INVESTIGATION OF COLOUR MEMORY

INVESTIGATION OF COLOUR MEMORY PH.D. THESES Tünde Tarczali Supervisor: Dr. Peter Bodrogi Doctoral School of Information Sciences University of Pannonia 2007

Introduction Colour memory plays an important role in many practical tasks related to the choice, identification and assessment of coloured objects. Customers of colour imaging products often prefer long-term memory colours or colour prototypes of familiar objects frequently seen in the past. Colour memory is also one of the factors responsible for the phenomenon of colour constancy. These facts motivated researchers of colour science to construct different psycho-physical methods to characterize human colour memory on a computer-controlled display. In memory matching techniques the remembered colour might differ from the original colour even if the viewing situation is the same. The aim of this thesis was to point out that these so-called memory shifts are significant in everyday situations of viewing photo-realistic images depicting sky, skin, or plants, or viewing standalone uniform colour patches of sky, skin, or plant colours. In many cases, significant memory shifts have been found. Several different psychophysical methods were found in the literature for the investigation of human colour memory. New experimental methods were developed and realized to achieve more stable and reliable experimental results. In this thesis experimental data resulting from different techniques were compared. For a given type of image context (e.g. human complexion), systematic colour shifts in human colour memory were observed, which could be explained by the existence of prototypical colours. In the experiments, the existence of these prototypical colour centres was confirmed. A cognitive theory of the memory shifts is presented.

New scientific results Developing new experimental methods to examine colour memory effects Ensuring equal adaptation conditions for colour memory experiments performed on computers 1. Applying blurred images in the memory matching phase of psychophysical colour memory experiments to ensure equal chromatic adaptation conditions of the observer in the observation/memorization phase and in the memory-matching phase 2,3,12,13 and thus to eliminate the perceptual artefact influencing the memory colours. 2. Applying greyscale background images to examine colour memory effects in order to ensure equal chromatic adaptation conditions in case of employing arbitrary number of original colour. 6,7,9,11 Developing new computerized experimental methods to ensure more precise examination of the colour memory effects 3. Developing a computerized colour mixing method changing hue, lightness and saturation components of the colours (i.e. according to the perceptional attributes of colours) in the memory colour mixing phase of the psychophysical examination of the memory colour shift to enable a more precise action of observing the memory colour shifts in the memory colourmixing phase. 2,3,7,9,11,12,13 4. Applying the simple decision rating experimental paradigm during colour memory experiments on computers to get fast, unambiguous results by eliminating the observer s fatigue. 6,7,8 2

New experimental results for short-term memory colours 5. Based on the new paradigm of the accuracy of the memory - the variability ellipses of the most important sky, skin and grass colours can be determined, analogous to the fundamental MacAdam ellipses - describing just perceptible simultaneous colour differences - as shown in the following figures 4,5,6,7,8 : -15-25 -20-15 -10-5 0 20-20 15 b* -25-30 10 b* -35 5 a* Sky L*=54-40 0 10 15 20 a* 25 30 50 Skin L*=80 45 40 35 b* 30 25 20 15-50 -45-40 -35 a* -30-25 -20-15 Grass L*=50 Figure 1. Variability ellipses for sky, skin and grass colours in CIELAB a *, b * plane for memory experiment with image context. The original colours are depicted by a large plus sign, the mean decision colours of the "yes, the same" subsets is depicted by big crosses. The "yes, the same" subset is depicted by small crosses. Table 1. Length of the long and short axes of the variability ellipses in CIELAB units Long axis Short axis sky 6.2 3.8 skin 3.8 3.4 grass 8.0 5.0 3

6. The unambiguous proof of cognitive effect in colour memory: the shift towards prototypical colour is the strongest in case of Caucasian skin colour 6,7,8 as could be proved by determining the length of the axes of the variability ellipses obtained during the psychophysical experiments applied for examining the accuracy of the memory. Table 3. Mean of length of the axes of the variability ellipses of the simultaneous, the colour patch and the photo experiments at the simple decision method Simultaneous Colour patch Photo sky 3.0 5.8 5.0 skin 4.2 5.2 3.6 grass 4.8 10.4 6.5 New experimental result for long-term memory colours 7. A comprehensive three stage experimental method for determining long term memory colours is based on the following two paradigm, built one on the other: 1. First Stage: applying the method of constant stimuli where the memory colour is selected from several constant colour patches, 2. Second Stage: applying the method of constant stimuli once more for more exact results, 3. Third Stage: applying the mixing method to find the long-term memory colours of the subjects, through this method the mean of the results of the three series provides the accurate longterm memory colours of the observers. 1 4

8. Significant differences exist between long-term memory colours of European and Far- Eastern observers, illustrated by the following tables and figures 1 : Table 4. CIELAB values of long-term memory colours of Hungarian (H) and Korean (K) observers Skin Sky Grass Foliage Banana Orange H K H K H K H K H K H K L* 81.8 73.1 65.7 62.6 45.5 55.6 84.1 40.5 68.2 86.6 43.2 72.6 a* 10.5 8.6-10.4-10.0-40.4-45.7-10.6-33.9 27.7-10.5-39.1 24.5 b* 19.3 20.3-28.9-35.4 32.2 35.1 66.7 21.4 62.9 69.1 30.0 69.1 Figure 2. Long-term memory colours from Korean observers (open circles) and Hungarian observers (plus signs); in CIELAB a *, b * plane. Foliage is shown in a separate diagram on the right. Table 5. Significance of the differences between the long-term memory colours of Korean and Hungarian observers. T- test, bold numbers show significant differences, at the p=0.05 level. p Skin Sky Grass Foliage Banana Orange <L*> 0.000 0.397 0.000 0.306 0.019 0.000 <a*> 0.037 0.835 0.060 0.038 0.715 0.150 <b*> 0.147 0.166 0.240 0.007 0.324 0.000 5

Application The customers of colour imaging products often prefer long-term memory colours or colour prototypes of familiar objects frequently seen in the past. Therefore, it is important to determine the prototypical colour of frequently seen (or used) objects, like sky, grass, or skin. This is also important in everyday life. Just think about the advertisements on TV, or posters in the streets. Prototypical colours are the best-remembered colours (with the smallest shifts and the lowest interobserver variability). If an object is displayed in colour shades within the prototypical colour region, its perceived naturalness is high. The advertised object has greater influence on the customers if its colour is more natural. Therefore it is recommended to include in the imaging software s palette special sky-blue, grass-green and Caucasian complexion hue patches to help novice image processing engineers to select pleasing colours for their jobs. In the future further special patches should be added. 6

References associated with the thesis Journal: 1. T. Tarczali, D. S. Park, P. Bodrogi, C. Y. Kim. Long-term memory colours of Korean and Hungarian observers. Color Res. Appl. 31, p. 176-183. 2006 2. P. Bodrogi, T. Tarczali. Colour memory for various sky, skin and plant colours: effect of the image context. Color Res. Appl. 26, p. 278-289. 2001 Book chapter: 3. P. Bodrogi, T. Tarczali. Investigation of Colour Memory. Colour Image Science: Exploiting Digital Media, John Wiley & Sons Limited, 2002. Papers or abstracts in Conference Proceedings: 4. T. Tarczali. Psychophysical investigations on computer. Kandó Konferencia, Budapest, Hungary, 2006. 5. T. Tarczali, P. Bodrogi. Colour memory investigations on computer. 6 th International Symposium of Hungarian Researchers on Computational Intelligence, Budapest, Hungary, 2005. 6. T. Tarczali, J. Csikós, P. Bodrogi. Accuracy and variability of simultaneous and memory colour matching. 10th Congress of the International Colour Association, Granada, Spain, 2005. 7. T. Tarczali, J. Csikós, P. Bodrogi. Simultaneous and memory colour matching of colours of familiar objects. XXXth Jubilee Colouristic Symposium, Eger, Hungary, 2005. 8. T. Tarczali, P. Bodrogi. Cognitive Colour. 25th Session of the CIE, San Diego, USA, 2003. 9. T. Tarczali, P. Bodrogi. Cognitive colour. XXIXth Colouristic Symposium, Eger, Hungary, 2003. 10. T. Tarczali, P. Bodrogi. Psycho-physical study of colour memory. AIC 2002, Maribor, Slovenia, 2002. 11. P. Bodrogi, T. Tarczali. Variability ellipses for memory colour matching. XXVIIIth Colouristic Symposium, Tata, Hungary, 2001. 7

12. P. Bodrogi, T. Tarczali. Variability ellipses for memory colour matching. Lux Junior 2001, Dörnfeld/Ilmenau, Germany, 2001. 13. T. Tarczali, P. Bodrogi. Colour memory for sky, skin and plant. International Conference on Color in Graphics and Image Processing, Saint-Etienne, France, 2000. 14. P. Bodrogi, T. Tarczali. Corresponding colours: the effect of colour memory. Colour Image Science 2000, Derby, UK, 2000. 15. P. Bodrogi, T. Tarczali. Az érzékelt színminőség javítása, Képfeldolgozók és Alakfelismerők 2. Országos Konferenciája, Noszvaj, Hungary, 2000. 16. P. Bodrogi, T. Tarczali, Biros G. A színemlék jellemzése, XVIIth Colouristic Symposium, Tata, Hungary, 1999. 8