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This directory holds the source files for a 2-D vector graphics library:
GNU libplot. Seven different sorts of graphic output are supported:
metafile, Tektronix, HP-GL/2, xfig, Postscript, X11, and X11 Drawable. The
first five are supported via output streams. The last two are supported
directly, by calling X routines. The X11 driver pops up a window
(actually, a Label widget) on an X display, and draws graphics in it. The
X11 Drawable driver draws graphics in a `drawable' (a window or a pixmap),
which must be passed to it.
The names of the source files in this directory include as prefix a letter
indicating which driver they include code for. For example, the file
h_filltype.c is part of the HP-GL/2 driver, and contains the source code
for the function _h_filltype(). m=metafile, t=Tektronix, f=Fig,
p=Postscript, x=X11, y=X11 Drawable. The many files whose names begin with
`g' (e.g., g_relative.c) include generic code that is used by more than one
driver.
The library provides a uniform `virtual plotter' interface to each display
device. The C binding for the library is set up in api.c. Besides
defining 79 standard user-level functions, it defines the functions
newpl(), selectpl(), and deletepl(), which create and manipulate an array
of virtual plotters. Each virtual plotter is implemented as a C++-style
object, which means it is a rather large structure. We call each such
object a `Plotter'. Each Plotter includes both private data and public
methods: one method for each of the user-level functions in the libplot API.
Plotters are created by calling newpl() and destroyed by calling
deletepl(); an arbitrarily large number of Plotters may exist
simultaneously. But exactly one Plotter may be selected at any specified
time, by calling selectpl(). Which Plotter is selected determines which
low-level routines are invoked in response to calling the user-level
libplot functions. Throughout this library, the pointer `_plotter' points
to the currently selected Plotter.
For example, calling the user-level function filltype() will invoke
_m_filltype() if _plotter points to a Plotter that produces metafile
format, _t_filltype() if it is one that produces Tektronix format,
_h_filltype() if the format is HP-GL/2, _f_filltype() if the format is
xfig, _p_filltype() if it is Postscript, and _x_filltype() if the Plotter
is an X11 plotter, which directs the output of libplot to an X display
rather than to an output stream.
Adding support for a new display device, given the object-oriented way in
which this code is structured, is quite easy. An entry for any new device
must be added to the _plotter_initializations[] array in api.c. This entry
must contain an initialization for the corresponding type of Plotter
structure. The structures that initialize the existing types of Plotter
(e.g., _ps_default_plotter) are located in the files *_defplot.c. Most
Plotter methods are `generic'; in fact, each type of Plotter structure,
with the exception of the type that implements the metafile device driver,
is built largely from generic methods.
To see how easy it is to add or remove support for a new display device,
search for the symbol X_DISPLAY_MISSING. If this is defined, support for
the X display device will be dropped at compile time. The only occurrences
of `#ifdef X_DISPLAY_MISSING' are in api.c, the header file extern.h, and
*_defplot.c and *_defstate.c. The reason for its appearance in the C files
other than api.c is that the basic datastructures include X-specific
fields, which are omitted if X support is not included.
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