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STANDARDS PIPELINE

Vol.33 No.2 May 1999
ACM SIGGRAPH

CGM Background



For years, companies, industry groups and government organizations have used computer graphics metafiles for storing and exchanging 2D graphics. CGM is a format for digitally describing vector, raster and hybrid (raster and vector) graphic pictures very compactly. It has proven to be a very good format for the technical illustrations in electronic documentation, geophysical data visualization and other demanding two-dimensional graphics presentation applications.

About the same time the CGM standard matured, the World Wide Web began to explode and graphics began to play an increasingly important role. Until recently, graphics on the Web consisted of raster images – pictures represented as large arrays of colored pixels. The formats typically used to transmit these images are GIF, JPEG and more recently, PNG. However an alternate approach is to send the instructions (vector graphics) for drawing lines, circles, ellipses, curves and other shapes.  Among the advantages of describing pictures in vector terms are:

  • Vector graphics can be scaled easily and quickly while retaining the quality of the picture; unlike raster images, which scale poorly and display poorly at resolutions other than that for which the image was originally created.
  • In most cases, vector graphics require less bandwidth and can be accessed and viewed faster than raster images. 
  • Vector graphics can be edited and manipulated more easily than raster images.
  • It is easy to combine vector graphics with metadata (non-graphical data) for defining how pieces of a picture should react and behave in response to user interactions.
  • Metadata and text in vector graphics can be searched as easily as text in an HTML page.

Because of the inherent advantages of vector graphics, vendors and users of CGM began to experiment with using the WWW and CGM to publish 2D vector graphics on the Web. While some of these experiments were successful, the CGMs produced by one vendor could not be interpreted properly by viewers produced by another. 

The graphics are generally not the problem. The problem is with the metadata associated with the “graphical objects” within the metafile and the behavior associated with this metadata. For example, each vendor would invent its own metadata for defining hotspots and decided what their viewer would do when it encountered a hotspot.  Authoring tools and viewers were not interchangeable. These problems were the reasons that WebCGM was born.