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«Publications of the University of Eastern Finland Dissertations in Forestry and Natural Sciences TUIJA JETSU Modeling color vision Publications of ...»

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Tuija Jetsu

Modeling color vision

Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences


Modeling color vision

Publications of the University of Eastern Finland

Dissertations in Forestry and Natural Sciences

No 20

Academic Dissertation

To be presented by permission of the Faculty of Science and Forestry

for public examination in the Louhela Auditorium in Science Park, Joensuu, on December 7, 2010, at 12 o’clock noon.

School of Computing Kopijyvä Oy Joensuu, 2010 Editors: Prof. Pertti Pasanen, PhD Sinikka Parkkinen, Prof. Kai Peiponen


University of Eastern Finland Library / Sales of publications P.O. Box 107, FI-80101 Joensuu, Finland tel. +358-50-3058396 http://www.uef.fi/kirjasto ISBN: 978-952-61-0257-3, ISSN: 1798-5668 (printed) ISBN: 978-952-61-0258-0, ISSN: 1798-5676 (PDF) ISSNL: 1798-5668 Author’s address: University of Eastern Finland School of Computing P.O.Box 111 80101 JOENSUU


email: tuija@jetsu.fi Supervisors: Professor Jussi Parkkinen, Ph.D.

University of Eastern Finland School of Computing P.O.Box 111 80101 Joensuu


email: jussi.parkkinen@uef.fi Professor Timo Jääskeläinen, Ph.D.

University of Eastern Finland Department of Physics and Mathematics P.O.Box 111 80101 JOENSUU


email: timo.jaaskelainen@uef.fi Reviewers: Associate Professor Sérgio Nascimento, Ph.D.

University of Minho Department of Physics Campus de Gualtar 4710-057 BRAGA


email: smcn@fisica.uminho.pt Professor Olli Nevalainen, Ph.D.

University of Turku Department of Information Technology 20014 TURUN YLIOPISTO


email: olli.nevalainen@it.utu.fi Opponent: Senior Research Fellow Alessandro Rizzi, Ph.D.

University of Milan Department of Information Technology via Bramante, 65 26013 CREMA ITALY email: rizzi@dti.unimi.it


Theobjective of this research was to investigate the current state of human color vision modeling and to also address its problems and possibilities. All the following results strengthen the idea that color vision as a concept is complicated, and that the modeling of it well is a demanding task.

First, some existing color vision models were examined from a computational point of view. No model was able to replicate the performance of human color vision fully in every experiment, and our experiments showed that there are large differences in the properties of these models. At least some kind of nonlinearity had to be implemented in order to be able to compensate for the differences between different brightness levels. The nonlinear models also performed significantly better in a color classification task. The errors made by the nonlinear models were similar to the ones human observers also often make, meaning that if the color was not classified into the correct class, it was usually classified into one of the neighboring classes.

Another part of our experiments indicated that if the changes in cone ratio would not be compensated for at all in the later stages of the human visual system, the resulting color space would be very different for each individual. A small part of the research was related to the Retinex model and color constancy. Finally, we conducted a color naming experiment by focal colors of different sizes and durations, and measured the reaction time for each stimulus. The results of the color naming experiment showed that the connections between different colored stimuli and the actual color sensation can vary a lot depending on the parameters related to the stimuli.

UDC: 004.942, 159.937.51, 519.876.5, 591.185.6, 612.843.31 Keywords (INSPEC Thesaurus): colour vision; visual perception; colour;

visible spectra; vision defects; eye; modelling; computers; computer vision Preface I would like to start by thanking my supervisors, Prof. Jussi Parkkinen and Prof. Timo Jääskeläinen, for pointing me to the appropriate direction in the course of my studies and providing me the chance to be a part of Joensuu Color Group. I was given quite a free hand to find my own way, and I definitely learned a lot during these past years.

Next I want to thank all the former and current members of Joensuu Color Group for the cooperation in all scientific and notso-scientific matters. I had the great pleasure of sharing ideas and a wide range of different moments with you. The same applies also

to students, staff and alumni of the Biological and Physiological Engineering Laboratory at Toyohashi University of Technology, Japan:

thank you for welcoming me to your group for six months and introducing the various aspects of Japanese culture to me. I also want to thank Prof. Shigeki Nakauchi separately for the invitation to Japan and all the help during my stay. Kimiyoshi Miyata, Masayuki Ukishima, Mitsuyoshi Tashiro, Ryouhei Suzuki, Naoko Takekawa and the Arai family also deserve thanks for sharing their pieces of the Land of the Rising Sun with me. Arigatou gozaimashita!

I appreciate the valuable comments that the reviewers of this thesis, Prof. Olli Nevalainen and Prof. Sergio Nascimento, have kindly provided. Also my peer reviewers, Ville and Ilja, have given me important feedback, and the language review done by Prof.

Gregory Watson has helped my work remarkably. The financial support from Tekniikan edistämissäätiö and Finnish Concordia Fund for parts of my post-graduate studies is also gratefully acknowledged.

I am most indebted to my family and friends for providing indispensable counterbalance to the world of science during all the years I spent with my studies. It has been good to know that I could turn to you whenever necessary, no matter how far or near from each other we have been.

And last, but definitely not least, I am grateful to my dearest Ilja for being always there for me. Thank you for your endless love and support.

Joensuu November 4, 2010 Tuija Jetsu "If we knew what it was we were doing, it would not be called research, would it?" – Albert Einstein


This dissertation consists of an overview part and the following

selection of the author’s publications:

P1 Jetsu, T., Heikkinen, V., Pogosova, A., Jaaskelainen, T., and Parkkinen, J., "Comparison of color vision models based on spectral color representation", Color Research and Application, Vol. 34, Number 5, pp. 341-350 (2009).

P2 Jetsu, T., Heikkinen, V., Pogosova, A., Jaaskelainen, T. and Parkkinen, J., "Cone ratio in color vision models", in the Proceedings of the IEEE 14th International Conference on Image Analysis and Processing Workshops (ICIAP 2007), pp. 179-182, Modena, Italy, September 11-13, 2007.

P3 Pogosova, A., Jetsu, T., Heikkinen, V., Hauta-Kasari, M., Jaaskelainen, T. and Parkkinen, J., "Spectral images and the Retinex model", in the Proceedings of the 9th International Symposium on Multispectral Colour Science and Application (MCS07), pp. 80-87, Taipei, Taiwan, May 30-June 1, 2007.

P4 Jetsu, T., Essiarab, Y., Heikkinen, V., Jaaskelainen, T. and Parkkinen, J., "Color classification using color vision models", to be published in Color Research and Application. Available online since November 9, 2010. DOI: 10.1002/col.20632 P5 Jetsu, T., Komine, H., Nakauchi, S., and Parkkinen, J., "The effect of stimulus color, size and duration in color naming reaction times", in the Proceedings of the 11th Congress of the International Colour Association (AIC) 2009 (AIC2009), Sydney, Australia, September 27-October 2, 2009.

Throughout the overview, these papers will be referred to as [P1]P5].

AUTHOR’S CONTRIBUTION The contributions of the author of this dissertation for the publications [P1]-[P5] can be summarized as follows.

In publications [P1], [P2] and [P4] the author was responsible for the implementation or revision of the necessary functions and execution of the computations (excluding the implementation of original Bumbaca & Smith and Ingling & Tsou functions and the original classification algorithm for [P4], which were done by Yugo Imazumi, Anahit Pogosova and Yasser Essiarab, respectively) and analysis of the results. The author of this dissertation was also the main writer of these three articles.

In publication[P3], the author was co-supervising the M.Sc. candidate working with the topic, had implemented the original function used for color space conversions, proofread the manuscript and gave the conference presentation.

In publication [P5], the author planned and executed the psychophysical experiments, analyzed the results and was the main writer of the article.


–  –  –


1 Introduction Colors are a significant part of our everyday life. They play a major role, for example, in warning signs, marketing, quality control and medical applications. In applications where important decisions must be made based on color, it is necessary to be aware of all the facts that affect color perception. Color sensation is always a sum of at least three factors: the object under inspection, the light source under which the object is examined, and the observer him/herself.

Also, the surroundings of the object under consideration may affect the perceived color dramatically. Even though a lot of applications based on accurate color measurements and machine vision have been developed for different purposes (e.g. [1, 22, 26, 50, 55, 60, 61, 67, 76]), in many cases quality judgements are still made by a human employee. People can be trained to make judgements of uniform quality at a certain level, but, nonetheless, the color vision of an individual is always a subjective characteristic. This study aims at examining some parts of this sophisticated system in more detail, especially from a computational point of view.

Human color vision is a very complex system: even though there are differences in the anatomical and physiological properties between individuals [34, 66], most people are able to recognize and name the colors on a general level in the same way. Those individuals who make an exception to this common behavior are often suffering from some form of color vision deficiency. An example that is not directly related to color vision, but illustrates well the complexity of the visual processing in the brain, is a case of a patient who completely lacked activity in the visual cortex, but was still able to successfully navigate along a long corridor without bumping into any obstacles [16]. A similar case has also been reported with monkeys [39]. There is also a known case of a patient that has been able to consciously see colors being otherwise virtually blind [95].

1 Dissertations in Forestry and Natural Sciences No 20 Tuija Jetsu: Modeling Color Vision Human color vision is fundamentally trichromatic [49]. This means that on the retina, there are cones that are sensitive to three different wavelength regions. There are, however, a lot of individuals who have some kind of malfunction in one cone type. A common title for all these abnormalities is color vision deficiency (CVD).

Color vision deficient people usually have problems either with long or middle wavelength cones. Long wavelength cones receive most of the information from the red end of the visible spectrum, and cones sensitive to middle wavelengths have their peak sensitivity in the green region. A complete lack or malfunction of either of these cone types leads to a condition that is in colloquial language often called red-green color blindness. Such dichromatic or anomalous trichromatic color vision is actually rather common among most mammals [45]. An individual with this kind of condition has problems in differentiating between certain hues of red and green.

It is also possible, however very rare, that the short wavelength cones of an individual are malfunctioning, which causes problems mainly with the separation of certain blue and yellow hues [4, 49].

The genetics and the evolution of primate color vision has been researched a lot (e.g. [43–45, 66, 68, 70, 73, 93]). It is known, for example, that Old World primates have normally three types of cone photopigments and the set of their visual pigments is more or less uniform. At the same time, New World primates are more often dichromats and there is more variance in the photopigments [43, 63]. Generally speaking, it seems that trichromatic vision is almost always superior or at least equal in performance when compared to dichromatic. For example, Osorio and Vorobyev [69] have found that for identification tasks, the dichromat’s eye is almost as good as a trichromat’s, but for a trichromat it is easier to detect fruit against leaves. Also, in [68], the authors discuss differences between di- and trichromatic primates, and they state that "it would be interesting to find a natural situation where dichromat monkeys or humans enjoy any advantage over trichromats".

In addition to differences in the number of cone types active on the retina causing some kind of color deficiency, there can be variDissertations in Forestry and Natural Sciences No 20


ation in the anatomical structure of individuals with totally normal color vision. For example, it has been proven with adaptive optics imaging that there are remarkable differences in the cone mosaics between individuals with normal color vision [34, 91]. Also, genetic variations in the sensitivities of the long and middle wavelength cones are known to exist [66], and it is actually genetically possible for human beings to simultaneously have on the retina more than three different cone pigments types [46, 65, 66]. There seem to be at least some indications that the fourth photopigment could also yield to a richer color experience [46].

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