Z-axis Based Visualization of Map Elements Cartographic Experiences with 3D Monitors Using Lenticular Foil Technology

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1 The Cartographic Journal Vol. 50 No. 3 pp International Cartographic Conference 2013, Dresden Special Issue August 2013 # The British Cartographic Society 2013 REFEREED PAPER Z-axis Based Visualization of Map Elements Cartographic Experiences with 3D Monitors Using Lenticular Foil Technology Kai Bröhmer 1, Claudia Knust 2, Frank Dickmann 1 and Manfred F. Buchroithner 2 1 Institute of Geography, Ruhr-University Bochum, Germany. 2 Institute for Cartography, Dresden University of Technology, Germany kai.broehmer@yahoo.de The current advent of autostereoscopic monitors and TV screens using lenticular foil technology proves the increasing desire to visualize movies, pictures, graphics or even maps in true-3d. These techniques could expand the cartographic tool box drastically, implying that 3D is no longer limited to the well-known representation of landform characteristics. Major map design aspects of multi-image models have been analyzed and evaluated regarding their use in thematic cartography. For map design using lenticular foil technology empirical findings are necessary to position layers adequately along the z-axis of a 3D depiction. Hovering layers may generate additional information values, which seem to be able to compensate the shortcomings in 2D maps. Several parameters or dimensions of cartographic contents can be displayed simultaneously. True-3D display techniques must be implemented in a differentiated manner to achieve positive impacts on cartographic communication. However, lenticular foil technique opens perspectives for more diverse products in thematic cartography. Keywords: digital map, lenticular foil technology, thematic cartography, true-3d, 3D monitor INTRODUCTION For the visualization of cartographic contents autostereoscopic displays play a particular role because they allow the viewing of 3D map elements without any additional viewing aids like polarization- or shutter glasses. The advantages of lenticular multi-image models in cartography are mainly that they can widen the existing spectrum of cartographic representation means. With the foreseeable development of a mass market, which currently begins to emerge through the first commercially available 3D TV screens based on lenticular foils (Ott, 2012), it becomes obvious that autostereoscopic techniques are also going to play a significant role in cartography. Hence, it is even more recommendable to derive empirically tested design rules for the construction of lenticular (thematic) maps (Dickmann et al. 2009; Dickmann, 2010; Dickmann et al., 2012). A current research project jointly carried out at the Ruhr-University Bochum, Germany, and the Dresden University of Technology, Germany, is aiming at the derivation of information about the visual perception performance using autostereoscopic cartographic representations on the basis of tests. Here, the studies into the communication efficiency of thematic maps are of particular importance (Bröhmer et al., 2012; Knust et al., 2011). The investigations are mainly made on autostereoscopic 3D monitors. It can be well assumed that through the use of the third dimension by means of the lenticular 3D effects a decisive superiority in comparison to twodimensionally designed thematic maps is especially showing up when there exists a heavy graphic load. In these cases various map symbols and even the labelling could be placed in different distances from the viewer at different levels, one above the other. Due to the available lateral motion parallax for objects at different depths, the viewer can, despite superimpositions, still identify underlying map symbols (cf. Section on PROSPECTS FOR THEMATIC CARTOGRAPHY ). TRUE-3D BY MEANS OF LENTICULAR FOILS For natural vision the two-eyed perception of a 3D object is subject to the different perspectives displayed at the retinas with slight lateral shifts, the retinal disparities, due to the different viewing points. As opposed to this, artificial stereoscopic vision uses a trick. Maps are 2D objects, which per se cannot cause any depth sensation to the map user. This can only be materialized if separate images are offered to the left and right eye, i.e. two views showing slightly DOI: / Y

2 212 The Cartographic Journal Figure 2. Front view of objects at different distances to the projection plane causing different disparities. The position of the objects is corresponding to Figure 3 (draft: C. Knust) Figure 1. Principle of the lenticular foil technique (Knust et al., 2011, p. 2) different perspectives. The generated horizontal parallax has to correspond in size to the disparity of the retinas. Through the merging of these two images the viewer is in the position to mentally generate a 3D spatial image (cf. e.g. Kraus, 1997; Gründemann, 2004). Until recent the separation of the stereo-partners corresponding to the left and right eye has frequently been realized by means of anaglyph-, polarization- or shutterglasses. The use of these glasses over longer periods finds only little acceptance among map users (Buchroithner et al., 2004). The lenticular foil automatically separates the images, so that no viewing aids (glasses), which as already mentioned before are frequently considered inconvenient, have to be used (Figure 1). In contrast to the common 3D visualizations where an object is depicted on a 2D plane in a perspective way (i.e. in pseudo-3d) and thus not really protruding above this image plane, a stereoscopic visualization causes a truly three-dimensional sensation for the map user. Hence, the phenomenological impression of the 3D depiction [ is] much more lively and more impressive than the flat visualization on a normal 2D-display (Cours, 2004). Consequently it is no surprise that so far lenticular 3D effects have preferably been used for the generation of socalled physical maps where the georelief was the main object of presentation (cf. Kirschenbauer and Buchroithner, 1999; Kirschenbauer, 2004; Buchroithner, 2007). The use of the z-axis can significantly increase the information content of a map. The possibility to arrange map symbols in multiple z-levels, which can be perceived in different distances by the map-reader, may decrease the graphic load although the overall amount of information per unit of area increases. With the help of true-3d visualization it is possible to integrate additional thematic information hovering above the base map without occluding the elements of this base map below. Diagrams, for example, can hover above relief, line and areal elements of the base map. PROSPECTS FOR THEMATIC CARTOGRAPHY Since the human perception is basically 3D, thematic mapping should also be 3D, more precisely truly 3D. It is expected that the visualization of thematic content should be as similar to the corresponding 2D mapping design as possible in order to optimize the interpretation capability of the map reader who is normally not yet used to look at truly 3D thematic maps. The truly 3D highlighting of map elements by means of the lenticular foil technique is mainly based on the use of different physiological depth cues such as motion parallax and disparity. For more information about the depth cues as well as for some examples the reader is kindly referred to books like Okoshi (1976), Albertz (1997), Goldstein (1997) and to publications like Reichelt et al. (2010) and Knust and Buchroithner (2013). Figure 3. Top view of objects at different distances to the projection plane causing different disparities. For the sake of easier understanding the viewing direction of the left eye was assumed to be in the direction of the objects O, A, B, C and D. The position of the objects is corresponding to Figure 2 (draft: C. Knust)

3 Z-axis Based Visualization of Map Elements 213 Figure 4. Improved information transfer rate using 3D effects. Left: stereo-mate 1; centre: stereo-mate 2; right: superposition of both stereo-mates (draft: K. Bröhmer and C. Knust) The apparent motion of hovering map elements is changing, depending on the distance between the map element and the projection plane in the virtual 3D model. Objects in the projection plane have zero disparity, i.e. they are located at the same position in the left and in the right stereo-mate. Map elements with different virtual distance to this plane have more or less great lateral disparities between their location in the left and the right image. In Figures 2 and 3, only object O is located in the projection plane. By shifting the lateral viewing direction either by head movement, shifting or tilting of the map the objects with greater distance to the projection plane move faster than the objects, which are closer to the projection plane, because their motion parallax is greater. This effect also occurs using lenticular foil technique and enhances the depth effect. Due to the motion parallax effect for objects at different depths the map-reader is, despite super-positions, still able to identify underlying map symbols. Figure 4 displays an example using this advantage of multiple z-layers. Details, which are not visible in the left view, are visible in the right view and vice versa. As shown in Figure 4, the 3D map enables the mapreader to communicate the contents of multiple information layers at the same x-y-position on a map. Due to the 2D character of the map symbols in 2D as well as truly 3D maps the positioning geometry of spatial elements, which is defined by centre points or the positions of the axes, is in 2D maps often occluded. This leads to blurring and distortions during the visual-cognitive process regarding the locations and names of objects as well as the geometric and factual information about status and relationship in maps (Bollmann and Koch, 2001). This danger of partial or total occlusions of underlying map information caused by map symbols or diagrams which need a lot of map space (Imhof, 1972) can be effectively reduced using lenticular foil technique. As already mentioned by M. Buchroithner at a very early stage of lenticular map production (Buchroithner, 1998; Buchroithner et al., 2000) and again pointed out in Dickmann et al. (2012), the graphical variables of Jacques Bertin can be enhanced into the third dimension. Map symbols can be created which use the third dimension for visualizing additional parameters (see Figure 5). Their position on the z-axis, i.e. the distance to the map-reader, gives information about the scaling value, a quantity, or a parameter class, which the symbol belongs to. Different thematic contents could be positioned as 2D objects in separate layers hovering at defined different distances above the base map: Quantitative diagrams (e.g. class-divided pie charts or bar diagrams in x-y-direction) can be positioned in different layers according to a further quantitative value. Another possibility is the creation of a separate thematic data-relief ( value relief ) which hovers above the terrain ( topographic relief ), e.g. the total annual precipitation is as a thematic relief in a semi-transparent manner or as a grid hovering above a mountain range. Thus, the Figure 5. Arrangement of the different map symbols at multiple levels. Symbols concerning the number of in-commuters (blue squares) and out-commuters (red squares) are located at different heights depending on their value. The other map symbols are located directly on the two-dimensional base map (updated; based on Dickmann et al. (2012), p. 142)

4 214 The Cartographic Journal Figure 6. Scheme for the arrangement of thematic layers on lenticular foil monitors (updated; based on Bröhmer et al. (2012), p. 231) mountain range below the thematic relief remains visible for the map user. Three-dimensional effects can also be used for the visualization of specific spatial data in a user- and activityoriented way. For example, in complex-analytic maps (Arnberger, 1991) it is possible to arrange the different information classes according to their factual meaning and thematic relevance not only topographically (x-y-position) but also at separate elevation levels (z-position). In economy maps the symbols for raw materials production are arranged below those for the secondary economy sector. And both categories are, again, arranged below the third economy sector. Thematically more significant, and therefore cartographically highlighted map information is visualized closer to the map user than less important subordinate data. Clearly, restrictions due to the reduced resolution subject to the interlacing of the stereo-mates and aspects of the perception-psychological efficiency are, in this context, also playing a significant role and have to be considered adequately. This is still topic of the current research (cf. Section on INTRODUCTION ). EXAMPLES FOR THE USABILITY OF Z-AXIS BASED VISUALIZATION As already mentioned in the previous section, one possibility to use the third dimension for cartographic map design is to encode thematic information of a geoobject with its position along the z-axis, i.e. according to the height of this object relatively to the base map. Figure 5 shows an economy map with symbols of the economic structure in the x-y-plane. The z-axis is used to show additional information, namely the number of in- and out-commuters at municipal level. In a traditional 2D map this arrangement would immediately lead to difficulties in recognizing the different map symbols. Due to abundance and mutual superimposition of the symbols a heavy graphic load and, thus, also a heavy cognitive load of the map would occur. With the 3D-effect of the lenticular foil technique this can be reduced or even totally eliminated. In our case, the quantitative parameters (numbers of commuters) are encoded through their position on the z-axis. The value of a signature can be decoded by using the perception of a signature as near or far away from the eye of the map user and interpreting it according to the explanation in the legend. For example, near is higher above the base map than further away and thus, near means a higher quantity than far away. For easier interpretation, it is proposed to insert a vertical scale in the legend and, if possible, also close to the symbol or diagram itself or at the corners of the map (see Figure 5). EMPIRICAL STUDIES INTO THE USABILITY OF Z-AXIS BASED VISUALIZATION For the modelling of the test maps to be visualized on 3D monitors within the aforementioned empirical studies, three work planes were used to arrange the map elements (using 3D modelling software) correctly in different levels without blurring: the proximate plane (near plane), the projection plane and the distant plane (far plane) (Figure 6). The proximate plane is the foremost border plane for the visualization of the scene. All objects of the test map situated between proximate plane and projection plane will be seen in front of the monitor. The projection plane is identical with the monitor surface itself. The zone between proximate and projection plane determines the near range. On the opposite, the far range is the zone between projection plane and distant plane. All objects in this range will be seen behind the monitor plane (Spatial View, 2008). Especially for multi-user monitors, i.e. 3D monitors with more than two stereo-images, the changing sharpness along the z-axis is still a problem for the map design. Many monitors show, with increasing distance from the

5 Z-axis Based Visualization of Map Elements 215 Figure 7. Two of the eight blocks of the perception test regarding the distinction of different z-levels. The signatures are hovering at different distances above the satellite image, which is placed in the background (draft: C. Knust) projection plane, a reduced sharpness. This implies that objects, which are far behind the monitor plane or far in front of it, will, to a certain degree, be seen as blurred. This is not really problematic for simple map symbols like rectangles or circles, but for detailed or complex map symbols like diagrams, iconographic map symbols or fine lines and labelling it is. Consequently, for 3D map design for such monitors one may consider the following proposal: If complex map symbols are used, place them into the monitor plane in order to guarantee their legibility. Simple map symbol can also be placed in the proximateplaneasshowninfigure6. For single-user monitors with the same screen resolution as a multi-user monitor normally the picture sharpness is higher, because they use only two stereoimages instead of for example five or eight, like for the multi-user monitors. Thus, the resolution of each half image is higher, and the stereo-image will be perceived in a better quality. One objective of the current research project was to determine the ability of differentiation of multiple z-levels. The test pattern contained a satellite image in the distant plane. Two-dimensional signatures were placed in or in front of this plane. There were eight blocks. Figure 7 shows two of them. The signatures hovered at different distances above the satellite image, but within one block they were placed at the same distance. Four different but equidistant z-levels were shown: each two non-neighbouring blocks with signatures in the distant plane, in one plane between distant and projection plane, in the projection plane, and in the proximate plane. That implies the subjects had to perceive each of the four z-levels twice (two blocks) in order to answer correctly. The result for the tested single-user monitor corroborated the quality of its 3D visualization. Twenty-three out of 30 participants, i.e. 76.7% perceived all planes correctly. The others thought to see more than the four different planes, i.e. five, six or even seven planes. This means, that a distinction along the z-axis is possible on the tested monitor for at least four planes for many or even most of the map users. More in depths investigations are planned in order to verify these initial results. Further spot tests show that the two multi-user monitors used within the project do not give similar results regarding the determination of planes in different z-levels. It became clear that here the differentiation is more difficult and more error-prone than for the singleuser monitor. The reason is not only the different number of stereo-images. The tested multi-user monitors also require greater viewing distances (three to four times the viewing distance of the tested single-user monitor). This implies for the map design that either the number of z- levels has to be reduced or the distance between these different planes has to be enlarged. This enlargement is, however, limited because of the problem of the reduction of the picture sharpness with increasing distance to the projection plane. Thus, both parameters have to be balanced carefully in order to create a well legible cartographic depiction. VALUE-ADDING OF INFORMATION BY MEANS OF MOTION PARALLAX One important prerequisite for 3D vision is the different speeds with which two objects at different depths are viewed by an observer, if they are shifted along the observer s visual axis. As already mentioned, objects that are closer to the observer move relatively faster sidewards than the objects that are further away, if the observer moves laterally. According to this the images on the observer s retina do. This effect is simulated by the lenticular technique. The viewer can use it, to shift objects on a static map by moving his head in order to see which objects are located above each other. Thus it is possible to overlay map symbols in different spaces to the observer. Some empirical studies demonstrated that the use of the motion parallax achieves a higher information transfer. A test group of 30 persons was asked to identify point symbols on a diagram map, which were arranged vertically under lenticular floating diagrams. Up to 70% of the individuals were able to determine the correct position of all point symbols. However, problems occurred with comparatively complex and subdivided maps (Table 1). Especially with little symbols the subjects had to face perceptive problems (Figure 8). Table 1. Results of map reading test with map symbols at different levels Test map Positioning of the diagrams Percentage of correct answers (%) Response time (seconds) Rectangular correlation diagram Proximate plane Bar diagram Proximate plane Pie chart Projection plane

6 216 The Cartographic Journal Figure 8. Point symbols overlayed by bar charts (map detail) (draft: K. Bröhmer) The visibility of map symbols worsens with increasing distance to the projection (5 image) plane. In addition, the so-called cross-talk effect leads to an increased leaping and makes it more difficult for the viewer to clearly view the symbols which lie closer to him. The extrinsic load during the perceptive process seems to be too massive. This is reflected in the longer time-spans, which were needed by the subjects to solve the respective tests. CONCLUSIONS The use of the z-axis orthogonal to the map plane causes an increase in information for graphical low-key maps. This refers to the construction of multi-layered thematic maps making use of motion parallax. Initial empirical findings of a study prove, that the changeable central perspective caused by the lenticular autostereoscopic three-dimensionality allows the observer to discern superimposed (vertically staggered) map symbols in the horizontal direction. The advantage towards 2D maps is the possibility to perceive map symbols laying on the map basis behind diagrams or other map symbols needing big space requirements. However, at the current state-of-the-art, it is important to comply with comparatively big minimum dimensions for map symbols. Like Bertin s variables, the lenticular three-dimensionality offers the possibilities to create map symbols, which differ, according to factual importance, in both scaling level and graphical graduation. Furthermore, they provide another important advantage: as an additional variable they allow the save of input of an additional 2D-symbol category in the map. This implies that on the one hand the graphical load of a map can be reduced by means of true three-dimensionality. On the other hand, by maintaining the load, 3D map symbols can give additional information to the map user. BIOGRAPHICAL NOTES The author Kai Broehmer studied Geography at the Ruhr-University Bochum. During his master programme he focused on Digital Cartography and the visualization of maps. He worked as an Academical Staff in the research project, described in this article. He also did his master thesis Dreidimensionale Darstellung von themakartographischen Diagrammen auf einem autostereoskopischen Lentikularmonitor about the true-3d visualization of maps under the review of Professor Frank Dickmann. ACKNOWLEDGEMENTS The authors want to thank the German Research Foundation DFG (Deutsche Forschungsgemeinschaft) for funding this project (BU 949/23-1 and DI 771/5-1). Special thanks may also go to all students and colleagues participating in the empirical studies, which already took place within this project. Our gratitude shall also be expressed to all staff members and students who were involved in various project tasks and discussions. REFERENCES Albertz, J. (1997). Die dritte Dimension Elemente der räumlichen Wahrnehmung, in Wahrnehmung und Wirklichkeit Wie wir unsere Umwelt sehen, erkennen und gestalten, ed. by Albertz, J., Schriftenreihe der Freien Akademie Berlin, 17, Freie Akademie, Berlin. Arnberger, E. (1991). Thematische Kartographie, Westermann Schulbuchverlag GmbH, Braunschweig. Bollmann, J. and Koch, W. G. (eds.) (2001). Lexikon der Kartographie und Geomatik, 1, A Karti. Spektrum Akademischer Verlag, Heidelberg/Berlin. Bröhmer, K., Knust, C., Dickmann, F. and Buchroithner, M. F. (2012). Die Nutzung autostereoskopischer Monitore zur kartographischen Visualisierung von Diagrammen, Kartographische Nachrichten, 5/62, pp Buchroithner, M. F. (1998). Satellite Imagery and Hill Shading: Synergetic Use for a New Type of Mountain Image Maps, in Wiss. Arbeiten Fachrichtung Vermessungswesen Univ. Hannover, 227, Festschrift Univ. Prof. Dr.-Ing. Dr.h.c.mult. Gottfried Konecny on the occasion of his retirement, pp , Hannover. Buchroithner, M. F. (2007). Echtdreidimensionalität in der Kartographie: Gestern, heute und morgen, Kartographische Nachrichten, 5/57, pp Buchroithner, M. F., Schenkel, R. and Kirschenbauer, S. (2000). Display techniques for cartographic purposes: semiotic aspects, International Archives of Photogrammetry and Remote Sensing, Vol. XXXIII, Part B5, pp Buchroithner, M. F., Gründemann, T. and Habermann, K. (2004, Invited Paper). Theoretical modeling aspects of bitemporal true- 3D hardcopy production, Commemorative Volume for the 60th Birthday of Prof. Dr. Armin Grün, ETH Zurich, Institute of Geodesy and Photogrammetry, pp

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