In addition to text, still images, video and audio clips, an engineering or scientific document often contains diagrams such as block diagrams, Booch (or other CASE) diagrams, logic or circuit diagrams, Entity-Relationship-Attribute diagrams, and CAD drawings. Information contained in such a diagram is often more than what meets the eyes; for example, objects can be connected. Representing such a diagram as a bitmaped image, which is often done in a WWW environment, can lose considerable amount of information; e.g., do not know whether objects are connected together any more.
These diagrams can only be created and edited by specialized (and often expensive) tools that run in application-domain-specific environments. When they are put on the WWW and shipped through the use of MIME, the users need the specialized tools to view them. If bitmap is used to represent the diagram, a user can not easily communicate how modification should be performed on the diagram in a collaborative environment. Bitmap printing is slow and the quality of the printout is often poor.
These diagrams often can be represented in structured-graphics . A structured-graphics diagram is composed of objects having graphical representations. Primitive objects in a structured-graphics diagram are text, lines, polygons, arcs, ovals, splines, bitmaps, etc. Objects can be grouped together to form composite objects. Objects can have visible or hidden attributes. Objects can overlap each other, and therefore, have a stacking order within a diagram. A hyper-structured-graphics (or hypergraphics) document is a structured-graphics document with embedded hyperlinks. It is clear that there is a need for such documents to be linked together.
Tgif, developed at UCLA, is a hypergraphics editor/viewer on the WWW. It is a WYSIWYG drawing tool in nature. It supports primitive objects mentioned above and composite objects. Objects can be copied and pasted for easy sharing. It also allows attributes to be attached to objects in a diagram. In tgif, an attribute is represented in a very generic way; it is just a string in the form <attr_name>=<attr_value>. It recognizes the special attribute named href and treats the value of the attribute as a destination URL. In addition, tgif supports client-side interpretable scripts which can be embedded in attributes and used for giving instructions, animations, calculations, etc. Tgif's hypertext WWW home page is at http://bourbon.cs.umd.edu:8001/tgif/ and its hypergraphics home page is at http://bourbon.cs.umd.edu:8001/tgif/index.obj.
Tgif is also supported by popular plotting tools such as gnuplot and xgraph. There's also a tool called pstoedit which can convert PostScript(TM) files to tgif files. These tools make creating and maintaining engineering and scientific drawings in tgif even easier.