Recalling R1, we design our occlusion-aware techniques in a way that they support the duality of occlusion. This means that our solution considers and allows for em- ploying both the positive as well as negative aspects of occlusion. The ObjecTop system allows users to intentionally occlude digital items for the purpose of man- aging space or deliberately hiding items. Thus, once occlusion takes place, we do not automatically relocate the digital objects. Instead, the system informs users by showing unobtrusive visualizations of occlusion and representing occluded items through proxies.
Proxy
Informed by the literature analysis (3.1), we employ a proxy-like representation of occluded items (cf. figure 3.11) which is basically a semantically scaled represen- tation of the actual item. Proxies serve two main purposes – namely, delivering an adequate level of awareness to users about occluded digital items and providing an additional strategy to resolve occlusion (see access-supporting concepts). Typi- cally, they are placed on the nearest edge of the physical occluder and therefore the spatial relationship between occluded items and physical occluder is preserved. As an example, figure 3.10 shows that the media player is occluded and its proxy is visualized right on top of the occluder edge.
However, in situations where there is not enough space to place the proxy right on the nearest edge of occluder (due to, for instance, surrounding occlusion or tabletops’ edges) it is placed far from the actual object. Figure3.10illustrates an example that due to the lack of enough space near to the edge of tabletop, the proxy is located far from the occluded image object. To mitigate this problem, we designed a beam that connects the proxy with its actual occluded item to convey some spatial information about occlusion. The direction of the line indicates the orientation to which the occluded item is placed. The width of the line also gives a rough estimation of how far the proxy and the actual object are from each other. The beam helps users to approximate where an occluded object is located under a physical occluder and could potentially support users in moving the occluder to resolve occlusion (R5).
Figure 3.10: Proxy placement – beam connects the actual object and its proxy repre- sentation (Highlighted objects indicated by dashed lines on the book and document are not part of the interface and are added in Photoshop for ease of understanding.)
Drawing upon R3, proxies can represent different levels of awareness about the occluded object. It ranges from showing no information about the hidden objects to uncovering the original object with various intermediate levels in between. We represent this spectrum by three main levels as depicted in figure 3.11:
Presence provides presence information about an occluded item under the physical object. This includes the type, number and rough location of occluded objects. This level minimizes visual clutter, providing only an indication of the presence of occluded objects.
Figure 3.11: Different levels of detail represented by the proxy
Identity showing information about the type and identity of the occluded objects – such as the icon and the name – so that users can identify the occluded object.
Interactivity shows detailed preview information – such as a thumbnail or minia- ture version of the occluded object. Similar to the thumbnail view on the Windows Taskbar, it provides basic functions so that users can interact with an occluded object without retrieving it. Although this representation occu- pies more space than the other awareness levels, it supports a high level of engagement with the occluded objects.
Full access the occluded object is detached from the halo and moved to a position outside the area of the occluder.
The selection of varying levels of detail depends on the importance or relevance (R4) of the occluded objects to the context of the user. Since tabletops enable a spatial arrangement of digital and physical objects on their surface, the degree of importance of occluded objects is determined by their proximity to the user. Based on the in-depth analysis of Sellen and Harper on physical workspaces [Sellen 2003] as well as findings from the study on reachability for tabletops [Toney 2006], we feature three functional zones based on the distance to the user:
Active nearest zone to the user within the arm reach. Objects placed on this zone (physical or digital) are highly related to the context of users and frequently used. In this zone, when a physical object occludes a digital one, the highest level of awareness (i. e. the interactivity level) is shown by the proxy. Thus, the user can quickly find and engage with the occluded objects that are highly related to her context.
Figure 3.12: Proxy representations on each functional zone
Intermediate is a zone at arm’s reach for temporarily placing objects that are not needed. Once occlusion occurs, the proxy provides awareness such that users can easily identify digital objects
Storage this is the farthest area on tabletop where the user can reach objects by leaning over the tabletop and is used for long-term storage of objects. In the case of occlusion, only a halo is represented indicating the presence of occluded objects.
These zones are illustrated in 3.12. By mapping different levels of detail to the different zones, the system offers a trade-off across the tabletop zones between user’s intention as well as introducing visual clutter by the proxy representation. This means that although the interactivity level introduces more visual clutter in the active zone, it facilitates user to effectively interact with the frequently used occluded digital and physical objects. For the objects occluded farthest away and less frequently needed by the user, the system provides low-level representation thus, less visual clutter is introduced.
Displaying the basic proxy representation enables design of access- and organization- supporting techniques discussed in the next sections.
Figure 3.13: Accessing an occluded document by dragging out the proxy