Time matters in soft proofing

Transcription

1 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, ACTA GRAPH. 171 UDC Research paper Received: Accepted: Time matters in soft proofing Jon Y. Hardeberg, Peter Nussbaum, Sylvain Roch, Ondrej Panak Author Jon Y. Hardeberg The Norwegian Color Research Laboratory Gjøvik University College Gjøvik, Norway jon.hardeberg@hig.no Abstract One of latest developments for pre-press applications is the concept of soft proofing, which aims to provide an accurate preview on a monitor of how the final document will appear once it is printed. At the core of this concept is the problem of identifying, for any printed color, the most similar color the monitor can display. This problem is made difficult by such factors as varying viewing conditions, color gamut limitations, or the less studied color memory shift. Color matching experiments are usually done by examining samples viewed simultaneously. However, in soft proofing applications, the proof and the print are not always viewed together. This paper attempts to shed more light on the difference between simultaneous and time-spaced color matching, in order to contribute to improving the accuracy of soft proofs. A color matching experiment setup has been established in which observers were asked to match a color patch displayed on a LCD monitor, by adjusting its RGB values, to another color patch printed out on paper. In the first part of the experiment the two colors were viewed simultaneously. In the second part, the observers were asked to produce the match according to a previously memorized color. According to the obtained results, the color appearance attributes lightness and chroma were the most difficult components for the observers to remember, generating huge differences with the simultaneous match, whereas hue was the component which varied the least. This indicates that for soft proofing, getting the hues right is of primordial importance. Key words Color matching, color memory, LCD, soft proofing, visual match 1. Introduction Soft proofing is the concept of simulating the appearance of a printed document on a computer monitor (Katoh, 1994; 1995; Katoh and Nakabayashi, 1997; Gatt et al,., 2004; Sugiyama and Kudo, 2005). With a correctly calibrated and characterized workflow, according to color management principles, soft proofing should be straightforward (Gatt et al., 2004). Indeed, the colorimetric characterization of the monitor and the printing press should make it possible to predict accurately the monitor RGB values that will match any printed color, provided that the color gamut of the display is large enough. This approach would typically result in a colorimetric match between monitor and print. However, experience shows that colorimetric match does not

2 2 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, 1-10 give satisfactory results. Many factors affect the appearance, including the viewing conditions, spatial image-dependent effects, and variations of color vision properties between observers. The primary use of soft proofing is to replace the commonly used "hard proof" systems in which the final print (typically from an offset press) is typically simulated on a Cromalin system or an inkjet printer. Soft proofing can thereby save time and money for the print professional, since it allows the proof to be examined by many different people in different locations at the same time (using dedicated software such as Dalim Dialogue or ICS Remote Director). Another application of soft proofing is the previsualization by the designers of what the actual prints will look like once they get through the whole workflow and the press. It would be very valuable for designers to have an accurate tool for this, since it would help them choose colors that are more likely to render the artistic effect they want to build. In this application the designers will typically look at the printed copy several days after they created their document, probably also under considerably different viewing conditions. Therefore, they just remember the colors they chose, so color memory plays a role. The color moves in our memory toward so called prototypical colors. Those prototypical colors are represented by our long term memory colors of familiar objects, such green grass, blue sky, skin colors. The observer tends to categorize perceived original colour and to remember only that category. The discrete set of those categories is called cognitive color, and short-term color memory shift is explained by cognitive effect. (Bodrogi, 1998; 2006; Bodrogi and Tarczali, 2002; CIE 166:2005, Bonnardel and Herrero, 2006). The color memory is influenced by many factors. The context in which the color is seen, has an important effect on the memorising of colors. (Bodrogi and Tarczali, 2001) The duration of the time delay is another important factor in color memory investigation. (Pérez-Carpinell et al., 1998; de Fez et al., 1998; 2001) Also, in the print house, it is often difficult to get a well calibrated monitor anywhere near the press, mainly because of lack of well controlled environment. Therefore, when the printed copies are being printed and compared to a soft proof, there is usually a distance to cover (the monitor will be in another room), and some time elapses - the hard proof and the soft proof are not seen next to each other. It is therefore interesting to investigate the effect of time spacing on the visual appearance of colors. The experiments done by Pérez-Carpinell and de Fez shows, that light original color are remembered as lighter, the dark colors as darker. (Pérez-Carpinell et al., 1998; de Fez et al., 1998; 2001). The memorized color has higher chroma than the original color, almost for all investigated colors. It was also found, that the differences increased with time. The chroma increasing correspond to other published works (Bodrogi, 1998, Bodrogi and Tarczali, 2001). In the literature (Bodrogi, 1998) it has also been found that short-term colour memory for chroma becomes less accurate when the chroma of the original colour increases. In this study, the single color patches did not show a significant correlation between original lightness and lightness shift as shows colors in photorealistic image. In all color memory experiment the hue attribute was the best remembered. Let us discuss why a colorimetric match does not correspond to a perceptual match. According to basic color science, two colors having the same measured colorimetric values should appear the same. But actually, two colors having the same CIE XYZ values will match in color for an average observer, provided that certain constraints are followed. Among these constraints are factors such as the retinal locus of stimulation, the angular subtense, and the luminance level (Fairchild, 2005). Also, the two stimuli must be viewed with identical surrounds, backgrounds, size, shape, surface characteristics, illumination geometry, etc. And this is where the difference appears: in the soft proofing point of view, it is almost impossible to get a match between all these parameters. Although the viewing set up is based on the standard condition of the graphic art industry (ISO 3664:2000), achieving identical surrounds or backgrounds in a print house is practically impossible, the illumination geometry of a viewing booth can barely be reproduced by a monitor (since the latter one emits light whereas the paper reflects the light of the viewing booth), the characteristics of the surface of a screen and those of a sheet of paper are completely different, etc. The phenomena that create this difference between the colorimetric match and the visual

3 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, match are called color appearance phenomena. In the last decades there has been a significant effort in color research to develop color appearance models (CAM) to take into account such phenomena (Fairchild, 2005). In this paper we investigate into the difference between simultaneous and time-spaced color matching, in order to contribute to improving the accuracy of soft proofs. In the next section we describe our experimental setup, in which observers were asked to match a color patch displayed on a LCD monitor, by adjusting its RGB values, to another color patch printed out on paper. In the first part of the experiment the two colors were viewed simultaneously. In the second part, the observers were asked to produce the match according to a previously memorized color. In Section 3, we present and discuss our result, before drawing some preliminary conclusions in the final section. 2. Methodology and experimental setup The aim of this work was to evaluate the effect of time spacing on color perception, in particular on color matching. Therefore, it was needed to carefully control all the other viewing parameters of the color matching experiment. Thus, it was divided in two parts: In Part A, the aim was to find the best visual match achievable when the two stimuli were viewed simultaneously. All the parameters were set so that the colors on the paper and on the screen could correspond. In Part B, the aim was to evaluate the effect of time spacing: with all other parameters being set the same as the in Part A, the target color was shown for 30 seconds to the observer, and then removed. Consequently the observer was asked to produce a match to the stimulus according to the previously memorized color. monitor profile was tested. A set of 124 CIELAB colors was converted to RGB values using the ICC profile. The RGB values were then displayed on the monitor and the corresponding XYZ values were measured using the Minolta CS-1000, converted to CIELAB using the measured white as reference white point (relative colorimetry), and compared to the original CIELAB colors. The obtained average E* ab is quite high (5.18), but it is worth mentioning that the accuracy of our experiment does not depend critically on this accuracy, since the matching colors are directly measured with the spectroradiometer. A set of 10 test colors were selected so as to contain memory colors e.g. skin colors. Furthermore colors from the RGB-cube s surface and pastel colors were also chosen, see Figure 1. Uniform patches of these colors were printed with a HP DeskJet 1220 inkjet printer using HP Glossy Photo paper. The resulting colors were measured using the spectroradiometer. A total of 12 observers with normal color vision and ages from 22 to 65 took part in the two experiments. 9 observers were chosen for each part, but in total: 6 among them carried out both parts, 3 carried out just the first part, and 3 carried out just the second part, see Table II. Figure 2 shows the geometrical setup of the experiment. The angle between the viewing booth and the screen was set so that the viewing of both colors simultaneously was comfortable. The keyboard was placed left to the viewing booth and the mouse right to the screen so that the observers had to keep both hard and soft copy in their viewing field while trying to make the match. The hard copies were viewed in a VeriVide viewing booth, with illuminant D50 selected. The color of the various elements in the viewing field were measured, see Table II. 2.1 Setup The monitor used in this experiment was a HP 2023 LCD display, set in 24 bits color mode and controlled by a Dell computer. The colorimeter EyeOne from GretagMacbeth and the profiling application ProfileMaker 5.0 was used for monitor calibration and characterization. In a preliminary experiment (Roch, 2006), the accuracy of the 2.2 Experimental procedure The matching tool developed for this experiment allowed users to control the RGB values of the displayed color patch by using both the mouse and the keyboard. Thus, no level bar appeared on the screen, making the image closer to what could be seen in the viewing booth (Figure 3).

4 4 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, 1-10 Figure 1: Colors used in the experiment. The colors in the image (from right to left, from top to bottom) are in the same order than in the table. The defined values correspond to the values chosen before printing, and the measured values correspond to the colors printed out and placed in the experiment environment Table I: Observers. The last column indicate the observer s experience with color: from 1 to 5, 1 meaning no experience at all and 5 meaning very familiar with color coordinates Before starting the color matching experiment, the observer could train with some test colors. Once the training was done, the experiment was carried out as follows: 1. Use the buttons 1, 2 and 3 and the mouse wheel to make a match with the color on the paper. 2. Press the "Esc" key, and choose one button to qualify the accuracy of the match (on a 6- category scale from not perceptible to unacceptable color difference). 3. The next color is placed in the viewing booth, as close as possible to the previous one. 4. Click on Ok in the message box, press Esc to make the bottom banner disappear, and proceed to the next match. Once the 10 colors have been displayed, the results are saved as a text file. Figure 2: Geometrical setup of the experiment Table II: CIELAB values of the surrounding colors measured with the Minolta CS-1000 spectroradiometer, using the preset calibration white point Figure 3: During the matching operation, only the color patch, the paper white and the black of the viewing booth s background were displayed on the screen. Therefore, the image viewed by the observer was as close as possible to that seen in the viewing booth

5 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, Results and analysis 3.1 Perceptual match versus colorimetric match The RGB values chosen by the observers to match the colors in the viewing booth were recorded in a text file, and the corresponding XYZ values measured afterwards. To do so, a software tool was developed to display the RGB values on the monitor and measure the XYZ coordinates using the Minolta spectroradiometer (Roch, 2006). The XYZ values were then converted into CIELAB values using relative colorimetry. The printed target colors were directly measured with the spectroradiometer and converted to CIELAB coordinates using also relative colorimetry. In Figure 4 we report the CIELAB coordinates found by the different observers in the simultaneous color matching experiment (Part A). As indications of the distributions corresponding to the different target colors, variation ellipses have been manually drawn so as to enclose all the matching points. We note that the variation ellipses are smaller near the achromatic axis, meaning that the observers defined the visual match more accurately in terms of chroma and hue in this region. This is expectable, since human eyes are very sensitive to hue variations around the achromatic colors. It is also interesting to notice that the variation ellipses are very circular in this region. Considering the other colors, it is clear that the variations are much bigger for chroma than for hue. The observers were more able to agree on a visual hue than a visual chroma. In Figure 5 we compare the visually matching colors (represented by their average value) with the colorimetric matches, that is, with the points representing the colorimetry of the printed colors. We see that the shift between the visual and colorimetric matches appears greater in the reddish colors, at least in terms of chroma and hue (a* - b* plane). Tables III and IV give quantitative information about the difference between the colorimetric match and the visual match. The first observation that one would make is that the E* ab for the blue is very high, mainly because of a shift of chroma. Figure 5 shows that the variance between the observers is also oriented in the direction of chroma. This would mean that considering this color, a chroma shift in the final print would be less noticeable than a hue or lightness shift. Thus, it seems that such a shift would still make an acceptable print. This, in the light of the difficulties of the screen gamut to match those of the presses especially in the blue colors, would be much valuable. It is definitely something that should be investigated further. It is also interesting to notice that the ΔH average value is the lowest among the 3 attributes (except for the blue color). This also indicates that the visual match is defined more accurately in terms of hue, and that a shift in hue will be more noticeable than a comparable shift in the other attributes. Bonnardel and Herrero (2006) came to a similar conclusion in their recent paper. Table III: CIELAB and color difference values for the experiment in Part A. The perceived match is, for each color, the average of the data given by all the observers. The colorimetric values are the measured values of the printed colors

6 6 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, 1-10 memory experiment than in the simultaneous (cf. Figure 4). The biggest difference concerns the colors near the achromatic axis, which had very low variances in the first experiment. It is very interesting to see that, once again, the ellipses are mainly oriented in the direction of the chroma, meaning that human eyes may be less accurate with chroma matching. The spreading of yellow color is in memory experiment mainly bigger in hue attribute. That can be reported as less capability to remember hue of yellow color, what could be similar to the published work (Pérez- Carpinell et al., 1998). The brown color has bigger distribution in all attributes, what describes worse memorizing of brown color. The both skin colors were mixed with very similar values of hue and chroma. The lightness values are not far away as well. It can be explain by only one skin prototypical color. Therefore the distinction between dark and light skin tone is minimal. The skin colors attend to be more yellow, this follows the description of Barletson s results (Bodrogi, 1998) as well as Bodrogi and Tarczali s (2001) results. Figure 4: Color matching data and variance ellipses in two different views of the CIELAB color space for the simultaneous matching experiment (Part A) Table IV shows that the shift between the visual matches (average result of the 9 observers, for each color) of the two experiments (Part A and Part B) is noticeable. Table IV: ΔE* ab between the visual matches of the two experiments Part A and Part B Figure 5: Colorimetric match of the target colors (i.e. printed colors) and the observer s average visual match for the simultaneous matching experiment (Part A) 3.2 Influence of time spacing In Figure 6 we show the CIELAB coordinates found by the different observers in the timespaced color matching experiment (Part B). As expected (Bodrogi, 1998; de Fez et al., 1998; 2001), the variance ellipses are bigger in the 3.3 Evolution of the visual match Figure 7 illustrates the evolutions of the visual match between the two experiments. It is interesting to notice that the lightness value was the one to get the biggest shift between the two experiments, meaning that it is the lightness coordinate that was the most difficult to remember

7 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, for the observers; this did not follow the previous results (Pérez-Carpinell et al., 1998; de Fez et al., 1998), where chroma has the greatest contribution on color difference. This trend is confirmed by the linear regressions presented in Figure 8: the variations of E* ab and variations of lightness and chroma are much correlated (R = 0,826), whereas hue and E* ab variations are almost not correlated at all (R = 0,162). Finally, Table V show that the difference between the colorimetric match and the visual match is not very far from the first experiment (average ΔE is 17,30 in Part A and 21,26 in Part B). Also, it is interesting to see that the trends concerning the ΔL, ΔC and ΔH values seen in the first experiment are confirmed, the average ΔH being even more significantly lower than the other two attributes lightness and chroma. Table V: CIELAB and color difference values for the time-spaced color matching experiment (Part B). The perceived match is the average of the colors given by the observers. The colorimetric values are the measured values of the printed colors Figure 7: Evolution of the visual match between the 2 parts of the experiment (a-b plane). The black arrows indicate the direction and size of the shift induced by the time spacing 3.4 Influence of observer experience Figure 6: Color matching data and variance ellipses in two different views of the CIELAB color space for the time-spaced matching experiment (Part B) Finally, the experience of the observers with color had little influence (Figure 9). The naïve group even obtained smaller E on some colors. This could be due to the fact that the matching method used was too much constraining for the observers.

8 8 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, 1-10 Figure 8: Linear regressions of the 4 different functions E = f(lightness), E = f(chroma), E = f(hue), and E = f(lc) 3.5 Parameters that might have changed the results This experiment was conducted with the best intention to have accurate data. But unfortunately, some parameters were not as much under control as one would have wanted, creating variations and uncertainty among the results. These parameters were: Two colors were a little out of gamut (Green and Cyan, see Figure 10): the gamut of the monitor used to build the colors was measured without surrounding light in a previous experiment, and was re-measured after the experiment, once the colors had been already used. Instability of the viewing booth luminance: the viewing booth used for this experiment could not maintain a consistent luminance. It was adjusted regularly, but nevertheless this could have changed the results a bit. Paper white match quite approximate: the paper white was matched using a spectroradiometer, converting the CIELAB values into RGB by using the monitor s profile in Matlab, and then by adjusting the color with the matching tool. Note, however, that only one person made this match, and it was not necessarily considered an accurate match by the other observers. Difficulty of the matching method: adjusting a color by modifying the RGB values is quite demanding and quite difficult (Henry et al., 2006). Sometimes, observers might just have not been able to find the match. Eye strain: visualizing colors on a LCD screen puts quite a hard strain on the eyes. Observers were allowed to take a break whenever they wanted, but it might have not been enough. The time effect and adaptation mechanism has influence on mixed color (Henley and Fairchild, 2000). 4. Conclusions and perspectives This work presented in this paper aimed to investigate the difference between simultaneous and successive color matching, in order to contribute

9 J. Y. HARDEBERG ET AL.: TIME MATTERS IN SOFT PROOFING, ACTA GRAPH. 19(2007)1-2, Figure 9: Influence of the experience with color of the observers for Part A (on the left) and Part B (on the right). The Expert group was composed of observers having an Experience of 4 and 5 (see Table I), and the Naïve group was composed of observers having between 1 and 3 In a first part of the experiment the two colors were viewed simultaneously, while in a second part, the observers were asked to produce the match according to a previously memorized color. According to the obtained results, the color appearance attributes lightness and chroma were the most difficult components for the observers to remember, generating huge differences with the simultaneous match, whereas hue was the component which varied the least. This indicates that for soft proofing, getting the hues right is of primordial importance. Figure 10: Target colors which are out of gamut to improving the requirements for soft proofing. A color matching experiment setup was established in which observers were asked to match a color patch displayed on a LCD monitor, by adjusting its RGB values, to another color patch printed out on paper. Note, however, that the presented experiment had several limitations, in particular concerning the number of colors and the number of observers. Thus the results here should only be interpreted as trends rather than exact psychophysical data. Note, however that recent work by Zhang and Montag (2006) seem to be well aligned with our results. References [1] Bodrogi, P., (1998), Shifts of short-term colour memory, PhD thesis, University of Veszprém [2] Bodrogi, P. and Tarczali, T., (2002), "Investigation of Colour Memory. " In Colour Image Science: Exploiting Digital Media, Chapter 2, John Wiley & Sons [3] Bodrogi P. and Tarczali, T., (2001), "Colour memory for various sky, skin, and plant colours: Effect of the image context, " Color Research & Application Vol. 26, No. 4, p [4] Bodrogi, P., (2006), "Color Memory", CGIV 2006 Final Program and Proceedings, pp , Society for Imaging Science and Technology

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