CGM and Web Schematics

June 22, 1998

The Computer Graphic Metafile (CGM) format is a well established ISO standard for complicated schematics -- thousands and thousands of vectors or lines defining, say, a Boeing 747. There has been ongoing work at W3C for over a year on CGM for the Web, including active liaison with industry groups such as the recently formed CGM Open group. Expect to see some announcements about CGM and the Web in the next couple of months.

However, the community of people who want to exchange CGM is fairly focussed -- primarily the aerospace, automotive and defense industries. CGM is an industrial-strength format but it has not seen much use on the Web.

"The CGM work is for large scale CAD applications," explains Bob Hopgood, co-editor of the Web Schematics on the World Wide Web submission. "CGMs get generated by large CAD packages; you never write them by hand."

"We were looking at a simple schematic graphics addition to the Web that would fit into current Web philosophy," explains Hopgood. "The assumption is people should be able to write it as easily as they write HTML, and that the graphics should not be treated as second class citizens."

The Web Schematics on the World Wide Web submission aims at representing very simple flow charts and simple graphic representations, sort of the PowerPoint, flowchart version of schematic diagrams and the like.

The submission defines a SCHEMATIC element that encloses any number of graphical elements, including LINE, SPLINE, BOX, TEXT and POLYGON with a minimal set of attributes. There is also a GROUP element that can be used to position a set of graphical elements.

An example adapted from the submission looks like:

<SCHEMATIC>

  <BOX shape = "rect">Rectangle</BOX>

  <BOX shape = "ellipse">Ellipse</BOX>

  <BOX shape = "diamond">Diamond</BOX>

</SCHEMATIC>

These three box elements specify a rectangle, an ellipse, and a diamond on a line and each element contains the string identifying it.

Also, because CGM is not expressed in XML, applying style sheets to CGM graphics is difficult. These factors and others seem to point to the need for a vector graphics tagset for XML, one which is able to make use of XML namespaces and stylesheets to mix text and graphics.

The Web Schematics submission is not really competitive with VML and PGML; rather it was designed to provide a profile for a simpler diagramming mechanism than the CGM format standard could provide.