Segment Header Parsing
----------------------

Segment header records may contain one of more of the following options:

**-G**\ *fill* 
    Use the new *fill* and turn filling on
**-G-** 
    Turn filling off
**-G** 
    Revert to default fill (none if not set on command line)
**-W**\ *pen* 
    Use the new *pen* and turn outline on
**-W** 
    Revert to default pen :ref:`MAP_DEFAULT_PEN <MAP_DEFAULT_PEN>`
    (if not set on command line)
**-W-** 
    Turn outline off
**-Z**\ *zval* 
    Obtain fill via cpt lookup using z-value *zval*
**-Z**\ *NaN* 
    Get the NaN color from the CPT

Custom Symbols
--------------

We allow users to define and plot their own custom symbols. This
is done by encoding the symbol using our custom symbol macro code
described in Appendix N. Put all the macro codes for your new symbol in
a file whose extension must be .def; you may then address the symbol
without giving the extension (e.g., the symbol file tsunami.def is used
by specifying **-Sk**\ *tsunami/size*. The definition file can contain
any number of plot code records, as well as blank lines and comment
lines (starting with #). The module will look for the definition files
in (1) the current directory, (2) the ~/.gmt directory,
and (3) the **$GMT_SHAREDIR**/custom directory, in that
order. Freeform polygons (made up of straight line segments and arcs of
circles) can be designed - these polygons can be painted and filled with
a pattern. Other standard geometric symbols can also be used. See Appendix
:ref:`App-custom_symbols` for macro definitions.

Bugs
----

No hidden line removal is employed for polygons and lines. Symbols,
however, are first sorted according to their distance from the viewpoint
so that nearby symbols will overprint more distant ones should they
project to the same x,y position.

The module cannot handle filling of polygons that contain the south or
north pole. For such a polygon, make a copy and split it into two and
make each explicitly contain the polar point. The two polygons will
combine to give the desired effect when filled; to draw outline use the
original polygon.
