WebBook and Web Forager

In the WebBook and Web Forager interface [28], web pages are collected into books, where books can be organized on the desk or on the shelves. Pages and books can also be left hanging in the air. Bookmarks attached to the lower edge of the book which provides direct access to certain pages in the book. Thus, the lowest level nodes of the presentation hierarchy are the page and bookmark nodes, under the book node at the second level. At the third level are the desk, the shelf and the air nodes (Figure ). While links can also be considered as a node, for the simplicity of the discussion they are omitted. Let's examine each node and link, and their presentation hierarchy model attributes and functions along with factors influencing them.

• Page node: Page is a singular node, thus its capacity is 1. Its space requirement can be changed dynamically as the user resizes the page. The representation function simply draws the contents of the web page (I).
• Bookmark node: The capacity of a bookmark node is 1. Its space requirement depends on the length of the bookmark label. The representation function simply draws the label on the lower edge of the book.
• Book node: Theoretically, there is no limit on the number of pages a book contains, however its capacity is limited by the human capabilities to search for pages linearly. Books typically use a page-sized screen space when they are closed, and twice that amount when they are open. Pages in a book (I) are organized in a linear order where the layout function may simulate the shuffling of pages in a rapid manner. The representation function of the book draws a colored cover with the title printed on the front cover.
• Desk node: The capacity of the desk node is limited by its size which is fixed. The layout function is responsible for organizing ages and books (I) arbitrarily overlapping on the two-dimensional desk space. Its representation function draws a three-dimensional view of the desk on the screen.
• Shelf node: The capacity of the shelf node is limited by the number of rows and the quantity of books it can hold in each row. Its space requirement is also fixed. Books can be organized into shelves at different levels and in a linearly ordered fashion on each shelf. Its representation function draws a three-dimensional view of the shelves.
• Air node: The capacity of the air node is limited by the visibility pages and books due to occlusion, and by the human capabilities to recall the locations of them. Its space requirement is also fixed, filling up the rest of the screen space. Pages and books (I) can be organized arbitrarily in three-dimensional space of the air node with appropriate scaling. Its layout function also allows the user to bring a page or book in the air closer.

• Bookmark-Page link: This link indicates that a bookmark can be used to access a page. Typically, clicking on the bookmark brings the associated page into view. Its footprint depends on the textual label of the bookmark.
• Bookmark-Book link: Bookmarks of a book have very low access costs because they are always visible, thus they also have high footprints.
• Page-Book link: Users typically access pages in a book by turning pages one by one or shuffling them in a rapid manner. Since pages are organized linearly in the book, access costs grow as the number of pages in the book increase. Users can also access pages from the bookmarks attached to the lower edge the book, giving them direct access to certain pages. Bookmarks typically improve access performance. The footprint of a page in a closed book is rather low since only the book title and the cover color contain information that might lead to the pages sought. An open book has a higher footprint since page contents are shown. However, only two of the pages are shown at the same time.
• Page-Desk link: In order to access pages from the desk users need to drag it from the desk and leave it in the air. The footprint of a page is quite high, as it is basically a small size image of the page contents.
• Page-Air link: Similarly, users drag a page in the air in order to access it, thus the access cost is not much. The footprint of a page in the air is also quite high due its visibility.
• Book-Desk link: Access cost and the footprint attributes are similar to that of the page-desk link.
• Book-Shelf link: In order to access a book from the shelves, users similarly drag the book from the shelf. The foot-print of a page is visible as books are organized in such as way that their covers face the user. However, reading the book titles may be hard.
• Book-Air link: Access cost and the footprint attributes are similar to that of the page-air link.

Web Accessibility