Use of Depth Cues for the Annotation of 3D Geo-Virtual Environments

An increasing number IT applications and systems applies 3D geo-virtual environments as user interfaces, that facilitate the intuitive communication of spatial information, whereby depth cues play an important role for mental reconstruction of the depicted scene. These applications require the integration of application-specific information, represented by annotations such as textual labels or symbols, in analogy to labeling in traditional 2D maps. This contribution presents first results on how to improve information transfer with respect to annotations used within. We found that automated annotation techniques, used in traditional 2D map presentations, if applied to 3D geo-virtual environments, can damage depth cues and, thereby, possibly harm the human perception. Illustrated by examples, we show the override of different depth cues and discuss possible solutions. Finally, we propose a number of user tests, whose results are needed to improve the quality of automatic label placement techniques in the future. Introduction In cartography and GIS, a growing number of applications are being extended towards 3D geo-data modeling, management, and visualization. This trend can be explained by several reasons. First, an increasing number of companies offer a fast, cost-efficient and detailed process for the 3D data acquisition, so that 3D data becomes widely available. Second, the availability of 3D data models enforces research on techniques that take advantage of the additional dimension and increased precision. Finally, specialized applications and geoservices, e.g., Google Earth, NASA Worldwind, Microsoft Virtual Earth, lead to a growing user bases becoming familiar with 3D geo-virtual environments. Annotations are essential elements to enhance geo-virtual 3D environments with meta information about visualized objects, such as application-specific, thematic, or attribute information. Annotations occur in the form of text labels, iconic symbols, or 3D geometries. In contrast to a separate legend, they effectively provide information in place or near the reference, allowing a viewer a more direct and error-free access. The way how annotations are added to a depiction depends on the intended tasks for the result. The annotation process includes the selection of the visible annotations (e.g., thematic, spatial, or by the view frustum), the presentation style (e.g., font type, font size, or color), as well as the placement position. In general, annotations should be placed legible, over or close by the object they are referencing, and without overlaying other scene elements or annotations. For the automated annotation placement on two dimensional maps a number of techniques have been developed [CMS95], [ECM96], [EKW03], but little is known whether these techniques can be applied on 3D geo-virtual environments without adaptations, or how they need to be extended. This contribution presents first results on how to improve information transfer with respect to annotations used within. Evaluating existing implementations, we found that techniques used in traditional map presentations and illustrations can disturb depth cues and thereby harm the perspective impression. In the following, we illustrate these cases by examples and discuss possible solutions. For the further development of automated annotation techniques, we propose some user test related to different perception issues. 3D Cartographic Information Transmission Multimedia maps and 3D geo-virtual applications follow semiotic guidelines, similar to traditional cartography, with the basic intention to maximize benefit in gaining and transmitting spatial related information. For this task, multimedia, interaction and 3D with their support of different sensual modes, enhance information transfer for a bidirectional communication process [Gol99], which become important on graphical restricted displays, such as small scale displays [Rei03]. At the same time demands for high geometric precision of map elements and perceptibility of the content have to be met [Mac01]. For instance, a precise use of localization for a dense set of annotations leads to overlapping and thus to illegibility. Selection and repositioning has to be done to keep perceptibility. On the other hand, repositioning may lead to a damage of topology (neighborhood), especially when small scale maps with high information depth are created. The grade of multimedia, interaction, precision, perception, and abstraction depends on the used interface. For example, the traditional interface “paper” provides high precision, well-defined perceptional procedures for the elements and a high grade of abstraction, but lacks of multimedia, interaction, and immersing information transmission. In constrast, digital displays provide limited resolution, which cause perceptional discrepancies, but a high grade of multimedia and interactivity. An immersing information transmission with low textual abstraction can be established on digital displays with 3D geo-virtual environments, which provide a kind of natural spatial recognition by physiological and psychological depth cues [Alb97]. 3D geo-virtual environments visualized through 2D media, e.g., depictions based on perspective projections of 3D scenes, make use of depth cues, which are basically independent from the human ocular system and only focus on cognitive procedures. The damage of these depth cues causes misleading information transfer and, in most cases, makes intuitive perception impossible. In case of annotations used in 3D geo-virtual environments various cases of disturbed depth cues occur and an demand for specific solutions. A classification of these cases helps to clarify applicability and dependencies of annotations for cartographic purposes within 3D geo-virtual environments. 3D Geo-Virtual Environments The extension of concepts, algorithms, and applications towards the third dimension represents still a major point on the research agendas in the geo science and computer science communities. Frequently, the keyword “3D” often refer to different aspects: • 3D Data Models: Modern data acquisition techniques, e.g., laser scanning, or stereoscopic aerial photography, permit a fast, high-detailed, and cost-efficient production of geo-data that overcome the spatial limitation of planar surfaces. As a result, 2.5D digital terrain or surface models and 3D city models, including buildings, city furniture, and vegetation, becomes widely available and more usable, facilitated by the evolving standards, e.g., CityGML [KGP05]. In addition, the demand for 3D geo-data is increased by applications that already benefit from 3D geo-data, e.g., virtual city planning applications, noise simulations, or spatial visibility calculations for advertisement and security monitoring. • 3D Visualization: Printed maps and GIS applications commonly use twodimensional representations, e.g., 2D points, line, or polygons, to depict spatial information, like points of interest, railroads, or land-use data. Driven by the increasing availability of 3D data models and modern graphics hardware, included in today’s standard computers, more and more applications visualize these 3D data sets directly. After the preparation for a 3D scene graph representation (e.g., for terrains, buildings, or vegetation) and the definition of a virtual (perspective) camera, images can be generated for every position and viewing direction. • 3D Presentation Media: Independently from the contents, paper maps and computer screens are two dimensional presentation media. If they show perspective views on 3D objects, we are, however, still able to mentally reconstruct these scenes. A step towards an immersive, more real-world experience, can be reached by using 3D presentation media, such as stereoscopic viewing techniques, e.g., color coded stereo, shutter glasses, or head mounted displays. In the following, we use the term 3D geo-virtual environments to refer to the most common applications using 2.5D and 3D geo-data today: the 3D visualization using a perspective view transformation, generating depictions on a standard computer screen. Annotation of 3D Geo-Virtual Environments The extension from 2D cartographic maps to 3D geo-virtual environments implicates a number of changes related to annotation techniques. Now, not only point, line, and area features, but as well volumetric objects, e.g., buildings, demand for annotations [MD06]. Additionally, annotations them self are no longer restricted to two dimensional representations, such as symbol images or texts. The term “Geo” in 3D geo-virtual environments point out to the differences when annotating virtual environments that are based on a virtual terrain or globe. For example, in virtual illustrations the object of interest is normally centered in the depiction and rotating and zooming are the dominant navigation techniques. Therefore, it is obvious to look for white-space around the silhouette of the object, which than can be used for the annotations. In contrast, navigation techniques related to geo environments are often using analogies from the real world, e.g., walk/drive for virtual pedestrian/car, or the flying metaphor for birds-eye sceneries. Here, only regions showing the sky are directly available as annotation white-space. Commonly 2D text and images are used as annotations, so applying these to 3D geo-virtual environments raises the question about the principles and rules that ensure a usable combination of both in one depiction. We found two fundamental approaches here: screen space annotation and the scene embedded annotation techniques. The screen space techniques add annotations after the perspective view transformation, such as in a separate overlay layer to the depiction. Hereby, the search space for algorithms is reduced to two dimensions, a fact that eases the annotation placement and supports the legibility of annotations by concept. In co