Human perception of structure in shaded space-filling visualizations Pourang Irani 1 Dean Slonowsky 2 and Peer Shajahan 1 1 Department of Computer Science, University of Manitoba, Canada and 2 Department of Statistics, University of Manitoba, Canada Correspondence: Pourang Irani, Department of Computer Science, University of Manitoba, 556 Machray, R3T 2N2, Canada. Tel: þ 1-204-474-8995; Fax: þ 1-204-474-7609; E-mail: irani@cs.umanitoba.ca Received: 15 August 2005 Revised: 21 December 2005 Accepted: 6 January 2006 Online publication date: 10 April 2006 Abstract Very early in the object recognition process the human visual system extracts shading information. While shading can enhance the visibility of structures, it can have a negative impact on the judgment of sizes of elements in a structure. In certain visualization systems the underlying hierarchical structure is not noticeably explicit, such as in space-filling techniques. We hypothesize that in such cases, shading can make the structure more explicit. In this paper, we report the results of two experiments designed to investigate the effects of shading information on extracting the structure in space-filling visualizations. In the first experiment subjects performed better with the visualization tool with shading on structure-based tasks. Our results do not show that shading impairs users’ judgment on size-based tasks. A subjective evaluation shows that users preferred interacting with the system when shading was available. The second experiment was designed to investigate further users’ capacity to identify structural elements within the space-filling visualization. A sub- structure recognition task was employed in this experiment and results show that users are capable of identifying sub-structures quicker and with fewer errors when the visualization tool was equipped with shading information than without. The results of both experiments provide evidence that shading information can be used to effectively obtain structural information from space- filling visualizations. Information Visualization (2006) 5, 47–61. doi:10.1057/palgrave.ivs.9500113 Keywords: 2½-D Representation; structure-from-shading; shading and perception; space-filling visualization; TreeMap; CushionMap Introduction Hierarchies are common and abundantly available in our daily informa- tion managing activities. They describe the relationships among entities in file systems, organizations, and social structures. Hierarchies are organized into levels with a root at the top most level, and all other components at sub-levels. To adequately navigate or locate components within a hierarchy the structure needs to be evident to the user. A popular and common representation of hierarchical data is the node-link tree (Figure 1). The structure of the hierarchy is explicit and visually clear to the user (i.e. we can clearly see all the child–parent relationships in the hierarchy). However trees are not space efficient. A significant amount of space remains unused in the background as a result of creating an adequate layout for the nodes. Research in Information Visualization has developed a range of displays known as space-filling systems to make more efficient use of the display space available to the user. These systems are characterized by their compactness and effectiveness at showing the sizes of elements in a hierarchy. The basic space-filling representation divides the display area Information Visualization (2006) 5, 47–61 & 2006 Palgrave Macmillan Ltd. All rights reserved 1473-8716 $30.00 www.palgrave-journals.com/ivs