Editing Graphics development with vplot (section)

==Being tricky about the order things are plotted==
The external variable
<syntaxhighlight lang="c">
extern int (*genreader) ();
</syntaxhighlight>
can be changed in dev.open to point to your own routine for processing
input files. You should only need to change this variable if for some
reason you want to be able to change the order or way in which input files
are handled, or if you want to know the actual file names of the input plot
files. (You might find this handy if you are creating an interactive vplot
editor, for example.)

The default input file handling routine, which is what you get if you
ignore this section of the documentation, is the following. Frontend
already does the job of finding all the input files, verifying that
they exist, and opening them for reading.
<syntaxhighlight lang="c">
gen_do_dovplot (nn, inpltin, innames)
int     nn;
FILE ** inpltin;
char    innames[][MAXFLEN+1];
{
    int     ii;

    for (ii = 0; ii < nn; ii++)
    {
	pltin = inpltin[ii];
	strcpy (pltname, innames[ii]);
	dovplot ();
	fclose (pltin);
    }
}
</syntaxhighlight>
"Nn" is the number of input files. "Inpltin" is an array of nn stream
pointers each of which point to an open (but unread-upon) input stream.
"Innames" is the corresponding array of strings giving the associated
name of the input file for each of the streams in inpltin.

The external integer "buffer_input" can be set to NO in dev.open to
assure that all the input streams are unbuffered. The external integer
"allow_pipe" can be set to NO in dev.open to assure that all the input
streams are seekable on.

It is possible to change most variables set by command line arguments
between calls to dovplot without ill effect. There are comments in
init_vplot at the end of the section of code where command line arguments
are processed listing which variables have been explicitly set up to be
changeable in this way.

Variables controlling the mapping between the device's and vplot's
coordinate system can also be changed, but the subroutine
"reset_parameters()" in frontend must be called afterwards to
reinitialize all the related variables that may need it.

./filters/vplib/vpdovplot.c is an example of a routine that makes
2 passes through a vplot file, doing things differently the second
time around.