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### 12.1 External Filters

An external filter is a stand-alone program that works on the output of Form Designer and translates the output to the target language. The filter can elect to work on the `.fd` or the C output or both simultaneously. However, in non-testing situations, the c output from Form Designer probably should be deleted by the filter once the translation is complete.

By default, Form Designer only outputs the `.fd` and C files. If the presence of `-ada`, `-perl`, `-python`, `-fortran`, `-pascal` or `-scm` command line options to Form Designer is detected, then after emitting the default output, Form Designer invokes the the external filter with the root filename (without the `.fd` extension) as an argument, together with possible other flags, to the filter. Any runtime error messages are presented to the user in a browser. The filter name by default is `fd2xxxx` where `xxxx` is the language name (such as `fd2perl` etc.), which can be changed using the `-filter` command line option (or equivalent resources).

The resources that are relevant to the filter are listed below

Resource Type Default
language | string | C
filter | string | None