Our main effort so far has revolved around the creation of a server that collects and merges personal bookmark files of participating users. Besides personal bookmark files, we have included one general purpose subject guide in our initial experiments as well, namely, Yahoo , whose role we will subsequently explain. Such a database combined with a World Wide Web server, which we call a Group Asynchronous Browsing (GAB) server, can then provide access to a merged subject tree structure in various ways. This collection of tools is intended to address the issue of how to utilize the browsing activities of others to discover resources, some of which themselves may be guides to further World Wide Web resources. Secondly, we have created a tool that can monitor resources of interest and alert users when significant updates appear. This tool, which we call WebWatch, serves the purpose of resource discovery by drawing users' attention to changes to a known document which itself may be a subject guide to other World Wide Web resources. These tools go part way towards meeting requirements for providing asynchronous, community-based browsing on the World Wide Web.
In the remainder of this paper we will first discuss a vision and the current status of our GAB server. Next, we will describe a client application we have created to visualize and browse over merged subject trees in the GAB database that is built on Pad++ , a substrate for creating zoomable human computer interfaces. Then we will briefly discuss WebWatch and conclude with comments on limitations and future research.
Figure 1 is an abstract example of such a merged subject multitree, where each subject tree is indicated with a different color and the categorized URLs are indicated as capital letters at the trees' leaves. Subject headings would be attached to each of the nonterminal nodes of the trees. This example only shares tree structure at the terminal nodes of the trees, which is consistent with the sharing of links that occur across personal bookmark or hotlist files, but it should be noted that multitrees can share nonterminal nodes as well.
One of our goals then is to explore World Wide Web services that might be based on such merged subject trees. Assuming that a server is involved, there are several issues that must be addressed. First, there is the data gathering problem, i.e., the server must acquire access to useful subject trees. To date, we have assumed that, with one exception, these are in the form of individual bookmark files that can be created with widely available browsers. For each service that a server then might offer, there is the issue of how to define a client request as well how to define a server response. We have considered both browsing and querying services that make use of structural relationships in the merged subject trees. Various user interfaces are of course possible, some of which are afforded by existing World Wide Web clients and some of which would require customized browsers.
To date our implementation has focused on the server response side for standard World Wide Web browsers as well as for a customized client. The data gathering problem we have finessed so far by experimenting only with World Wide Web users within our own organization whose personal bookmark files are accessible within our local file structure. However, note that this local scenario may well be useful for intra-organizational information sharing that is deliberately kept proprietary. For wider public access, a forms interface such as can be seen in Webhound  could be used for users to submit their bookmark files to a GAB server. We started with bookmark files associated with the widely available Netscape Navigator. Our choice of Netscape is not to be taken as an endorsement of this product, however. Any tool for hierarchical organization of World Wide Web resources would do. Specialized clients that incorporate more sophisticated personal indexing facilities such as Paint  or Simon  would make our proposal easier to realize in one sense; however, choosing a commercial client that is in widespread use brings with it obvious advantages.
We have so far explored three services:
The first two services involve a "top-down" request on the client side that specifies which subject trees to include by simply choosing them from a list. Once this data is received on the client side, it may be browsed there. We envision that this sort of request, refined to include searches on other features of the participating user community and augmented to include the choice of specifying individual subject headings, might be useful for browsing over a relatively unconnected GAB subject tree. The hypothesis is that if one can choose a promising set of subject trees, perhaps by identifying a suitable group of people, it may be fruitful to simply browse over the merger of the trees looking for new resources. A possible indicator of quality of URLs in this scenario is the number of occurrences of a particular resource across the collected group. In other words, if a resource is indexed by many people, it may indicate that the resource is likely to be a better one compared to ones that are not so widely included.
Figure 2 shows a screen dump of an HTML document, being viewed through a commercial World Wide Web browser, that is the result of a query to the GAB server to include a set of participating users' bookmark files (along with Yahoo). The subject headings are indicated as paths from the root of the bookmark tree files. URLs that appear in other GAB subject trees are evidenced as "See Also" crosslinks that appear indented below the URLs in question. The user can then browse to potentially related URLs by clicking on a cross reference link that will jump to that portion of the document that includes other subject listings that also included that same URL. So, for example, note that the URL titled "The Mother-of-all BBS", categorized under Kent Wittenburg's bookmarks with the subject path "WWW Resources:Navigation" also appears under Will Hill's bookmarks under the category "Search Engines." A click on the latter link will jump to that category of Will Hill's, where related resources may then appear.
One of the weaknesses of this browsing method is that users can easily become disoriented about where they are in the information space--an intra-document instance of the well-known "lost in hyperspace" problem. Our informal observation is that this phenomenon invariably occurs when one makes an abrupt jump to another location in the document that is off screen initially. While there are various ways to address this navigation issue in the context of dynamic generation of sets of HTML documents, the Pad++ multitree browser, discussed in the next section, is motivated in part as a visually-oriented solution to this problem.
An alternative to the top-down query method for a GAB server is for
the server itself to do the navigating and return just those resources
which are presumed to be the best for the client's purposes.
Here the client's request comes in the form of one or more URLs, and
the GAB server responds with the "close relatives" of those URLs.
This is a query-by-example scenario where the measure of relevance is
defined structurally. Our starting point for the relevant relation
over nodes in a multitree is the sibling relation, defined as follows.
A slightly more distant relation over nodes in a multitree is the
cousin relation. It is essentially the transitive
closure of the sibling relation, but we find it convenient to distinguish
siblings from cousins and so define it as follows.
An issue we encountered early on in our experiments was the problem of sparse connectivity in a merged subject tree database. If the original subject trees share no common resources, it is difficult to leverage the collective subject trees with structural relations in the ways we have been describing. We conducted some informal experiments to determine how much sharing of URLs there was across actual user bookmark files in our organization. It turned out that the amount of sharing was surprisingly low. Even within our local computer graphics and interactive media group at Bellcore, a group who all use the World Wide Web regularly and who would presumably have common interests, we found a very low incidence of shared resources.
As a reponse to this problem, we came up with the idea of using the cousin relation in conjunction with one or more general purpose subject indices to provide the glue that connected a group of personal bookmark files. The general idea here is to increase connectivity across a relatively sparsely connected GAB database by taking advantage of the presumably larger coverage of one or more general purpose subject trees (which can be merged into a single multitree). The tree colored green in Figure 1 is meant to suggest the role of the general purpose subject multitree. It will tend to cover more of the World Wide Web than any relatively small collection of personal bookmark files, which are suggested by the red and blue trees. Thus, even though one may find no common linkages across the personal bookmark file collection directly, there is still a means of indirectly finding related resources by going through the green tree. Our initial experiments used Yahoo  in the role of general subject multitree.
Figure 3 is a screen dump of a page automatically generated by our GAB server in response to a query containing the URL whose title is "Bellcore Information and Sciences Technologies Home Page." There were siblings found in only one user's bookmarks, but there were a greater number of cousins from three of the participating users. The implication is that a subject category of Yahoo contained a link to the resource "Bellcore Information and Sciences Technologies Home Page" and that other resources that appeared under this category were shared in the subject trees of the three other users. We include a link to the mediating Yahoo subject category as well, since users may be interested in jumping there directly for futher browsing.
Our hypothesis is that the more general "cousins" relation will afford more connectivity in a GAB server, and it has been borne out by our initial observations. Ultimately, the right choice of which relation to use may have to fine-tuned and could well differ across specific instances of GAB databases. It would of course also be possible to combine other evaluations of the usefulness of some potentially new resource with these purely structural relations, e.g., community-based recommendations .
Pad++  and its forebears  provide a substrate for creating interactive graphics on an infinitely scalable surface with smooth zooming and panning. It is hypothesized that hierarchical relations can be naturally captured with scale: information that is deeper in the hierarchy can be represented as being smaller on the Pad++ surface. Treemaps , a method for visualization of hierarchical information based on spatial containment, seemed to us to be the most promising layout strategy for zoomable multitree data. Most other alternatives that occurred to us required some form of dynamic changes to object positions during user interactions. Smooth animation for arbitrary object movement or generalized warping was not yet well-supported in Pad++ at the time we did this research. Also, dynamically shifting layout brings with it a host of other issues involving cognitive continuity that we supposed might be handled by general panning and zooming features alone if reasonable static layouts could be found.
Figure 4 shows a screen capture of our Pad++ multitree browser together with a standard HTML client that may be controlled through Pad++. The Pad++ client is automatically invoked as a helper application from the file returned by the GAB server, which it tags with a Pad++ MIME type.
Each user subject tree in the Pad++ visualization is assigned a unique color hue. The coloring algorithm chooses points of equal distance from the continuum of hues proportional to the number of users. Color saturation is bound to hierarchical depth; it is reduced at each lower level of the tree. Both intermediate and terminal nodes of the multitree are represented as rectangles. Their names are displayed in the translucent status bar. A double click with the left mouse button will cause the system to smoothly zoom/pan to the rectangle of interest. In addition to system-controlled zooming and panning, the user is also able to control panning and zooming directly. As the user zooms in, nodes higher up in the hierarchy slowly dissolve while revealing more of their descendants.
Figure 5 is suggestive of the system behavior during a zoom in to a subject category of one of the users. The actual application simply narrows the frame as the view is smoothly zoomed towards its destination. If a user wants to look at the contents of a resource itself, she double-clicks on its name and a message to open that resource is conveyed to the standard World Wide Web browser.
Multitrees are of course not just trees, which treemaps are designed to support, and so a solution for adapting treemaps to the more general graph structure of multitrees was required for this visualization. For purposes of layout, we created a tree out of our original multitree by (1) adding a root node whose immediate children were the root nodes of each of the subject trees in the GAB database and (2) for each shared node, adding an additional level in the tree consisting of cross references to the subject headings containing those shared nodes. This is a graphical rendition of a structure that is also evident in the HTML nested lists of Figure 2. The cross references are then color-coded to the subject trees which contain them. Besides the participating users subject trees, we also include cross-references into Yahoo, which are colored red.
Consider the visualization encoding exemplified in Figure 5. "Navigation" is a subject heading. It is assigned a rectangle colored by its host subject tree. This green rectangle contains several other green rectangles that represent the URLs categorized as navigation resources. Those URLs that appear elsewhere in the multitree structure under different subject categories enclose additional rectangles of different colors, one for each cross-referenced subject heading.
As for navigating, clicking on a cross-reference rectangle invokes a system-generated pan/zoom that smoothly zooms out until both the point of origin (i.e., the cross reference) as well as the destination are in the view and then zooms in on the destination, after which the shared node blinks. We believe such automatically computed pan/zoom trajectories  are a very significant feature of Pad++ navigation and uniquely characterize this browsing and visualization approach. The hypothesis is that one can overcome the "lost in hyperspace" feeling garnered from abrupt transitions in hyperspace by always getting an automatically generated bird's eye view as a transition when making large jumps from one part of the space to another. Note that this hypothesis is only sustainable if the user is in fact able to acquire some familiarity with the global layout -- thus the importance of maintaining continuity through relatively static spatial positioning.
Our Pad++ client has other features as well. With a slider the user is able to rescale the treemap, which has the following sigificance. A value of 100% yields the classic no-offset treemap (where only terminal nodes are afforded real estate), whereas a value of, for example, 40% yields a layout where adjacent tree levels differ in size by orders of magnitude. Imagine being able to stretch and compress a tree arbitrarily as one is looking down from the top. As the tree stretches, the levels that are lower in the hierarchy get smaller and further away. This leaves more screen real estate that might be used to display information about the local levels one can see clearly. It also diminishes the load on the perceptual system by making the overall display less cluttered. There is a need for further research in exploring these layout options through "semantic zooming," a concept in the Pad community that signifies that the system might choose to display information differently depending on how much screen real estate is afforded a particular object.
We developed WebWatch as a portable service to help users keep abreast of new developments in their favorite World Wide Web pages. WebWatch makes it easy to monitor any Web page in HTML format. On a daily basis, WebWatch alerts its users to new Web resources as they appear at monitored locations. WebWatch outputs an HTML formatted page which reports added and/or deleted resources at any monitored location in the Web. For example, Figure 6 shows the result of a WebWatch report for the resource "America's Cup On-line."
Reported are anchors that are "new for me here since the last time I checked" This type of service is most useful in a large rapidly changing and unpredictable information environment.
How does it work? Users of WebWatch use Xcut&paste to copy and paste in anchors into their WebWatch subscription page. This is an HTML page whose anchors point at resources to be monitored for the user. Entry of an anchor into a WebWatch subscription file activates the monitoring of the Web location pointed at. WebWatch fetches the last modified date (if available) of the resource and compares it with its own page records. If the resource has been modifed more recently than WebWatch records, or if WebWatch has no record, WebWatch fetches the document to diff against the last fetched copy it has.
Nightly, a chron file wakes up WebWatch which is an Emacs batch-mode program. WebWatch relies upon Emacs W3 functionality. It fetches and stores new copies of all monitored HTML World Wide Web resources. Old and new versions are diff'd to identify additions and deletions. These are analyzed for anchors pointing at resources. New anchors added to monitored documents will contain URLs pointing at new resources. Deleted anchors will contain old URLs that no longer appear as resources in the monitored resource. Having collected these newly added resources and newly deleted resources, WebWatch generates an HTML page which encodes a tree whose leaves are added and deleted resources and whose upward paths are HTML heading levels from monitored resources. Only changes show up in each tree.
When is a new anchor really new? What constitutes a real change is a semantic question. Currently, WebWatch reports new links if the anchor in question has both a new reference location and name. If an anchor has only a new reference location and not a new label, we consider the change merely a house-keeping change. If an anchor has only a new label in the document but points to exactly the same reference location as last time, we consider the change only a cosmetic name change and do not report it as a new resource.
WebWatch appears to work well for the simple task for which it was designed. Monitoring locations for changes has been useful to the small group of current users. One user was able to purchase Van Halen tickets in a timely manner by monitoring the Van Halen concert schedule page and then ordering as soon as a nearby concert was announced. Two other users monitored news from the America's Cup races. It seems evident that there is a demand for some monitoring functionality on the World Wide Web, and we expect future browsers and/or servers to contain resource monitoring features that take into account personal interests, responsibilities, and history of interaction. As is discussed by LaLiberte and Braverman , there are still issues involving scalability that must be addressed when "what's new" services become commonplace on the World Wide Web.
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2. Clearinghouse for Subject-Oriented Internet Resource Guides, http://www.lib.umich.edu/chhome.html.
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8. Mark J. Johnson, SIMON HOMEPAGE, http://www.elec.qmw.ac.uk/simon/.
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16. The World-Wide Web Virtual Library: Visual Languages and Visual Programming, http://cuiwww.unige.ch/eao/www/Visual/Visual.Programming.biblio.html.
17. The Webhound WWW Interface, http://rg.media.mit.edu:80/projects/webhound/www-face/.
18. Yahoo, http://www.yahoo.com.
Kent Wittenburg [http://community.bellcore.com/kentw/home-page.html]
Bellcore, Room MCC 1A-332R
445 South St.
Morristown, NJ 07962-1910
Duco Das [http://community.bellcore.com/duco/home-page.html]
Stevens Institute of Technology & Delft University of Technology
2628 BL Delft
email: firstname.lastname@example.org or afstc029@IS.TWI.TUDelft.NL
AT&T Bell Labs, Room 2B-402
600 Mountain Ave.
Murray Hill, NJ 07974
Bellcore, Room MCC-1A348R
445 South St.
Morristown, NJ 07962-1910