Editing Graphics development with vplot (section)

==SEPlib tricks==

If you don't use the seplib versions of the filters, you can ignore
this section.

Pen filters have to do some tricky things with SEPlib since the
standard defaults are inappropriate. Normally seplib wants to send
the header to standard out. Since the output of Pen filters are usually
not redirected, this would dump the header on your screen and possibly
interfere with your plot. By the same token, the data output usually
SHOULD go to your screen, instead of being saved in a file or sent
down a pipe. The normal SEPlib method of self-documenting also
has to be subverted in order to be consistent between the SEP and non-SEP
versions of the programs.

To accomplish this, several things are done:

A library "tseplib" is provided that contains routines for
which the standard seplib versions had to be modified. This
should be linked AHEAD of seplib so that the modified versions
get taken. The routines are documented as to how they differ and why.

"OUT" and "HEAD" are defined to be the external variables "sepoutwhere"
and "sepheadwhere", respectively. These are normally "/dev/tty" and
"/dev/null". (These values are grabbed from the routine "sepwhere"
in tseplib.) If these values are inappropriate for your filter (unlikely
to be the case for most filters; see Vppen for an example of one) you can
create your own version of this routine and make sure it gets linked in
ahead of tseplib.

If head=/dev/null and this was the default in sepheadwhere and
standard out is redirected, then it is assumed the user really wants
some sort of header despite the fact that the Pen filter killed the
real one and so a "fake" one will be created for them by frontend.
Frontend will only actually write something into it if "out"
is not standard out (to avoid mixing header and data). The
device-dependent code may do so if it wishes, though (see Raspen for
an example). The device should only use "Puthead" to write to the header,
to make sure everything is done properly. The "fake" header has the
advantage that information can be added to it at any time.
Normal SEPlib headers must be closed before any data can be written.
Thus only fake headers can have "n3=number_of_frames" added onto
the header. (This is why Raspen uses fake headers.)