining the space to be a convex sum of these bases is shown to be a stringentconstraint. Moreover, no other constraints need to be imposed on the solution space, i.e.the constraint guarantees that the estimate is smooth and positive. Further, because noother assumptions about the linearity of the reflectance space are made the method isguaranteed to account for nonlinearities.

The contributions of this paper can be summarised as follows:

1. A method to estimate a small number of convex bases, which encapsulate thespectral data is developed,

2. All the calculations are carried out in a 31 dimensional space, i.e. no data com-pression is required,

3. Using as few as 10 bases is shown to completely account for all the data in the 24patches Macbeth colour checker,

4. By imposing the constraint that any reflectance needs to be a convex combinationof the bases derived, the 264 Esser calibration target reflectances are estimatedbased on their corresponding camera responses.

5. It is shown that for 95% of the surfaces the actual reflectance lies inside the metamerset.

Biography

Ali Alsam received his PhD in computational colour science from the University of EastAnglia (UEA), UK, in 2004. His research at UEA, which was supervised by Prof. Gra-ham D. Finlayson, focused on the problem of camera spectral calibration. Ali’s generalresearch interests include; computational colour science, device calibration, multispectralimaging, colour constancy, image enhancement, regularization, optimization, numericalanalysis, and computational geometry. Currently, Ali is working as a researcher in theNorwegian Color Research Laboratory at Gjøvik University College.

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Usage of a liquid crystal tunable filterfor multispectral imagery

Ambroise Marin, Audrey Roman, Jon Hardeberg, and Loïc NusillardLE2I, University of Burgundy, Dijon (France)

This work is a first step in a thesis about multispectral imagery contribution forartificial color vision and industrial quality control.

Why LC tunable filter?

The most of multispectral acquisition system are based on a system of filters that canbe placed in front of the camera : the filters wheel. A set of n filters are mounted onthe wheel and an engine rotates the wheel from 0 to 360◦ and then changes the filter infront of the camera. The most important drawback of this system is the limitation ofthe number of filters by the space needed to increase this number. Another limitationfor filters wheel based systems is the time to switch from a filter to an other is randomlychosen. The use of a LC tunable filter to replace the filters wheel allows to get a largenumber of different spectral bands in a limited time with a compact system.

How to mount the CRI varispec tunable filter?

The most important problem we had with the CRI varispec filter is the thickness of 5cmwhich reduces dramatically the camera angle field of view. It also produces vignettingon the resulting image, i.e. signal reduction on the image borders. Common solution tovignetting phenomenon is to place the filter between the sensor and the lens. Actually,it implies that near infinite placed object approximation is no more applicable and theobject must be placed at a distance of less than 10mm and must be smaller than 1mm.One solution is to insert a Barlow negative lens between the lens and the filter, but itdramatically increases focal length and the camera has to be placed at least at 8m toobject. The solution we propose is to replace this Barlow lens with a positive lens whichconjugates the CCD plan and the image plan of the primary lens [fig. 1]. This systemallows to change the primary lens easily and also to provide and wider angle for theglobal system equivalent lens.

References

1. Bruce Walker, "Optical Engineering Fundamentals", SPIE Press Vol. TT30, 1997

2. R. Shannon, "The Art and Science of Optical Design Robert", Published August1997, ISB 0521588685

Figure 4: Two positive lenses mounting.

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Spectral Inkjet ReproductionJéremie Gerhardt and Jon Y. HardebergThe Norwegian Color Research Laboratory


Based on the idea of creating a reflective physical image, in which the spectral re-flectance of the original scene is reproduced, we suggested that it was possible to repro-duce multispectral color images faithfully on printed media [1], using a multi-channelimage reproduction system [2]. The goal is to reproduce images with a spectral matchto an original scene, or a reference image, in order to eliminate the problems of theconventional metameric matches that can be achieved with four-color printing processes.A metameric match is only correct under a given viewing illuminant, while a spectralmatch is correct under any illuminant.

I will present the settings we made to drive spectrally our inkjet printer, how tocharacterize spectrally our device. We first made the forward model of the printer usingthe Yule-Nilsen spectral modified Neugebauer model. The steps to build the model andthe operations to use it will be described.

References

1. L. A. Taplin and R. S. Berns, Spectral color reproduction based on a six-color inkjetoutput system, in Proc. of Ninth Color Imaging Conference: Color Science andEngineering, Systems, Technologies and Applications, IS&T, Springfield, 2001, pp.209-213.

2. J. Y. Hardeberg and J. Gerhardt, Characterization of an eight colorant inkjetsystem for spectral color reproduction, Proc. CGIV 2004 - Second European Con-ference on Color in Graphics, Imaging and Vision, Aachen, Germany, April 2004,pp. 263-267.

Biography

Jérémie received his Bachelor degree in Electronic in 2000 and his Master degree in ImageProcessing in 2002 from University Pierre and Marie Curie in France. His master thesisproject was on wide format inkjet printing with the use of diluted inks, this project wasmade part time in the printing company Océ PLT in Créteil in France. He started hisPhD in the field of spectral color reproduction in September 2003 at Gjøvik UniversityCollege in the Norwegian Research Color Laboratory, he is involved as a PhD studentfrom Ecole Nationale Supérieure des Télécommunictions in Paris. He took part in thesecond European Conference CGIV 2004 in Aachen.

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Multispectral Gamut MappingArne M. Bakke, Ivar Farup, and Jon Y. Hardeberg



We propose a method for performing spectral gamut mapping, whereby spectral im-ages can be altered to fit within an approximation of the spectral gamut of an outputdevice. Principal component analysis (PCA) is performed on the spectral data to reducethe dimensionality of the space in which the method is applied. The convex hull of thespectral device measurements in this space is computed, and the intersection between thegamut surface, and a line from the center of the gamut towards the position of a givenspectral reflectance curve is found. By moving the spectra that are outside the spectralgamut towards the center until the gamut is encountered, a spectral gamut mappingalgorithm is applied to spectral data from the Macbeth color checker and test images.The spectral gamut is visualized by approximating the intersection of the gamut anda 2-dimensional plane, and plotting the resulting outline along with the center of thegamut and the position of a reflectance curve.

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Fast color-based browsing of large image databasesReiner Lenz, Thanh Hai Bui, Björn KruseDepartment of Science and Technology,

Linköping University, Norrköping (Sweden)

Overview

The amount of available digital information is growing very fast and the developmentof tools to handle these huge amounts of information is currently among the hottestresearch topics. The most popular and powerful engines (like Google) are text-basedbut research on the development of search engines dealing with visual information isreceiving more and more attention. Traditional approaches use keywords to describe animage and text-based engines for searching the database. This is however difficult, costly,time consuming and the result depends on the subjective judgement of the creator of thekeyword database. Automatic extraction of image content features and content-basedsearch (CSE) has thus received a lot of attention recently. We have developed severalversions of a fast browser that uses color and textural properties of images to browse avery large online image database. In this paper we will describe some features of thisbrowser and we will describe our experiments in which we use color and texture basedsimilarity measures to explore structures in a corresponding set of keywords that are usedto index the images in the database.

Introduction

Our image database that can be searched through the search engine contains now 617.415images. We have several versions of the search engine online and the user can browsethe database using keywords or content features. In our experiments we mainly concen-trate on color properties of images, on texture and their combination into color/texturefeatures. The user can also provide the web-address of an image. In that case the searchengine will fetch the image, analyse it and search for similar images in the database. Thecurrent version of the search engine can be found at http://media-vibrance.itn.liu.se/vinnova/cse.php. The images are small versions of images that are offered for saleby Matton AB, an image provider with sales office in a number of European countries andthe USA. From our search engine, the user can also directly access the original imagesand additional information at the Matton website.

Matton, the original provider of the images provided us also with a set of keywordsfor each image in the database. For each image we thus have the original keywordsand the content-based features computed from color/texture histograms. We can thusinvestigate the relation between content-based features and the corresponding keywords.In the full paper we will describe

• our methods for defining characteristic color and texture features,• very fast implementations for the computation of these features,• the compression methods for the statistical descriptors,• the application of statistical methods to define similarities between keywords and

images and• the results of our data-mining techniques to explore the color/texture induced struc-

ture in the keyword space.

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We will also briefly describe some implementation aspects: database storage and queriesare implemented with MySQL 4.0, and all computations are done with Matlab 7.0.

Experiments

The current keyword database consists of more than 13 millions records; each containing areference to the image file and its assigned keywords. Every image is usually characterizedby several keywords, and the same keyword can be used to describe several images.

We first extract a list of keywords, which are considered to be more meaningful. Alist of unique keywords (without duplicates), having more than 200 occurrences in thedatabase (i.e. at least 201 images in the database are associated to such a keyword) wasextracted with help of a simple SQL query. This list contains 6.081 distinct keywords.In our experiments we use the statistical descriptors of the images to investigate thestructure in this space of 6.081 keywords.

A typical experiment is the following: first we choose to describe each image in thedatabase with the help of a statistical descriptor (a vector) of its color/texture properties.Then we use each of the 617.415 images in the database as a query image and find itsclosest neighbor with respect to the given color/texture descriptor. This gives a list with617.415 pairs of similar images. For each such pair we now have a set of keywords anda content-based similarity measure. This content-based similarity measure is then usedas a similarity measure between the keyword pair from the two lists. Doing this for allquery/match image pairs and their keywords results in an accumulated similarity valuefor every pair of keywords in the short list (6.081 keywords). In this way we can use thecontent-based similarity to define a similarity between keywords.

Table 1 gives a few examples of keyword pairs with the highest similarity values basedon 105.000 images in the database. The keywords in each pair come from two separatekeyword lists, one from the query image and the other from the closest neighbour imagefound by color/texture searches:

horizontal---horizontal color---Caucasian outdoors---horizontal people---indoors Photography---horizontal people---male woman---color woman---lifestyle female---Caucasian Caucasian---business

white---adult woman---vertical woman---woman woman---smiling lifestyle---adult people---men outdoors---Caucasian smiling---Caucasian men---Caucasian people---outside

person---Caucasian woman---female woman---outdoors horizontal---blue woman---business vertical---adult woman---horizontal women---color vertical---lifestyle outside---outdoors

male---Caucasian people---inside women---vertical portrait---Caucasian horizontal---adult girl---Caucasian indoors---Caucasian white---horizontal inside---Caucasian vertical---Photography

We can see that certain keywords like Caucasian, horizontal and vertical (i.e. land-scape and portrait type images) appear very often in the database. The woman-womanpair indicates that if the query image shows a woman then also the retrieved imagecontains a woman with a high probability. More results will be described in the finalpaper.

References

1. L.V. Tran, "Effient Image Retrieval with Statistical Color Descriptors", Doctoralthesis-Linköping University Electronic Press, (2003)

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Characterization of color camerasusing multispectral imaging

Stephen WestlandCentre for Colour Design Technology, University of Leeds, Leeds (UK)

This study describes work to investigate methods for the recovery of reflectance spec-tra from the responses of trichromatic camera systems and the application of thesemethods to the problem of camera characterization. The recovery of reflectance fromcolorimetric data is an ill-posed problem and additional constraints are required to finda unique solution. In this study a novel method for reflectance recovery is introducedthat finds the smoothest spectrum consistent with the colorimetric data and a linearmodel of reflectance. The various methods were tested using data that were obtainedusing an Agfa StudioCam digital camera. The camera was used to image a set of targetswith known CIE tristimulus values that included the Macbeth ColorChecker DC chart,the Macbeth ColorChecker chart and a selection of chips from the NCS (Natural ColourSystem) system. The new method performed better than the three other methods for re-flectance recovery that were evaluated. The performance of the multispectral approacheswas compared with traditional methods for camera characterization such as polynomialtransforms and artificial neural networks. However, none of the multispectral methodswere able to out-perform the traditional methods. The polynomial and neural methodsgave statistically indistinguishable performance.

Biography

Stephen Westland obtained degrees in Colour Chemistry and Colour Physics at the Uni-versity of Leeds before working as a Colour Physicist for Courtaulds Research. In 1990he moved to Keele University where he carried out post-doctoral research in CognitivePsychology before taking up a Lectureship in Colour Vision. He was appointed as aReader in Colour Imaging at the Colour Imaging Institute at Derby University in 1999.In 2003 he was appointed to his current position as Professor of Colour Science andTechnology at Leeds University where he is Director of the Centre for Colour Scienceand Technology.

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Challenges for projection display tilingJon Y. Hardeberg



In recent years there have been made rapid advancements for projection displaysystems. Improved image quality, especially higher resolution and luminance, alongwith size and weight reduction have widened the areas of application to include areassuch as cinema, home and public entertainment, advertising, simulation, and informationdisplay.

It is well known that different color imaging devices reproduce color differently. Whilethis is quite obvious when comparing fundamentally different devices such as printers andmonitors, it is also true for devices of exactly the same type. Even two projection displaysof the same make and model will have different colorimetric characteristics, for exampledue to variations in the characteristics of the lamp.

To achieve consistent reproduction of color it is therefore necessary to perform somesort of color correction for each individual projector. The theory and practice of colormanagement, a concept well known in the graphic arts, provides a framework that allowsfor such corrections. Color consistency is achieved by mapping the color space of eachindividual device into a device independent color space such as CIEXYZ.

Generally, an exact mapping does not exist, and therefore different analytical char-acterization models are employed to different devices to best approximate this mapping.This process of determining a suitable mapping function and optimizing its parametersfor a given device is called colorimetric characterization. While there exist well estab-lished characterization models for CRT displays [1], the colorimetric behaviour of LCDdevices [2] and particularly DLP devices [3] are less well understood.

Due to the considerable spatial non-uniformity typically found with projection dis-plays, a conventional model for colorimetric characterization is only valid at the positionthe characterization data was measured. In a study of two LCD and DLP projectors [4,5], measurements of 25 spots over the images revealed poor spatial luminance uniformityfor both projectors. The intensity of the dimmest spot relative to the brightest was onlyabout 20% for the DLP and 30% for the LCD projector.

This non-uniformity is an important problem that needs to be solved in order tosuccessfully tile several projectors to achieve a seamless high resolution image [6, 7]. Wepresent here two approaches to solve the problem.

Non-uniformity correction using a high-end colorimetric camera

In [8], a colorimetric camera was introduced and evaluated as a supplement to the tradi-tional spectroradiometer. Inspired by the fact that we are now able to conveniently collectcolorimetric data with high spatial resolution, we proposed a new global characterizationmodel which enabled consistent color reproduction over the entire display.

The performance of the global characterization was evaluated based on two criteria,the absolute characterization accuracy of a color displayed at the center and the relativeaccuracy across the display. The absolute accuracy was tested by displaying 20 randomcolor patches at the center and gave an average color difference ∆E∗

ab = 3.66 between

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measured and predicted color. The relative accuracy across the display was tested by us-ing uniform tristimulus values as input, and measuring non-uniformities in the displayedimages. The average color difference across the display for a set of 12 test images was∆E∗

ab = 2.59.

Non-uniformity correction using a webcam

In a recent study [9] we investigate the feasibility of using an inexpensive webcam tocorrect the videoprojector non-uniformity. Two main approaches were proposed andevaluated, the colorimetric characterization approach and the global approach. Bothapproaches were based on displaying images, which should ideally have uniform colordistribution, capturing the displayed image with a webcam, and using this capturedimage, creating a correction function, which is then applied to images, in order to correctthem.

Our results show that the feasibility of the proposed methods depends a lot on thequalities of the equipment involved. For inexpensive webcams it is generally difficultto obtain reliable device-independent color measurements needed for the colorimetriccharacterization approach (average ∆E∗

ab = 12.27). Rather surprisingly, the simple globalapproach often gives visually good results.

References

1. R. S. Berns, R. J. Motta, and M. E. Gorzynski, CRT colorimetry. Part I: Theoryand practice, Color Research and Application 18, pp. 299-314, 1993.

2. Y. Kwak, L. W. MacDonald, Characterisation of a desktop LCD projector, Displays21, pp. 179-194, 2000

3. David R. Wyble and Hongqin Zhang. Colorimetric characterization model for DLPprojectors. Proc. IS&T and SID’s 11th Color Imaging Conference, pp. 346-350,2003.

4. L. Seime and J. Y. Hardeberg. Colorimetric characterisation of LCD and DLPprojection displays, Journal of the Society of Imaging Display 11 (2), 2003.

5. J. Y. Hardeberg, Colorimetric Characterization of Projection Displays, ElectronicImaging, 14(2): pp 5+9, May 2004

6. M. C. Stone, Color Balancing Experimental Projection Displays, Proc. IS&T/SIDNinth Color Imaging Conference, pp. 342-347, 2001

7. A. Majumder, A practical framework to achieve perceptually seamless multi-projectordisplays, PhD dissertation, University of North Carolina, 2003 (http://www.ics.uci.edu/~majumder/)

8. J. Y. Hardeberg, L. Seime, and T. Skogstad, Colorimetric characterization of pro-jection displays using a digital colorimetric camera, Projection Displays IX, Proc.SPIE 5002, pp. 51-61, 2003

9. G. Menu, L. Peigne, J. Y. Hardeberg, P. Gouton, Correcting projection displaynon-uniformity using a webcam, To be presented at Color Imaging: Processing,Hardcopy, and Applications X, Electronic Imaging Symposium, San Jose, Califor-nia, January 2005

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Color Image Quality in Projection Displays: a case studyMonica Strand, Jon Y. Hardeberg, Peter Nussbaum



Recently the use of projection displays has increased dramatically in different appli-cations such as digital cinema, home theatre, and business and educational presentations.Even if the color image quality of these devices has improved significantly over the years,it is still a common situation for users of projection displays that the projected colorsdiffer significantly from the intended ones. This study presented in this paper attemptsto analyze the color image quality of a large set of projection display devices, particularlyinvestigating the variations in color reproduction. As a case study, a set of 14 projectors(LCD and DLP technology) at Gjøvik University College have been tested under fourdifferent conditions: dark and light room, with and without using an ICC-profile. Tofind out more about the importance of the illumination conditions in a room, and thedegree of improvement when using an ICC-profile, the results from the measurementswas processed and analyzed. Eye-One Beamer from GretagMacbeth was used to makethe profiles. The color image quality was evaluated both visually and by color differencecalculations. The results from the analysis indicated large visual and colorimetric differ-ences between the projectors. Our DLP projectors have generally smaller color gamutthan LCD projectors. The color gamuts of older projectors are significantly smaller thanthat of newer ones. The amount of ambient light reaching the screen is of great impor-tance for the visual impression. If too much reflections and other ambient light reachesthe screen, the projected image gets pale and has low contrast. When using a profile,the differences in colors between the projectors gets smaller and the colors appears morecorrect. For one device, the average ∆E∗

ab color difference when compared to a refer-ence was reduced from 22 to 11, for another from 13 to 6. Blue colors have the largestvariations among the projection displays and makes them therefore harder to predict.

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Projection displays and colour appearanceLindsay MacDonald


Research at Derby over the past four years investigated colour appearance in projectiondisplays. The performance of two LCD-projector characterisation models was tested forfour liquid crystal based displays - two LC projectors and two LC flat panels. Datameasured from a series of test colours, indicated that all LC-based displays showed sim-ilar characteristics, including an S-shaped tone characteristic curve and poor channelchromaticity constancy. It was proved that the new characterisation models based ona hyperbolic function fit the tone curve very accurately with only four coefficients perchannel for any kind of LC displays.

A series of psychophysical experiments was conducted using the magnitude estimationtechnique. This resulted in the CII-Kwak data set, which comprises 5 groups i.e. P(Presentation), M (Monitor), C (Cinema), A (Ambient), F (Filter), with a total of 20phases. For each phase, 11 observers participated on the average and each colour wasassessed in terms of lightness, colourfulness and hue for 40 test colours except for GroupP, which had 32 colours per phase. The CII-Kwak data set covers 4 different kindsof displays (LCD projector, LCD monitor, 35-mm slide projector and CRT monitor),3 different backgrounds (white, mid-grey and black), 2 surround conditions (dark andaverage), 2 stimulus sizes (2◦ and 10◦) and luminance levels of reference white rangingfrom 0.1 to 154 cd/m2.

A new colour appearance model, Kwak03, based on the CIECAM02 model, wasdeveloped especially to perform better in predicting colour appearance phenomena underthe dark surround condition. Notable changes made in Kwak03 in comparison with otherCIECAM97s based models were:

1. The dynamic function is not used in the Kwak03 model. Instead, colour appearancechange by luminance factor is compensated at a later stage.

2. For the achromatic signal A, the ratios between three types of cone signals, R′k :

G′k : B′

k, are modified from 2:1:0.05 to 2:1:0.5. This modification does not haveany physiological evidence to support it but improves the performance of lightnesspredictor significantly.

3. The effect of colourfulness change caused by background luminance factor is re-modelled to fit the author’s data set, which contradicts the predictions of othercolour appearance models.

4. The effects of rods and stimulus size are included in the Kwak03 model. Therod signals are included in the achromatic signal function, which is capable ofpredicting the Purkinje shift. The size effect is included in the lightness predictor,which subsequently affects chromatic predictors.

The Kwak03 model was also tested with the same data sets used to test other colourappearance models. It was shown that the model performed best among all the colourappearance models studied. More importantly it also showed the best performance inpredicting a range of colour appearance phenomena.

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Digitization of stained glass windows – the VITRA projectLindsay MacDonald


The European collaborative research project IST-2000-28008-VITRA (‘Veridical Imag-ing of Transmissive and Reflective Artefacts’) was successfully completed on 31st August,2004. It has developed an innovative system for high-resolution digital image acquisitionfor conservation. Using a robotic platform to carry both camera and lighting, it cancapture colorimetric images up to 15 metres above floor level, thus eliminating the needfor scaffold towers. Potential applications include wall-paintings, tapestries, friezes andstained glass windows in heritage buildings such as churches, cathedrals, palaces andmonuments. In the course of the project a number of significant technical innovationshave been made.

The objects of VITRA project can be classified into two main groups: (1) reflectiveobjects, where the illuminating light source and the camera are on the same side of thetarget, e.g. a wall painting; and (2) transmissive objects, where the light source and thecamera are located on different sides of the target, e.g. a stained glass window. Generallyspeaking, transmissive objects have a larger colour gamut than reflective objects. In otherwords, their density range is greater and their colour is more saturated, which affects theperformance of characterisation model.

Two glass panels, with 50 stained glass tiles each, were specially constructed for thetransmissive characterisation. The glass tiles are 1.3 by 1.8 inches (3.4 by 4.6 cm) in size,arranged in 5 rows by 10 columns, to give an overall panel dimension of approximately 16by 10 inches (40 by 25 cm). Before they were leaded together, the glass tiles were mea-sured individually using a Macbeth Color-Eye 7000A spectrophotometer in transmissionmode. After correction for the non-uniform luminance profile of the backlight, the fullthird-order polynomial again gave the best performance for colour characterisation. Themean ∆E∗

ab with a test set of 50 colours was 3.8 with a maximum of 17.Image stitching software was developed to identify corresponding ’tie-points’ in suc-

cessive overlapping image frames, and then to ’blend’ the component images into a singlelarge image on a common coordinate grid. Where the subject is non-planar, some suit-able geometric transformations, or judicious ’rubber-sheet’ stretching of images may benecessary to produce a seamless result. A set of new image processing functions wasdeveloped for images in L*a*b* colour space, including: (1) an ortho-rectification func-tion for perspective correction, to make sure that parallel lines don’t cross; (2) up-rightrotation to ensure that the vertical line is perpendicularly upright to the ground level;(3) multiple tie-points stitching, in which corresponding tie-points are matched to eachother and the image is warped if necessary; (4) lightness balance of the luminance of twostitched images by remapping the original lightness of each pixel to the new value usinga linear interpolation.

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Crisatel Multispectral Imaging SystemFrancis Schmitt

Ecole Nationale Supérieure des Télécommunications, Paris (France)

Crisatel is a European project started in September 2001 in the field of conservationand restoration of canvas paintings. It has involved private enterprise, research insti-tutes and museum laboratories. One of its main achievements is a multispectral imagingsystem allowing the capture and the processing of paintings at ultra-high resolution (12000 x 30 000, 13 bands: 10 in the visible, 3 in the infrared). We will describe the ac-quisition system, the camera calibration, the reconstruction of the spectral reflectance ofthe pigment layer imaged on each pixel and applications such as simulation of illuminantchanges.

Biography

Francis Schmitt received an engineering degree from the Ecole Centrale de Lyon, France,in 1973 and a PhD degree in physics at the Université Paris VI in 1979. He has been amember of Ecole Nationale Superieure des Telecommunications (ENST) since 1973. Heis Full Professor at the Image and Signal Processing departement.

His main interests are in computer vision, 3D modelling, computational geometry,picture analysis and synthesis, colorimetry, multispectral imagery. He is author or co-author of about 150 papers in these fields.

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høgskolenigjøviksrapportserie,2004nr.9 proceedingsfrom ... · ﬁrst colour slide show ( 17.th of...

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