An Examination of Color Theories in Map-Based Information Visualization

Transcription

1 An Examination of Color Theories in Map-Based Information Visualization Sussan Einakian a,b, Timothy S. Newman b a California Polytechnic State University, CA, USA b University of Alabama in Huntsville, AL, USA Abstract The suitability of certain classes of color combinations for overlay of data attributes in map-based information visualization are considered here through user evaluations. The color combination classes considered include (1) harmonious colors, (2) opposing (i.e., opponent) colors, (3) high saturated colors, (4) low saturated colors, (5) high lightness colors, (6) low lightness colors, and (7) disharmonious colors. The evaluations focus on noticeability, in particular of the first six classes versus disharmonious colors since earlier work has suggested disharmonious colors may be advantageous. The first class, of which there are a variety of artistic color theories, is of special interest here. Keywords: color harmony, color theory, color opponency, information visualization, map overlays 1. Introduction 5 Color can be a valuable cue in presentation and visualization. For example, color can be used to visualize musical notes [7] and to differentiate features on a map [9]. Suitable color backgrounds can also make vegetables appear more attractive at a grocery store [27]. Many visualizations have used color as a visual cue, especially to represent one attribute of data. Use of color can be valuable for several reasons, including (1) allowing more attributes to be displayed than Corresponding author address: tnewman@cs.uah.edu (Timothy S. Newman) Preprint submitted to Journal of Visual Languages and Computing August 7, 2017

2 if only grayscale was used, (2) allowing variation (or distinguishing levels) to be displayed in an intuitively meaningful way, (3) possibly making structures or trends more readily discoverable, and (4) allowing annotations or labels in a different color than the rest of the visualization. Although color can be readily observed by standard observers, a visualization may be more useful if color is used in a safe or prudent way. Safe usage has many aspects. One aspect is the need to put the right color in the right place, as described by Tufte [31]. For example, using the same color for the background and display of an attribute would not be a safe choice. The second aspect is the need to use a suitable theme in the right place. For example, if using the combination of a colorful foreground theme on a colorful background was visually stimulative but did not foster ready discovery of trends by visualization users, such a combination would be imprudent, or possibly even unsafe, for visualization. Here, we consider the application of certain theories of color combinations in map-based information visualization. In particular, we explore if known artistic color theory principles can be useful in such visualizations. The issues considered here are related to larger questions of which color combinations can create effective impressions of different types (or levels) of information for users Harmony One class of theories explored here are theories about harmonious color combinations. Such theories posit that there are colors that are aesthetically pleasing when used together. In addition, such theories include rules about how to select color combinations that are harmonious. Some visualization researchers have shown interest in using harmonious colors according to artistic color theories. For example, Wang and Mueller [34] have suggested that use of harmonious color combinations may be suitable in visualizations that employ color cues. Artistic color theorists typically define a harmony with respect to some color space, for example, with respect to Goethe s hue circle [33]. For example, Goethe believed that colors whose hues are located on opposite sides of his hue circle could be regarded as a harmonious pair. 2

3 One motivation some have voiced for using harmonious color combinations in visualization is that since such colors are aesthetically pleasing together, visualization using harmonious groupings of colors may well engage viewers and thus promote discovery of significant features or phenomena [33]. In addition, since some color harmonies are specified by formulae or other formal relationships, visualizations that use harmonious color combinations may be able to use automatic, formulaic means to select colors. However, other work [6] has found that disharmonious color combinations may be advantageous over harmonious color combinations, in particular in mapbased visualization. In this paper, we report user evaluations that explore certain aspects of both harmonious and disharmonious color combinations for presentation of data attributes in map-based information visualization. In all, three models of harmonious colors are considered and two models of disharmonious colors are considered. We also report evaluations that considered alternative color combinations, including opponent colors, high (and low) saturated colors, and high (and low) lightness colors. A primary focus is consideration of the noticeability (visibility) of structure(s) (or phenomena) Current Practices There is currently a diversity of practice in applying various color combination schemes in map-based visualization. For example, we obtained a series of weather forecast visualizations maps from the National Oceanic and Atmospheric Administration s (NOAA) National Weather Service. The color overlays on these maps were found to follow a variety of color theories or properties: some overlays used harmonious colors (specifically, Itten harmonies, which are explained in the Section 2.1), some overlays used disharmonious colors, and other overlays used saturated colors. An example of a NOAA map with overlays using all three of these color theories or properties on the same map is shown in Figure 1. 3

4 Figure 1: Weather forecast map from the National Oceanic and Atmospheric Administration s (NOAA) National Weather Service [41] 1.3. Organization 70 The paper is organized as follows. First, in Section 2, related work is discussed. Then, the performed evaluations are described in Section 3. Results and analysis follow in Section 4. A discussion of results, based on consideration of perceptual difference, is presented in Section 5. Finally, the conclusion is presented in Section Background and related work Over the years, many artistic color theorists have proposed theories of color relationships in paintings and other art works. Many of the early color theories were strongly motivated by personal philosophy. More recent theories about color relationships in artistic, printed, and other presentation media have often utilized knowledge of how the brain processes or perceives color. Some of the key theories which could have applicability in map-based information visualization are described in this section. 4

5 2.1. Color harmony theory In addition to the Goethe theory about color harmony, a few other theories that describe or determine a set of colors that are harmonious when they appear together have been proposed Itten Harmonies One of the well-known models for color harmony is the Itten theory [14]. That theory uses Itten s hue circle (color wheel), which consists of twelve evenly spaced colors. His hue circle was based on three subtractive (i.e., color of pigments) primary colors of blue, red, and yellow. These primary colors in the Itten hue circle form an equilateral triangle. The equal mixture of two primary colors produces a secondary color, and these are also evenly spaced on the hue circle. The Itten hue circle s six other colors are mixtures of primary and secondary colors. Itten described rules of color harmony from artistic perspectives. In the Itten theory, color harmonies are based on the relative position of colors on the hue circle and the relationships among them. Specifically, in Itten s theory there are harmonious combinations of two, three, four, and six colors. Colors separated by 180 degrees on his hue circle are considered to be two-color harmonies (also called dyad or complementary harmonies). Any three colors forming an equilateral triangle on his hue circle are three-color harmonies (also called triadic harmonies) [38]. Colors forming a square (or rectangle) on his hue circle are considered to be four-color harmonies (also called tetrad harmonies). Colors forming a hexagon are considered to be six-color harmonies (also called hexad harmonies) [4, 14]. Itten s color harmony theory has also been used by Matsuda [17] and Tokumaru et al. [30], who introduced new sets of harmonious color combinations, which we describe next Matsuda Harmonies The Matsuda [17] color harmony theory is based on Itten s color model [4] and on patterns of popular colors in the fashion industry from The theory categorizes these color patterns into eight hue template types and 5

6 Figure 2: Matsuda s hue template types (duplicated from [4]) ten tone distribution types, creating a total of 80 color patterns. The hue templates, which are shown in Figure 2, are named the i (Similar harmony), V (Adjacent harmony), L (Intermediate harmony), I (Complementary contrast), T (Complementary half circle), Y (Three point contrast), X (Multi complementary contrast), and N (Neutral value contrast) hue template types [17]. Each hue template is defined by a radial relation on the hue circle rather than on any specific set of colors. Figure 2 shows the radial relations for each type in gray. Any combination of colors having a radial relation shape matching that of some template is an instance of that template type. (That is, any rotation on the hue circle of a given type of hue template is another instance of that template [30].) The Matsuda theory has been described by and used by Cohen-Or et al. [4], Tokumaru et al. [30], O Donovan [25], etc Nemcsics Harmonies 125 Another theory for color harmony is what we will call the Nemcsics [19] theory. His theory views colors as possessing three traits: hue (A), saturation (T), and luminosity (V). The Nemcsics theory defines hue as the dominant wavelength of light, saturation as the degree of purity (colorfulness) of a color, and luminosity as the degree of brightness of a color. The Nemcsics theory uses the ColorOid color system and that system attempts to describe color in an aesthetically uniform color space. The ColorOid was developed based on the 6

7 results of a series of experiments. It organizes color in a system that is cylindrical in shape. It has luminosity as its vertical axis. On this axis, the range is from white to black. It has saturation as a direction quantity, expressed as distance from the vertical axis. On this dimension, the lowest saturated colors are nearest to the vertical axis and the highest saturated colors are farthest from that axis. The ColorOid color system defines hue to be associated with radial location on the cylinder [15]. There are 48 ColorOid basic colors, spaced in a circle (the Nemcsics hue circle) such that if one pair of colors has the same spacing as another pair of colors, each pair will produce a comparable difference in sensation in the visual system. These 48 basic colors have wavelengths between 450 and 625 nm [19, 22, 23]. Each basic color specifies a half plane of constant hue. Colors are positioned on each half plane based on two other traits, saturation and luminosity. The circle of the 48 basic colors is partitioned into seven categories: yellow, orange, red, violet/purple, blue, cold green, and warm green. Each category is further subdivided into seven hues, except for red, which is subdivided into six hues. Figure 3 presents the ColorOid color space [40]. Figure 3: ColorOid Color space (duplicated from [40]) The Nemcsics theory considers color contrast in determining harmony. It 7

8 considers colors to be harmonious when they have a satisfactory contrast relationship in at least one of the three traits (hue, saturation, and luminosity). Hue contrast is related to the distance of the color hues in the Nemcsics hue circle (i.e., contrast tends to increase with distance). Saturation contrast depends on hue and lightness, and it corresponds to the difference in the perceived hue due to the differences in lightness [23]. Luminosity contrast is related to lightness contrast (i.e., the difference of perceived lightness values). The Nemcsics theory considers that the hue contrasts that are harmonious are color pairs with hue angles between 30 and 40 degrees or between 130 and 140 degrees. It also considers color pairs whose hue angles are between 70 and 90 degrees or close to 150 degrees to be not harmonious [20, 21] Other Harmony Models and Past Studies One early theory is the Munsell [18, 3] theory. In this theory, each color has three basic traits: hue, value, and chroma. The hue trait describes the (single) dominant light wavelength associated with the color. The value trait indicates the degree of lightness; it distinguishes light and dark colors. The chroma trait indicates color purity. Color purity indicates the degree to which a color is free of any achromatic color. An achromatic color is one that is not characterized by a single wavelength, such as black, white, or gray. In the Munsell theory the key factors in determining harmony in a visual work are color strength and area (A). Color strength is the product of the color s value (V) and chroma (C). A strong color has a larger product of value and chroma than does a weak color. Color area is the amount of the visual work (i.e., image) occupied by the color [18, 10]. The color area factor is tied to the scene rather than to a property of the color itself. In the Munsell theory, a combination of two colors is said to be balanced (and thus harmonious) if the ratio of the color areas (i.e., the area of the scene that has that color) is inversely proportional to the ratio of the color strengths: A 1 /A 2 = V 2 C 2 /V 1 C 1, where A 1, V 1, C 1 are the area, value, and chroma, respectively, of the first color and A 2, V 2, C 2 are the area, value, and chroma, respectively, of 8

9 the second color [40]. For example, for two colors in an image to be balanced and thus harmonious, the strong color needs to occupy less area in an image than the weak one [40, 24]. In addition, the theory considers two colors to be harmonious if their mixture produces neutral gray [18]. One more recent color harmony theory is the Ou and Luo [26] theory. That theory is based on psychophysical studies on human participants. In their studies, 1,431 color pairs were displayed against a median gray background. The pairs were formed from 49 chromatic colors and 5 achromatic colors. The 49 chromatic colors were based on seven universal colors and seven color tones. Their universal colors were black, white, gray, red, orange, yellow, green, blue, purple, pink, and brown, which are colors that many languages contain names for, as described in a study by Berlin and Kay [35]. Their color tones were vivid, pale, dull, dark, light-grayish, grayish, and dark-grayish. The five achromatic colors were white, light gray, medium gray, dark gray, and black. In the studies, participants viewed color pairs one by one on a CRT display in a darkened room and determined which pairs were harmonious and which ones were not harmonious. One study of the Munsell [18] and Nemcsics (ColorOid) [21] color harmony theories (by Szabo et al. [28]) has also been described. Their study presented combinations of two and three colors, displayed against a gray background, to participants. For the paired combinations, two square color patches were displayed side by side. For the triplets, three square color patches were displayed in a triangular formation on the display. One pair or triplet was displayed on one half of the display. That color pair or triplet followed the Munsell color harmony model. On the other half of the display, another color pair or triple was displayed. That color pair or triplet followed the Nemcsics color harmony model. For each color pair or triple, the observers rated their impression of color harmony from the most harmonious to the least harmonious color combinations. In addition, Szabo et al. [28] have considered human impressions of two sub-classes of harmonies, the monochromatic harmonies and dichromatic harmonies. A monochromatic harmony is a set of colors with the same hue, but 9

10 possibly differing chromas or values. A dichromatic harmony is a set of colors of complementary hues (i.e., the hues of the colors are separated by 180 degrees on the hue circle) but possibly differing chromas or values [28]. Szabo et al. found that, for the monochromatic harmonies, observers considered Munsell harmonies colors to be more harmonious than Nemcsics ones. But they found that for dichromatic harmonies (with equal chromas, but possibly differing values), the observers considered Nemcsics harmonies to be more harmonious than Munsell ones Color Combination Generators 220 Some automated tools that generate color combinations that follow certain color theories exist. For example, Hu et al. [12] have described a tool that uses the similarity and contrast of one or two traits of a color to create harmonious color schemes. In addition, Gramazio et al. [8] have developed Colorgorical, which is a web based tool that can automatically generate color palettes based on color perceptual distance, color name difference, etc Saturated Colors Saturation can also be used as a visual attribute in map-based visualization (e.g., for integer data, for which it is suitable), although it is considered to be an advanced topic [1] and thus is not used very often [1]. Saturated colors in images do tend to catch a viewer s attention [29], and high-saturation colors can be used to make important features stand out on a map [9, 5]. In addition, use of saturation differences may be prudent when there are many categories of qualitative data to be displayed on a map [32] Opponent Color Theory 235 Another type of color relationship that potentially could offer value in information visualization is use of opposing (opponent) colors. Theories of opposing colors were first proposed some time ago (e.g., Hering s [11] theory). The opponency of interest to us here is the chromatic opponency (red versus green and 10

11 240 blue versus yellow) of the human visual system. Chromatic opponency is the result of opponent neurons that have an excitatory response to one small range of wavelengths and an inhibitory response to another small range of wavelengths (i.e., the wavelength representing the opponent color) [11]. 3. Experiments and Methodology In this section, we describe our experiments to explore effectiveness of opponent, high (and low) saturated, and high (and low) lightness color combinations for map-based information visualization. Our emphasis is comparison versus disharmonious color combinations, building off our prior work [6]. Five classes of experiments were performed and are described here. One supplemental investigation was also performed. (Results of experiments are presented in Section 4.) These experiments used user perceptions to primarily test one aspect of the effectiveness of each sort of color combination: visual attentiveness (noticeability) to aspects of the visualization. These experiments involved overlay of a color label on either a single- or multi-color background. The background and overlay colors formed color combinations considered by the users. It is worth noting that other parameters (which are beyond our scope here) are also involved in noticeability, including the position, size of a feature of interest, hue contrast, etc Software and Tools Suitable visualizations following each model were viewed by the participants. All participants viewed the visualizations on the same moderate resolution display. The display was color-calibrated at least once per week or after every fifth participant, whichever came first. This calibration was performed using the Windows 7 display color calibration and the PowerStrip [42] calibration packages. In addition, room lighting was identical for all participants. Finally, participants had the opportunity to adjust the seating and display positions for best visibility. 11

12 Figure 4: Crowded Symbology Map A (before additional overlays) Figure 5: Crowded Symbology Map B (before additional overlays) 3.2. Experiment One: Itten Harmonies vs. Disharmonies Experiment One was designed to evaluate the noticeability of the well-known Itten color harmonies versus disharmonious color overlays. In it, the participants viewed visualizations on two maps, called Crowded Maps A and B, using label overlays colored harmoniously versus ones using label overlays colored disharmoniously. The maps are shown in Figures 4 and 5. They already contained labels for some well-known places. We overlaid additional labels (for less wellknown places) for the experiment. The harmoniously colored combinations were chosen according to Itten s rules of harmony. The disharmoniously colored combinations were colors separated by 80 or 150 degrees on the hue circle from the background color (since our prior work [6] had found colors with those sepa- 12

13 rations were disharmonious). Thirteen harmonious or thirteen disharmonious labels were overlaid on each crowded map. Examples of the two maps with harmoniously colored label overlays are shown in Figures 6 and 7. Figure 6: Crowded Map A (after added overlays) Figure 7: Crowded Map B (after added overlays) 280 The participants were asked to recite the overlaid labels in perceived noticeability order. The time to complete this task was recorded for each participant. These times were taken as the base measure for the degree of noticeability. Twenty participants (10 Female and 10 Male) took part in Experiment One Experiment Two: Matsuda Harmonies vs. Disharmonies 285 Experiment Two considered the noticeability of the Matsuda i-type har- monies versus disharmonious color overlays. These overlays were also considered 13

14 versus overlays that were neither known harmonies nor known disharmonies. In the experiment, participants viewed visualizations that involved three colored maps of the U.S. state of Alabama. On each map, 12 labels were overlaid, each in a different color. The colors were those whose positions on the hue circle were offset by 30, 60, 80, 90, 120, 150, 180, 210, 240, 270, 300, and 330 degrees clockwise from the background color. The labels at the 30 and 330 degree positions formed Matsuda i-type harmonies with the background color. The labels at the 80, 150, and 210 degree positions formed disharmonies with the background color. The participants were asked to recite the overlaid labels in perceived noticeability order, from most to least noticeable, for each map. The times until the recitation were recorded for each label. The participants were limited to 30 seconds to recite the label overlays. For any label not seen by participants a value of 31 seconds was recorded. For each user, the average time until recitation was determined for each label color, resulting in 12 measures per participant. Participants were also asked to report which color overlay on each map was (1) the most pleasant and (2) the most distinct. Overall averages per label were also determined. Thirty participants (15 Female and 15 Male) took part in Experiment Two, although 8 of the participants could not see 3 or more of the labels Experiment Three: Nemcsics vs. Itten Harmonies Experiment Three considered if the Nemcsics Harmonies and Itten harmonies had differing noticeabilities. In it, the participants viewed other visualizations involving labels overlaid onto colored maps. Five base maps were utilized in the experiment. Two of them are the Crowded Maps A and B. The other three maps (maps of the world), which are shown in Figures 8-10, displayed certain features of political subdivisions using color codings. Those three maps are termed the non-crowded maps. Relatively unknown place names were utilized for the labels we overlaid to minimize possible biases from personal knowledge. 14

15 Figure 8: (Non-Crowded) Map #1 before overlays Figure 9: (Non-Crowded) Map #2 before overlays Five of the ten maps used in the experiment utilized label colors forming harmonious color combinations with the background based on the Nemcsics rules. The other five maps were identical except for using harmonious combinations based on the Itten rules. Two examples of the visualizations used in this experiment are shown in Figures 7 and 11. The participants were asked to recite the overlaid labels in perceived noticeability order. The visualizations were presented in a randomized order to limit possible biases based on order. For all maps, the elapsed time for reciting each label was recorded for each participant. These times were taken as the base measure for the degree of noticeability. Participants were also asked to report which color overlay on each map was the most noticeable. 15

16 Figure 10: (Non-Crowded) Map #3 before overlays Figure 11: Map #3 (non-crowded) after harmonious overlays Forty participants (20 Female and 20 Male) took part in Experiment Three Experiment Four: Disharmonious vs. Opponent Experiment Four considered the noticeability of opponent colors versus disharmonious color overlays. In it, the participants viewed four solid-colored maps of the U.S. state of Alabama with overlaid labels (i.e., with different colors and labels from those in prior experiments.) The background colors of these maps were red, yellow, green, and blue. Examples of two maps before adding any label overlays are shown in Figure 12. The disharmonious color combinations for label overlays were colors separated by 80 degrees or 150 degrees on the hue circle from the background colors. The opponent color combinations for 16

17 label overlays were chosen based on chromatic opponency (i.e., red-green and blue-yellow). (a) Map1 with red background (b) Map2 with yellow background Figure 12: The U.S. state (Alabama) map with two different background colors Participants viewed a series of map pairs. In each pair, one map used a disharmoniously colored label and the other map used either another disharmoniously colored label or an opponent color label. For each pair, participants reported which label was the most noticeable. The total number of participants choosing each label type as the most noticeable one was then determined. The opponent colors in these pairwise comparison were chromatically opponent with the background color. The disharmoniously colored labels used colors separated by 80 degrees or 150 degrees on the hue circle from the background color. Twenty one participants (10 Female and 11 Male) took part in Experiment Four Experiment Five: Saturation, Lightness, and Disharmonies Experiment Five considered some alternative color combinations: high and low saturated colors, and high and low lightness colors. These were compared to one another and to disharmonious color combinations. In the experiment, participants made pairwise comparisons. The labels were overlaid on eight colored 17

18 weather maps of the entire United States. These maps were from the National Oceanic and Atmospheric Administration s (NOAA) National Weather Service. The disharmoniously colored combinations were chosen as in the prior experiments (i.e., at positions separated by 80 and 150 degrees from the background color), and these colors level of saturation and lightness were 42% and 58%, respectively. The color combinations for both the high and low saturation label overlays had a 58% level of lightness. The color combinations for both the high and low lightness label overlays had a 42% level of saturation. (These saturation and lightness levels are the colors S and L values in the HSL color model.) In all trials, the same label was displayed at the same location, but displayed using a color suitable for the trial. Figure 13 shows examples of one map with an overlay colored disharmoniously with the background (left) and another map with an overlay in a low saturation color with respect to the background (right). In this paper, Figure 13 is shown zoomed-in for presentation purposes. (a) Overlay with disharmonious color (b) Overlay with low saturated color Figure 13: Weather forecast map (zoomed-in) with added label overlay (Perkins) using disharmony and low saturated colors 370 Participants were shown a series of the same map pairs, in each case with one map using a disharmonious color label and the other map using either a high (or low) saturated color or a high (or low) lightness color label. For each pair, participants reported which label was most noticeable. The total number of 18

19 participants choosing each label type as the most noticeable one was recorded. For each user, the percentage of times each color combination type was also 375 determined. Twenty six participants (13 Female and 13 Male) took part in this experiment. 4. Results and Statistical Analysis In this section, the raw results and statistical evaluation of results for the 380 five experiments are presented. The analysis involves statistical testing of sig- nificance at the 95% confidence level Experiment One Results and Analysis Table 1 shows the average response times (in seconds) for Experiment One s 385 task of finding the 13 labels colored harmoniously (according to the Itten Harmony Model) and the 13 labels colored disharmoniously on each of the two crowded maps. Table 1: Average time (sec) to find the label overlays on the two crowded maps, Disharmony vs. Itten Color Label # Scheme #1 #2 #3 #4 #5 #6 #7 #8 #9 #10 #11 #12 #13 Map1 Disharmony Harmony Map2 Disharmony Harmony Table 2 includes the overall average response times for each of the color theories tested. The mean value of the average response times for finding labels colored harmoniously is higher than the mean value of the average response 390 times for labels colored disharmoniously. The table also shows the two sample t- Test results to determine statistical significance of this difference. The difference is statistically significant; there is strong evidence that label overlays colored disharmoniously are more noticeable than labels colored harmoniously (at least according to Itten s harmony theory). 19

20 Table 2: Two sample t-test for Disharmonious versus Itten-harmonious overlays Harmonious Disharmonious Mean Variance Observations T-Stat t-critical SIGNIFICANT Experiment Two Results and Analysis The second experiment considered the Matsuda i-type harmonies, disharmonies, and labels not definitively harmonious or disharmonious (according to the Matsuda theory). Table 3 shows the results for the 22 participants who recited all, all but one, or all but two of the labels. Average response times (in seconds) are shown for finding the labels, broken out by the color s offset (clockwise) on the hue wheel from the dominant color of the background. In this trial, the labels whose colors were 150 degrees from the background color tended to be the first one noticed, whereas the labels whose colors formed a Matsuda i- type harmony with the background (i.e., especially the labels whose colors were positioned 330 clockwise from the background) tended to be noticed later. Table 3: Average Time (sec) to find the colored labels Time To determine the statistical significance of these results, two sample t-testing was performed on the average times for the i-type harmonies (30 and 330 degrees) versus those for the disharmonies (80, 150 and 210 degrees). (Since colors at the 150 and 210 degree positions both represent 150 degree separations, we aggregated them to form composite 150 degree disharmony results.) Table 4 includes these averages as well as the result of two sample t-testing on them. There was a statistically significant difference between noticeability of label overlays done using i-type color harmonies and disharmonies; overlays 20

21 using disharmonious colors are likely inherently more noticeable. Table 4: The two sample t-test for i-type harmonies vs. disharmonies for response times i-type Harmonies Disharmonies Mean Variance Observations T-Stat t-critical SIGNIFICANT 415 We also did a t-testing of the response times for the i-type hue template combinations against the ensemble of the other combinations. Those test results are shown in Table 5. There was a statistically significant difference in the times to notice overlays using i-type color harmonies versus non i-type colors; overlays using i-type harmonies are likely inherently less noticeable. Table 5: The two sample t-test for i-type harmonies vs. non i-type colors i-type Harmonies Non i-type Colors Mean Variance Observations T-Stat t-critical SIGNIFICANT Lastly, we were curious if the difference in the two different types of disharmonies was significant, so we performed t-testing on the average response times the 80 degree color separations versus the 150 degree separations. (Again, the 150 and 210 degree position results were aggregated.) Table 6 shows these t- Test results. There is a statistically significant difference in the times to notice overlays using 150 degree disharmonies versus the 80 degree ones; 150 degree disharmonies seem to be the more promising ones Experiment Three Results and Analysis Table 7 shows the average, minimum, and maximum response times (in seconds) for Experiment Three s task of finding the harmoniously colored labels 21

22 Table 6: The two sample t-test for 80 degrees vs. 150 separation disharmonies. 150 degrees 80 degrees Mean Variance Observations T-Stat t-critical SIGNIFICANT 430 on the 5 maps. The columns labelled AN denote times for task completion for Nemcsics harmonies. The columns labelled JI denote times for task completion for Itten harmonies. Results are broken out by map. Table 7: Time (sec) to find labels overlays using harmonious color combinations. Map1 Map2 Map3 Map4 Map5 AN JI AN JI AN JI AN JI AN JI Mean Min Max Std. Dev Table 8 summarizes the overall minimum, maximum, standard deviation, and average response times (in seconds) for each harmony theory. The tasks 435 using the Itten harmonies were completed somewhat faster than the ones using the Nemcsics harmonies. Table 8: The overall time (sec) for finding the overlays using harmonious color combinations. Color Space Mean Min Max Std. Dev. AN JI Our t-testing of statistical significance of these results are shown in Table 9. The difference in results for Itten versus Nemcsics harmonies was not statistically significant. Thus, it may be no better to use Nemcsics harmonies for 440 visualization using map overlays than to use Itten harmonies. 22

23 Table 9: The two sample t-test for Nemcsics vs. Itten harmonies for all five maps. Nemcsics Itten Mean Variance Observations T-Stat t-critical NOT SIGNIFICANT (We also studied if there were any differences in male and female responses and between those with normal vision and corrected-to-normal vision. In both cases, no statistically significant differences were observed.) 4.4. Experiment Four Results and Analysis 445 Table 10 shows counts of participant choices for Experiment Four s task of stating which label of each pair was most noticeable. The columns labelled Opp denote results for chromatic opponent colors. The columns labelled 80 and 150 denote results 80 degree and 150 degree disharmonies, respectively. Table 10: The count of participants responses for label noticability Pairwise Noticeability Comparisons Opp 80 Opp Map Map Map Map Table 11 shows the overall averages and two sample t-testing results for 450 the pairwise tests of opponent colors versus the 80 degree disharmonies. Here, the averages are the average selection count (e.g., 2.52 for opponent means the average person chose the opponent color label as the most noticeable one for 2.52 of the 4 maps). There is a statistically significant difference between these color combinations; label overlays using opponent colors are apparently more 455 noticeable than the 80 degree disharmonies. Thus, opponent colors maybe a better choice than 80 degree disharmonies for overlay colors. 23

24 Table 11: The two sample t-test for Opponent versus Disharmonious (80 degrees separation) Opponent 80 degree Mean Variance Observations T-Stat t-critical SIGNIFICANT 460 Table 12 shows the overall averages and two sample t-testing results for the pairwise tests of 80 versus 150 degree disharmonies. Counts here have a similar meaning as in the Table 11. There is a statistically significant difference between label overlays using colors separated by 150 degrees and colors separated by 80 degrees on the hue circle; just as in Experiment Two, label overlays using 150 degree separations are more noticeable than labels using colors with 80 degree separations on the hue circle. Table 12: Two sample t-test for two Disharmonious (150 degrees versus 80 degrees on the hue circle), average by participant 150 degrees 80 degrees Mean Variance Observations T-Stat t-critical SIGNIFICANT 465 We also tested on counts of participant choices for noticeability of labels colored with 150 degree disharmonies versus opponent colors. There is no statistically significant difference between label overlays using colors with 150 degree disharmonies versus opponent colors. Thus, either opponent colors or 150 degree disharmonies may be good choices for overlay colors in visualization Experiment Five Results and Analysis 470 Table 13 shows the count of participants responses for the series of pairwise tests in Experiment Five. The columns labelled Dis are the results for the

25 degree disharmonies. The columns labelled HS are the results for the highly saturated colors. The columns labelled LS are the results for the low saturated colors. The columns labelled HL are the results for the high lightness colors. 475 The columns labelled LL are the results for the low lightness colors. It appears that label overlays colored disharmoniously are more noticeable than the others. It also appears that high saturation color overlays are more noticeable than low saturation, low lightness, and high lightness color overlays. For each user, we determined the percentage of times each color combination was chosen in each 480 pairwise testing type and used that in proportion testing. Table 13: Count of participants responses for label noticeabilities, disharmonious, high and low saturated, and high and low lightness. Pairwise Noticeability Comparisons Dis HS Dis LS Dis HL Dis LL HS LS HS HL HS LL HL LL Participants The disharmonious versus highly saturated color label test results are shown in Table 14. Here, since we have only proportion information, z-testing (the Z Proportions test) was used. Here, on average, users chose the disharmonious labels 65.3% of the time. For our sample size, this was not statistically significant; there is not strong evidence that label overlays colored disharmoniously are more noticeable than labels with highly saturated colors. Table 14: Z-Test (z Proportions Test) for Disharmonies vs. Highly Saturated overlays. Disharmony Highly Saturated Proportion Observations Z-Stat Z-Critical NOT SIGNIFICANT The disharmonious versus low saturation color label test results are shown in Table 15. Again, z-testing was used to test significance. The proportion of participants selecting the disharmonious color (92.3%) is a significant outcome; 490 there is strong evidence that label overlays colored disharmoniously are more 25

26 noticeable than labels with low saturated colors. Table 15: Z-Test (z Proportions Test) for Disharmonies vs. Low Saturated overlays. Disharmony Low Saturated Proportion Observations Z-Stat Z-Critical SIGNIFICANT 495 The disharmonious versus high lightness color label test results are shown in Table 16. On average, users chose the disharmonious color 96.1% of the time. The z-testing indicates this is a significant outcome; label overlays colored disharmoniously are more noticeable than labels with high lightness colors. Table 16: Z-Test (z Proportions Test) for Disharmonies vs. High Lightness overlays. Disharmonious High Lightness Proportion Observations Z-Stat Z-Critical SIGNIFICANT 500 The disharmonious versus low lightness color label test results are shown in Table 17. On average, users chose the disharmonious color 53.8% of the time. The z-testing indicates this is not a significant outcome; there is not enough evidence that label overlays colored disharmoniously are more noticeable than labels with low lightness colors. Table 17: Z-Test (z Proportions Test) for Disharmonies vs. Low Lightness overlays. Disharmonious Low Lightness Proportion Observations Z-Stat Z-Critical NOT SIGNIFICANT The high saturated versus low saturated color label test results are shown in Table 18. On average, users chose the high saturated colors 88.4% of the time. 26

27 The z-testing indicates this is a significant outcome; label overlays with high saturated colored are more noticeable than labels with low saturated colors. Table 18: Z-Test (z Proportions Test) for High Saturated vs. Low Saturated overlays. High Saturated Low Saturated Proportion Observations Z-Stat Z-Critical SIGNIFICANT High saturated colors also were found to have a significant difference noticeability over both lightnesses. In summary, it appears that disharmonious color combinations are more noticeable than low saturated, high lightness and low lightness color combinations. High saturated colors are also apparently a better choice than low saturation, high lightness, and low lightness color combinations, as well. 5. Discussion We next report an investigation into the relation between perceptual distance of colors and noticeability. Perceptual distance was one of the parameters used by Gramazio et al. [8] to create discriminable and preferable color palette. Their work used CIEDE2000 color difference to quantify perceptual difference in colors [8]. Our investigation here likewise uses the CIEDE2000 measure. Our investigation considered the perceptual difference between the 80 degree disharmonies and 150 degree disharmonies. It was aimed at finding an objective determination of which class of disharmony is most perceptually different and thus likely most discriminable. Table 19 shows the perceptual difference of these two classes of disharmonious colors for 34 disharmonious color pairs (i.e., 17 pairs of 80 degree disharmonies plus 17 pairs of 150 degree disharmonies). The average perceptual difference for the 150 degree disharmony pairs was The average perceptual difference for the 80 degree disharmony pairs was The larger perceptual 27

28 differences for the 150 degree pairs is the likely reason underlying our observations of users finding 150 degree disharmonies to be more noticeable than 80 degree ones. Table 19: Perceptual Difference for the 150 degree vs. 80 degree disharmonies Perceptual Difference of Disharmonious colors We also considered the perceptual difference between the 17 pairs of 150 degree disharmonies and 17 pairs of colors based on subset of the Itten harmonies (used in our Experiments 1, 3, and 4). Table 20 shows these perceptual differences. The average perceptual difference of the disharmonious color pairs is and the average perceptual difference of the Itten-harmonious color pairs is This perceptual difference deviation is the likely reason underlying our 535 observations of the superiority of the 150 degree disharmonies over the Itten harmonies. Table 20: Perceptual Difference of Disharmonious (150 ) vs. Harmonious colors. Perceptual Difference of 17 Disharmonious and Harmonious Colors Itten Conclusion 540 This paper has considered the noticeability of certain classes of color combinations for map-based visualization, with emphasis on the value of disharmonious color combinations vis-a-vis other color theories. The results of these experiments, based upon the participants responses, provide evidence that label overlays colored disharmoniously are more noticeable than ones colored harmoniously. Disharmonious color combinations also appear more noticeable than low saturated, high lightness, and low lightness color combinations. Opponent 28

29 colors and possibly highly saturated colors may merit more study. Our experiments also provide evidence that label overlays using opponent colors are more noticeable compared to 80 degree disharmonies. More study is needed, though, to determine the relative benefit of opponent colors versus 150 degree disharmonies. We have also considered perceptual distance, which was suggested by [8] as one of the parameters useful to create discriminable and preferable color palettes, to provide another perspective on discriminability. The 150 degree disharmonies are more perceptually distant than are the other color combinations, which we believe is the likely reason for their higher reported noticeability. Future studies of the relation of noticeability to color disharmony could be valuable. It also may be interesting to consider other classes of harmonies, especially Matsuda s two other harmonies, in future work. It may also be interesting to study if cultural background has any relationship to any color combination s utility in information visualization. It may also be interesting to explore if age or known eye disorders, such as glaucoma, cataracts, etc., affect the relationship between perceived readability and color separations. Acknowledgment 565 We are very thankful to K. Hayashida and K. Tsuda who translated part of Matsuda s book [17] from Japanese. We also appreciate P. O Donovan and K. Yatani who answered our questions regarding Matsuda s book [17]. The human participants approval was taken from IRB (Institutional Review Board) Human Subjects Committee. References 570 [1] A. Brown and W. Feringa, Colour Basics for GIS Users, Pearson Education Ltd., Harlow, UK, [2] G. M. Buurman, Total Interaction: Theory and Practice of a New Paradigm for the Design Disciplines, Birkhäuser, Basel,

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