4.10.2 Guide Line for Delegations

First of all, select carefully the applications you will use for the delegations. If a filter is known to cause problems, try to avoid it in delegations⁴. As a thumb rule, you should check that the PostScript generating applications produce files that start by:

%!PS-Adobe-3.0

a2ps needs the ‘%%BeginSetup’-‘%%EndSetup’ section in order to output correctly the page device definitions. It can happen that your filters don’t output this section. In that case, you should insert a call to fixps right after the PostScript generation:

########## ROFF files
# Pass the roff files to groff.  Ask grog how groff should be called.
# Use fixps to ensure there is a %%BeginSetup/%%EndSetup section.
Delegation: Groff roff:ps	\
   eval `grog -Tps '$f'` | fixps #?V!!-q! | #{d.psselect} | #{d.psnup}

There are some services expected from the delegations. The delegations you may write should honor:

the input file

available via the escape ‘$f’. You should be aware that there are people who have great fun having spaces or dollars in their file names, so you probably should always use ‘'$f'’. Some other variables are affected. Yes, I know, we need a special mechanism for ‘'’ itself. Well, we’ll see that later ‘;-)’.

the medium

the dimension of the medium selected by the user are available through ‘#w’ and ‘#h’.

the page layout

the number of virtual pages is ‘#v’.

the page range

the page range (in a form ‘1-2,4-6,10-’ for instance) is available by ‘#p’.

the verbosity level

please, do not make your delegations verbose by default. The silent mode should always be requested, unless ‘#?V’ is set (see the above example with groff).

If ever you need several commands, do not use ‘;’ to separate them, since it may prevent detection of failure. Use ‘&&’ instead.

The slogan "the sooner, the better" should be applied here: in the processing chain, it is better to ask a service to the first application that supports it. An example will make it clear: when processing a DVI file, dvips knows better the page numbers than psselect would. So a DVI to PostScript delegation should ask the page selection (‘#p’) to dvips, instead of using psselect later in the chain. An other obvious reason here is plain efficiency (globally, less data is processed).