Technically there are major differences between CGM and SVG, however, the graphics display is visually the same. So how do they differ? The big difference, the structure of the data.
The CGM file format is binary and therefore an illustration software is essential when creating or editing CGM graphics. The major benefit of the CGM format, the profiles methodology, profiles provide a way controlling the file format. CGM profiles have major industry support, for example, S1000D which uses the WebCGM profile. The additional benefit of profiles is the ability to validate the CGM file. The profile is a big factor in the data exchange process when interoperability is key.
The SVG data structure approach is very different, it is based on XML, providing many benefits. However, you will still require illustration software to create the SVG graphic. It is not very practical to edit the raw XML. The full SVG specification is very large, and not all the elements will be required for technical illustrations. The SVG Tiny specification is a more manageable subset of the Full version, and more suitable for technical illustrations. Currently, software exporting the SVG format does not follow any standard guidelines, especially for metadata and hotspots. Therefore, reliable and consistent data exchange is almost impossible. In some instances, the software exporting the SVG cannot open the same file reliably. This a major disadvantage, if the graphic is to be reused and maintained.
In conclusion, the big drawback of SVG is the strength of CGM, interoperability, and validity. The mass market does not really care about data exchange. However, the specification driven markets care deeply about the longevity of their data. The good news, SVG can be controlled via rules using a schema or DTD. There will be work required to develop and document the rules, but it is possible.
In the next part, we will discuss the display of CGM and SVG graphics online.
