The Visual Knowledge Builder: A Second Generation Spatial Hypertext

Transcription

1 The Visual Knowledge Builder: A Second Generation Spatial Hypertext Frank M. Shipman III, Haowei Hsieh, Preetam Maloor, J. Michael Moore Department of Computer Science Texas A&M University College Station, TX USA shipman@cs.tamu.edu ABSTRACT The development of spatial hypertext systems was driven by the need to lower users effort of expression. Users express categories and interrelationships through the visual similarity and co-location of information objects. The ease of changing a visual property or moving an object makes spatial hypertext better suited to tasks where the information continually evolves. But the implicit nature of the structure poses challenges for tasks in which the authors and readers are not the same set of people. The Visual Knowledge Builder (VKB) includes the ease of expression of earlier spatial hypertexts while adding greater support for long-term collaboration and tasks requiring explicit links. VKB includes a history mechanism that records the evolution of the spatial hypertext and local, global, and historical links for explicit navigational connections between chunks of information. The mechanisms added to VKB make spatial hypertext applicable in a much wider variety of tasks. In particular, VKB s global links enable wide-area distributed spatial hypertext using the existing infrastructure of the Internet. Versions of VKB have been in use for two years in tasks including note taking, writing, project management, and conference organization. Keywords spatial hypertext, distributed hypertext, history, link types, navigation, hypertext applications INTRODUCTION The development of spatial hypertext systems was driven by the need to lower the user s effort of expression. Traditional hypertext requires authors to create explicit representations of interrelations between information nodes in the form of links. Using node and link hypertext to express an emerging or evolving understanding means structuring and restructuring the material over time. This overhead makes traditional hypertexts ill-suited to many types of analysis and design tasks particularly, situations where a lot of information must be interpreted rapidly or where a group shares and restructures information in order to coordinate or reach consensus [13]. Spatial hypertext lets users express categories and interrelationships through the visual similarity and colocation of information objects. VIKI, an early spatial hypertext system [5], emphasized the expression and manipulation of information structures implicit in the layout of their components. The low effort required to change a visual property or move an object meant that VIKI users rapidly became facile at information triage, expressing task-oriented categorizations of information objects and relationships between them [6]. In information triage and information sharing tasks, the resulting spatial hypertext is not meant as a publication, but as a continual work in progress for an individual or small group. The hypertext is not the result of the user s work but a means to an end. The readers and writers of the spatial hypertext were one and the same group so there was less emphasis on the aesthetics of presentation and little support for understanding the hypertext. This limited the applicability of spatial hypertext systems like VIKI. This paper presents the Visual Knowledge Builder (VKB), a spatial hypertext system which builds on the successes and addresses issues and limitations of VIKI. The next section describes some of the particular issues we have aimed at addressing and our overall approach. After this is an overview of VKB and a detailed look at its more novel features, including navigable history. This is followed by a description of VKB s local and global navigational links. To give a sense for the range of tasks and situations where spatial hypertext can be of benefit, we then present a number of examples of VKB use and conclusions. ISSUES AND APPROACH First generation spatial hypertexts emphasize ease of expression over ease of comprehension. Rather than explicit, potentially labeled or typed, links interconnecting chunks of information, the information is colored, shaped, and arranged to indicate relationships. The exclusive focus on implicit relationships means that spatial hypertext systems are appropriate for a limited set of tasks. Challenges for the more general use of spatial hypertext include (1) the difficulty of interpreting changing visualizations, (2) the visualization interfering with visual source information, (3) the inability to express cross-space interrelations, and (4) the lack of means for expressing publishable information spaces.

2 One of the strengths of spatial hypertext is its ability to support the expression of evolving interpretations. But this ease of expression comes with a cost, namely that the comprehension of the visual representation requires understanding what the visualization means. Some users create legends explaining their visualization, but many do not. Also, visual languages evolve over time, leaving prior encodings ambiguous particularly when users change the meaning of a visual feature and do not go back to make prior visual expression consistent. A spatial hypertext system could try to enforce consistency but that undermines the flexibility that is spatial hypertext s greatest strength. As an alternative, we are exploring the recording, indexing, and playback of the evolution of a spatial hypertext. A navigable history supports comprehension by allowing the reader to go back to the state of the hypertext at which the visual coding took place. Another weakness of our prior spatial hypertext systems is their inability to display media other than text in the workspace. Other information workspaces, e.g., Pad [8] and Data Mountain [10], can display images, video, 3D renderings, etc. But the challenge has been to combine a space where these entities can be displayed with means for the visual interpretation of these entities as is found in spatial hypertext. This is partly a problem of attaching visual interpretation to visual content. Towards the inclusion of heterogeneous information, we have added the ability to include images both as background for the content and as foreground content, yet still allow visual attributes to be attached to these visual information types. The expression of a relationship between objects in a spatial hypertext requires information objects to either have related visual attributes or placement. A limitation of this media of expression is that an object has different relationships in different contexts and one visualization is unable to express these multiple interpretations. As a solution, VIKI allowed each information object to have multiple visual symbols that could each be colored, shaped, and placed independently to express various relations. Unfortunately, this concept of an object having many symbols did not catch on in practice. Computer scientists understand how to make use of multiple pointers to the same data structure or symbolic links and shortcuts in a file system, but this appears to be unintuitive for many others. This is particularly true when some information, like the text, abstract type, and attributes, is shared by multiple visual symbols and other information, like the color, shape, and size, is not. Rather than recreate the symbol/object abstraction, we decided to reintroduce links and explicit relations to the representation. Besides allowing the expression of relations to objects far away from the object in question, this allows the connection between short and long representations about a concept that we see in navigational hypertext. For example, many links on the Web go from a terse description of a concept or entity to pages that describe that entity in much more detail, such as links from a person s name in a document to that person s homepage. While spatial hypertext authors express multiple interpretations of a concept at similarly varied levels of detail, the expression of the relation between them is difficult in spatial hypertext. A final weakness of spatial hypertext we will consider is the inability to do what the Web does so well author, publish, and distribute information for others. We consider the Web as a massively interconnected set of hypertexts. The ability for an author to create a link to a page authored by someone he or she does not and may never know enables the various means of indexing information that is required for that information to be accessed and be of any use. VIKI and Web Squirrel [1] allow spatial hypertexts to point to external documents, but do not support external documents pointing to the spatial hypertext. They improve the collection and interpretation of personal or group resource lists but are unable to be interconnected to take advantage of the distributed social construction of knowledge that we see on the Web. To address this issue, we added features for authoring and distributing publishable spatial hypertexts. There is a focus on greater visual control in the form of more flexibility with fonts, colors, and backgrounds. In addition to the links internal to an information space discussed above, we introduced external links so that one spatial hypertext can link to other spatial hypertexts on the Internet. The result is a WYSIWYG authoring and browsing tool for wide-area distributed spatial hypertext. Such a system is applicable for a wide variety of tasks where prior spatial hypertexts were inappropriate this greater applicability is our focus for second generation spatial hypertexts. THE VISUAL KNOWLEDGE BUILDER The current incarnation of VKB expands upon the spatial hypertext concepts in VIKI. The areas of particular emphasis have been (1) improved support for visual expression and presentation, (2) easier inclusion of and interaction with information from other applications, and (3) implicit visual types for improved structure recognition. Visual Expression and Presentation VKB retains the ability to visually mark objects with background color and border width. However, the ability to create objects of different shapes was removed since odd shapes resulted in an unreasonable trade off in screen real estate compared to the usable area for textual data. To counter this loss, VKB augments the possibilities for visual encoding by adding other visual features. Figure 1 shows the VKB interface containing notes from a conference that make use of many of these new features. Borders can now be assigned a color that is independent of the background color. There are default color schemes that link background colors with border colors and change both when either aspect of an object's color is changed. However, this link between background and border color can be broken so that their colors can be assigned independently. In practice, background colors are visually dominant and are frequently used to indicate high-level categorizations, and border colors are used as modifiers or to indicate subcategories. VIKI provided six color pallets that could be edited by specifying personal RGB values for color palette items in a

3 create object create collection create local link visual modifiers history toolbar transparent objects collections objects Figure 1: Six collections containing notes in Visual Knowledge Builder configuration file. The cumbersome nature of this editing process meant that individuals rarely edited the color palettes. Also, these pallet definitions were per user, not per task and so the same set of colors would be used for all a user s activities. VKB provides a slate of 12 modifiable pallets that are stored independently for each VKB space. Users also can set the background and border color of individual objects using a color selection dialog. This change has resulted in a wider variety of colors being used in VKB spaces for task-specific forms of expression. VKB includes a number of authoring features to support presentation-quality spatial hypertexts. Greater flexibility in textual expression is added through the ability to change font size, style, typeface, and color. Moreover, there is often an advantage in blending the contents of objects rather than having the borders between objects be so obvious. To allow this, objects can be made transparent this causes the content of the object to be displayed on the background of the collection. Transparency allows objects to overlap and still be visible. Another addition to VKB is the ability to display images in the workspace. JPEG and GIF image files are recognized and may be displayed at their full size or as thumbnails within an object. These objects retain borders and backgrounds so the same methods and visual language can be used for interpreting objects containing text and images. A final presentation-oriented addition is that images can be set as the background for a collection, in effect adding another background color choice. This can be used for aesthetics or to arrange objects within the semantic context of an image, e.g., to place objects on a map, drawing, or schematic. Integrating Information A challenge for any information system is to make it easy for users to get information into the system. With the growing number of applications and information types, files available via local disk or network can be dragged from the operating system s desktop or file browsers into VKB. Double clicking on the object's border will open the file in its default application. Images, as well as other types of information, can be added to an existing VKB object by dragging the file housing the data into the object. If the file is not dragged into an object, a new object is created to house the data. Similar to files, URLs can be dragged into VKB. As with file attributes, double clicking on objects with a URL will display the contents of the associated URL in a Web browser. Another mechanism that eases the transfer of information into VKB is based on cutting and pasting text into objects. VKB users tasks often involve collecting text from documents in different applications into a workspace. To speed up this process, the contents of the operating

4 time of edit history player maximized collection history slider and counter Figure 2: Going back in hypertext s history during a project meeting. system s cut buffer can be automatically pasted into newly created objects. Users can control whether this feature pastes the contents of the clipboard into only one object, into each object created, or not at all (effectively turning the feature off.) Upon object creation, the text is highlighted so users may immediately type over the pasted content if it is not what they desired. Implicit Visual Types One feature shared by VIKI and VKB is the spatial parser [14]. This parser recognizes structures implicit in the layout of objects in the workspace using each object s position, size and type. Prior application of the spatial parser treated all untyped objects to be of the same type when grouped in the parse tree. VKB increases the effectiveness of the parser by adding a preprocessor that assigns implicit types to all objects without an explicit type. The implicit type assignment is based on the similarity of visual attributes including width, height, background and border color, and border width. Visually similar objects are considered to be the same type while dissimilar objects are considered to be of different types. While the type assignment algorithm is heuristic, the removal of the need for explicit types has increased the usefulness of the spatial parser since users often neglect to assign types explicitly while maintaining visual consistency. VKB uses the results of the spatial parser for hierarchic click selection and for making suggestions about formalizing, moving, and adding information. NAVIGABLE HISTORY IN HYPERTEXT Most current browsers allow the reader to return to hypertext nodes previously visited. However they do not allow the reader to witness the authoring of the hypertext. Trigg s history cards in NoteCards provided a limited view of the authoring process by creating a summary each authoring session [15]. VKB includes a history mechanism that extends this perspective into a new time dimension for the hypertext, that of its construction [11]. Users navigate through this history, perceiving the constructive time of the document, by moving between the current and prior states of a document. Navigable history supports (1) learning and interpreting authors' work practices, (2) recognizing patterns of activity in the information space, and (3) disambiguating specific actions and content. VKB's navigable history mechanism is similar to Reeves embedded history mechanism in INDY, a graphical environment supporting computer network design [9] and Hayashi's temporally threaded workspaces [2]. There are four mechanisms for returning to a prior state of the VKB information space: (1) rewinding and replaying the history, (2) dragging a slider representing the whole history of the hypertext, (3) selecting the start of a particular editing session, and (4) selecting a specific edit event on a particular object or collection. The interface of the first two of these mechanisms is found on the history bar, located under the visual edit bar in the interface. Figure 2 shows the results of going back in time in a project workspace. The left side of the history bar has controls similar to a VCR interface, providing the ability to play forward and backward through the changes to the workspace in either a step-by-step or continuous play manner. The system allows the user to select one of five speeds of playback.

5 The middle area of the history bar includes a slider for quickly navigating forward and backward through a lengthy history. The position of the slider is relative to the number of edits in the history, thus providing feedback on where the currently displayed state is relative to the cumulative effort put into developing the workspace and how fast the current play-forward/playback is moving relative to its history. The number of events in history and the number for the event that lead to the current state are shown next to the slider. Figure 2 shows the project space as it appeared after 1551 edit events out of a total of The time and date of the event that left the workspace in the displayed state is provided on the right of the history bar. This provides the absolute time of the modifications to the document. By noticing the date and time of changes through playback or while moving the slider, the user can determine the workspace s age and pace of changes. Gaps between time stamps also provide a sense of authoring and editing session lengths and frequency. VKB uses gaps in the history to recognize editing sessions. Figure 3 shows the editing sessions for the workspace in Figure 2. The user may select one to go back to or view the list of edits in that session in this tree view. Figure 3: List of editing sessions for project space. Finally, users may want to return to a particular event in order to interpret its meaning. For example, to return to the event that left the background color of the bug fixes object in Figure 2 bright red. A popup menu provides access to particular actions for every object and collection. Access to the history of the spatial hypertext adds constructive time to the existing notions of time in hypertext [4]. The multiple indexing and access methods provide readers with a view of the authoring process that resulted in the current hypertext. This allows readers to better interpret the meaning of the hypertext and to better understand the author. For a more in-depth discussion of the VKB history mechanism and its design, see [11]. NAVIGATIONAL LINKS IN SPATIAL HYPERTEXT Spatial hypertext emerged due to the limitations of and difficulties with using explicit links to interconnect chunks of information. Visual and spatial relations take less effort to create and are better suited for expressing more tenuous or ambiguous connections, particularly in situations where the authors and readers of the spatial hypertext come from the same group. Traditional links require more effort to create but are better at conveying explicit relations to an unknown reader. VKB includes both the expressive and easy to author visual representations of spatial hypertext and navigational links. Authors may choose their method for expressing relations to match the content being expressed and the context of the communication. Links between spaces are not new each page in a nodeand-link hypertext provides some ability for visual expression along with the ability to create links to other nodes. HyperMap uses conditional links between information spaces for students accessing educational materials [16]. Also, while lacking the expressive characteristics of spatial hypertext, other spatial information environments have included the ability to move between elements in an information space through combinations of multiple views and zooming, e.g. Pad s portals [8]. VKB includes two major categories of links, which we call local and global, and one twist to the traditional link providing it with a historical/temporal component. Local Links: Navigating within a Spatial Hypertext Local links in VKB connect an object or collection to another object, collection, or location within a collection. By clicking on the link indicator, the user is transported to that new location. The end point of the link is selected and displayed as close to the center of the VKB window as possible. The navigation up and down the collection hierarchy is animated to help provide the user with a sense of the distance and direction traveled, somewhat like an implicit warp coefficient [7]. From a system s perspective, VKB s local links are the spatial hypertext equivalent to links between sections of a Web page. The effort saved from such a link, the movement through the hierarchy of two-dimensional planes is much more complex than scrolling up/down on a page. Thus, the perception of the reader navigating a large spatial hypertext may be more like that of moving between pages due to the unpredictable nature of the motion and the amount of user effort duplicated by the link. Links from one object to another or to a collection enable the use of more traditional hypertext narratives. Links to distant or hidden collections facilitate take this action or view this perspective choices by the reader. Spatial hypertext without links can use collections to express some similar story structures but are limited to hierarchies. Navigational links simplify the author s task of creating choices for the reader and expressing connections across collections. These cross-hierarchy connections turn the hypertext s structure into a general graph (a rhizome).

6 Global Links: Interconnecting Spatial Hypertexts The VKB global link extends the idea that objects may point to information outside of the spatial hypertext. Where VIKI objects could point to Web pages and VKB objects can point to Web-based or file-based information, a global link is a pointer to another spatial hypertext. VKB s global links point not only to another spatial hypertext, but can include the identity of the resulting focus within that hypertext. Such a reference is crucial for linking a short reference to an entity or concept to a space about that concept. For example, an object in a spatial hypertext on Microsoft employees might have basic contact information for Jonathan Grudin. This object could have a link to a spatial hypertext about him (his home space instead of a home page ) or to a collection within a spatial hypertext about his contribution to computer-supported cooperative work. Global links can point to spatial hypertexts on the user s file system or available via the Internet. By interconnecting spatial hypertexts authored by an individual or collaborators, VKB users can create and manage larger scale spatial hypertexts. Traditionally, spatial hypertexts were expected to evolve to match their changing role. The ability to connect related spatial hypertexts enables interconnections between mature, expressive task-oriented representations in one spatial hypertext and the emerging expressions related to new tasks in another space. Global links using the file system create a locally distributed spatial hypertext. VKB links pointing to spatial hypertext delivered via the Internet currently use http to transfer data between the provider of a spatial hypertext and its reader. When a remote spatial hypertext is loaded, VKB enters read-only mode, which allows maximizing and minimizing collections, scrolling, and opening objects but does not allow moving, resizing, or otherwise editing the contents of the hypertext. Global links using http allow for distributed wide-area spatial hypertext. VKB s combination of links between spatial hypertexts with objects in spatial hypertexts pointing to Web pages promotes the vision of a global spatial hypertext alongside the node-and-link hypertext that is today s Web. Historical Links: Navigating through Space and Time A novel feature of VKB links relates to the recording of the history of each spatial hypertext. In addition to local and global links specifying an end point for the link, VKB allows the link author to select a point in the history of the workspace as well. Using this mechanism, an author can navigate the reader through both the space and the time of a hypertext. When a user takes a link that specifies a historical context, the workspace opens the hypertext, returns to the specified point in time, and centers the view on the end-point s object, collection, or position. Links to prior states of a hypertext enable the contextualization of events. Applications like design rationale, which create an information space that is used to make and record decisions, can use historical links to connect results to the context of earlier discussions. Historical link creation makes hypertext a more selfreferential media. By linking to prior states, a hypertext can discuss its own evolution, promote the exploration of prior states, and direct readers to important periods of the hypertext s emergence. Indeed, critical portions of a hypertext (such as who done it in a mystery) could be authored and then deleted to deliberately conceal them from the view of readers random navigation. The VKB Warp Engine: Supporting Navigational Links Authors and readers need support in creating links through and navigating in spatial hypertext. The VKB Warp Engine fulfills this role by helping create and record jumps through space and time. It records navigation via links for backtracking and generates anchors for global and historical links. Link authoring VKB includes separate authoring interfaces for authoring local and global links. Local links are authored using the link button on the main toolbar. The link author first selects an object or collection as a starting anchor and presses the start-link button, whose icon is replaced by the end-link icon. The author may navigate and edit the hypertext as desired prior to identifying the link s endpoint. Once the endpoint is identified, pressing the same button will finalize the link and return the button icon to start the next link. Figure 4: Pool of encoded global links in Warp Engine. Authoring global links is a reverse-jump process. The user must first determine the end point of the link and then attach this to the starting point. The end point of a global link, called a VKB Resource Locator (VRL), is represented by a VKB data location, an optional timestamp or history index, and an optional focus. Users may type VRLs into the textual attributes of the starting object or collection or use the Warp Engine s VRL pool, shown in Figure 4. Figure 5: Encoding the endpoint of a global link. To add an element to the VRL pool, the user navigates to the end point of the desired link (potentially including

7 moving through the destination hypertext s history and selecting a focus.) When the user presses the Encode button, a dialog for generating the VRL (Figure 5) appears with options for whether to include the timestamp and focus of the destination. To create a global link, the user just drags an element from the VRL pool onto the starting object or collection in the source hypertext. Logging navigation Like a Web browser, VKB preserves the user s navigation through spatial hypertext as a stack so users can go backwards and forwards along their traversal path. Unlike the stack in Web browsers, VKB records the state of the hypertext prior to navigation and the end-point of the navigation as separate elements in the navigation stack. This is in part due to the navigation record being incomplete by design. Navigation in a spatial hypertext is more varied than navigation in a node-and-link hypertext. Besides taking links, the user changes what is visible by maximizing and minimizing collections and scrolling. Currently, VKB only records link traversal in the navigation log. Consequently, when a user hits the back button, a record of the state prior to link navigation is required to recover the user s context prior to navigation. Web browsers similarly record the position of scroll bars at the point of navigation. Using this information, the reader returns to the position on the page the user left rather than to the point of initial entry. Web browsers treat this state information as an attribute to the page in the navigation log. This means each page is in the navigation stack once for each time visited. In contrast, the navigation stack in VKB includes both the point of entry and the point of exit for spatial hypertexts. Figure 6 shows a navigation stack with icons on the left indicating whether the element represents the source or destination of a navigational link taken by the reader. Figure 6: Navigation log in Warp Engine. In a spatial hypertext, the movement through collections can drastically alter the context of the reader. The recording of start and end states for link navigation provides more context points. It is possible that recording link navigation results in too few context points for some applications. In this case, the navigation log could record minimizing and maximizing collection events as well, thus providing more accessible context points for the reader. Caching remote hypertexts Like Web browsers, VKB fetches remote hypertexts and saves them on the local file system before displaying them. A simple lookup table is maintained by the VKB Warp Engine to keep track of the currently cached VRLs. If a requested VRL is not found in the local cache, it will be retrieved from the Internet. Unlike Web browsers, all cached files are cleared when the navigation log is cleared, i.e., cache files are cleared each time the user exits VKB. APPLICATIONS VKB has been under development for almost three years and versions of the system have been in use for two years. Here we present a summary of our experiences along with scenarios of use to show the applicability of new features. Note taking and Writing VKB has been used for many real-world tasks including note taking, writing, project management, and conference organization. In addition, VKB has been used in a more controlled environment in a study looking at collaborative writing practices. A more complete description and analysis of early experiences with VKB can be found in [12]. Figure 1 shows an example of note taking in VKB. This is a traditional spatial hypertext application there is one author who is also likely to be the hypertext s only reader. VKB s addition of greater variety in border and background colors, fonts, and transparent objects are commonly used during note taking tasks. An advantage of VKB for note taking has been the history mechanism. Notes record and remind the author of a presentation, discussion, or idea. Often, notes are incomplete representations of earlier impressions used for recall. The history mechanism can increase the value of notes in VKB by allowing the author to replay the note taking process, triggering more recollections than static notes alone. VKB has been used for a wide variety of writing. An early version of VKB was made available for download via the Internet. This version has been in use by members of the hypertext research community for over a year for work on papers, presentations, and artwork. While informal, the feedback has been very positive. Students in a local class on computers, education, and literacy used VKB for collaborative writing assignments, with two authors taking turns while writing. Navigable history can aid in such asynchronous collaborations by allowing the replay of a collaborator s sessions. Much as change bars in a word processor help collaborating writers identify modifications and areas of activity, witnessing changes to the hypertext provides an understanding of the focus and type of activity. Features commonly used in writing partly depend on whether the author expects the final writing to be read in VKB or whether VKB is being used for generating, collecting, and organizing ideas for a document eventually written in another application. In both cases, visual features

8 Figure 7: Demonstration of a spatial hypertext newspaper. result in implicit visual types for objects arranged in structures frequently recognized by the spatial parser. Writers of documents that involve collecting information (research papers) take advantage of the drag-and-drop and paste mechanisms for getting information into the system. Writing spatial hypertexts for others to read results in a greater use of visual features like transparency, images, and widely varied fonts. An example of one such writing is the spatial hypertext newspaper in Figure 7, which is visually similar to CNN.com. Collections here are used to recreate the sections of a newspaper or news site. Local links can take the reader from the overview to hidden collections and stories in the spatial hypertext and global links can take the reader to materials in associated spatial hypertexts or available from Web sources. A difference between node-and-link hypertext and spatial hypertext for information delivery or publishing applications like this is the ability for a single spatial hypertext file to reasonably contain more information. In effect, the hierarchy of collections provides an interface for readers to move from section to section and story to story. Information providers can decide how big the information chunks should be based on the hypertext s content and the provider and readers network connectivity. With pagebased hypertext, content needs to be divided in part based upon readers willingness to scroll. Organizational Activities A second class of VKB activity makes use of the facilities for collecting, organizing, and visually coding objects. Examples of these organizational tasks include project management, conference organization, and course design. In all of these uses, objects are created to represent taskrelated elements that must be acted upon. An example is the spatial hypertext used within the Walden s Paths project, shown in Figure 2. This hypertext has been used in weekly meetings since November, Figure 2 shows a collection with lists of prioritized tasks. Other collections include paper deadlines and notes about evaluation. The projected hypertext is edited during project meetings so the whole research group understands how discussion is reflected in the layout and content of the objects. Since project members are both the authors and readers of this information, there has never been a legend explaining the visualization. The use of colors and border widths seen in Figure 2 has changed several times during the 20 months of use. In long-term uses like this, the history becomes an intellectual record. This workspace can be replayed to witness the development of the concepts, software, and changing priorities of the project. If there is a question regarding how long ago a particular task was prioritized or

9 Figure 8: Cataloging is a natural task for spatial hypertext s combination of hierarchy and visual interpretation. initially generated, the workspace can be returned to when the object was moved or created. VKB was also used by the first author while planning new courses and the Hypertext 2000 Conference program. Much like moving index cards around on a wall, the ease of visual manipulation makes it possible to rapidly try out different arrangements of materials during weeks of the semester or conference sessions. Additionally, the visual coding of objects by type makes it easier to determine if the schedule breaks desired constraints (such as simultaneous presentations with similar content at a conference or many class assignments being due at the same time.) Similar experiences have been reported by other users. Figure 8 shows an imagined spatial hypertext portal similar to Yahoo! The use of objects placed in a hierarchy of collections mirrors the hierarchy used for categorization. How much of the hierarchy each spatial hypertext contains should again depend on presumed network connections and the depth of the hierarchy. Local and global links can be used to add a category in multiple locations in the hierarchy. So far, this mirrors the information provided by Yahoo! Spatial hypertext adds the ability to use visual attributes and spatial positioning to provide further information about sites cataloged. For example, sites can be spatially clustered in the plane to indicate similarity. Border width and font size could indicate high quality or well connected sites and border color could indicate how long it has been since the site was last checked to ensure accessibility and correct categorization. Johnson-Eilola discusses characteristics of similar uses of space in [3]. Writing Revisited The earlier writing experiences emphasized the ability for spatial hypertext to change the process of writing rather than to change the content of what is written. Navigable history and links to points in the history enable new forms of expression. Already, people have created hypertexts where the reader is supposed to watch the hypertext as well as read the hypertext. VKB objects can become actors as well as conveyors of information. They can be made to dance, chase, threaten, and attract one another as well as to jump from space to space through the hierarchy of collections. CONCLUSIONS Spatial hypertext emerged to address problems of using navigational and semantic links for representing emerging and evolving interpretations. Early spatial hypertexts emphasized the use of visual and spatial cues to the exclusion of explicit links, but this limited their usefulness

10 for other tasks. The Visual Knowledge Builder (VKB) expands the applicability of spatial hypertext by providing authors with a variety of methods for expressing content and interpretation and by providing readers with tools to aid navigation and comprehension. VKB improves visual expression and presentation, eases inclusion of and interaction with information from other applications, and increases the range of visual structures recognized. VKB s addition of features for authoring presentation-quality spatial hypertexts enables publishing applications. This allows more applications where the author is not also the reader of the hypertext. VKB s navigable history records the emergence of the spatial hypertext. Through a variety of access methods, readers can go back to prior states of the hypertext to interpret ambiguous visualizations or play through the history to witness the evolution of the space. This adds a constructive notion of time to the hypertext that authors can use to express their ideas and interpretation in new ways. The addition of navigational links provides authors with methods for expressing interconnections between elements within a single spatial hypertext. Navigational links enable a generic graph (or rhizome) of information to be created from the hierarchy of collections. Additionally, links can express elaborations like those that make the Web as a whole a useful information resource rather than merely a large group of separate information sources. VKB s global links go beyond providing shortcuts for navigating between locations in a single spatial hypertext document. They provide connections between spatial hypertexts, allowing authors to divide their work into multiple spatial hypertexts and to link to other authors spatial hypertexts. This allows for a wide-area distributed spatial hypertext alongside today s Web-based hypertext. By specifying a timestamp, local and global links can be created that take the reader to prior states of documents. Such historical links create the potential for hypertexts to be self-referential in new ways. Experiences with VKB use identify improvements in the support for traditional spatial hypertext tasks and advantages of spatial hypertext for current applications of node-and-link hypertext. New applications are likely to emerge as authors create hypertexts that require the integration of textual, spatial, navigational, and temporal expression. ACKNOWLEDGMENTS This work was supported in part by grant IIS from the National Science Foundation and by a grant from FX Palo Alto Laboratory. We thank Cathy Marshall and Susana Tosca for helpful discussions about this work. REFERENCES 1. Bernstein, M. Web Squirrel. Watertown, MA: Eastgate Systems, Hayashi, K., Nomura, T., Hazama, T., Takeoka, M., Hashimoto, S., and Gudmundson, S. 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