The Import/Export Library: 3.2 Writing Translation Libraries: Intermediate Formats

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Many libraries will want to make use of intermediate formats. For example, a company may have defined its own transfer format for its applications. The simplest way for it to translate files into the GEOS Metafile might be to translate the file into its own transfer format, then translate from this format into the Metafile format. GEOS supports this with its use of intermediate translation libraries.

An intermediate library is much like an ordinary translation library. Like other libraries, it must translate from its own format into the Metafile format. The only difference is that intermediate libraries are not called by the Impex objects; instead, they are called by other translation libraries.

For example, suppose FooWare, Inc., has a line of graphic FooApps which includes FooDraw, FooPaint, FooSketch, and FooScribble. FooWare has developed its own file-transfer format, FooInterchangeFormat (FIF); it has code written to translate any FooApp's files into FIF and back.

FooWare now wants to write translation libraries for GEOS. The first thing FooWare does is write an intermediate translation library which takes a DOS file containing FIF data and produces a VM chain containing a GEOS graphic Metafile, and vice versa. Once this is written, FooWare has an easy time writing the actual translation libraries. For example, the FooPaint translation library imports files by converting a FooPaint data file into the analogous FIF file. Since FooWare already has routines to do this translation, it just has to port existing code to the GEOS library. The FooPaint translation library then calls the FIF-to-Metafile intermediate translation library and gets the finished translation. Similarly, the FooPaint translation library exports data by calling the FIF-to-Metafile library to produce a FIF version of the data; it can then use ported code to produce an actual FooPaint file.

Note that any ordinary translation library can also be used as an intermediate translation library. For example, let's suppose that FooWare has code written to translate FIF files into PostScript files. This makes translating the documents even easier. The FIF-to-Metafile library can just use ported code to produce a PostScript version of the data; it can then call the PostScript-to-Metafile translation library, which is provided with GEOS. GEOS comes with translation libraries for many popular file-interchange formats; thus, many developers will be able to write translation libraries just by porting code from their pre-existing translators, then calling one of the GEOS translation libraries.

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