Users can define their own ship shape with a few limitations.
The server has an option -allowShipShapes to enable
user-defined ship shapes (the default is to enable it).
The client has a new option called -shipShape <decription>.
Where description defines the edges of the ship.  When the ship
is drawn the edges are connected in the order they are listed
in the description, with lines connecting them.  The first and last
point are also connected, so your first and last point should not
be the same.

The 3.1.0 version was the first XPilot version which featured
user definable ship shapes.  The format was changed in the 3.2.0
version to include more features and be more easily extensible.
First the old format is explained for completeness, then the new one.

The ship shape is defined as if it was rotated 90 degrees to the right.

For example, the following defines the default ship in old format:

    xpilot -shipShape '(3,1,2)(15,0)(-9,8)(-9,-8)'

Where the first parentheses surrounds a triple of numbers.
The first number of this triple denotes the number of points that
follow.  The second and third number of the triple define the two back
points of the ship, which are marked with the marking lights.
These two back points also determine where the thrust of the ship
originates, namely at the mean of these points.
All of the points follow, each one surrounded by parentheses.
The first point should be the top point of the ship as this is the
point where shots originate from (otherwise the ship will kill itself
by its own shots).

The definition should meet some requirements to enforce users to define
a ship which is not too small or too weird.  These restrictions are
arbitrary and open to discussion.  Currently they are as follows:
  1) each ship should have at least three different points and not more
     than twentyfour points.
  2) at least one point should be eight or more pixels left from the origin.
  3) at least one point should be eight or more pixels right from the origin.
  4) at least one point should be eight or more pixels above the origin.
  5) at least one point should be eight or more pixels below the origin.
  6) at least three points should be eight or more pixels away from the origin.
  7) no point may be more than fifteen pixels away from the x- or y-axis.

In the 3.2.0 XPilot version Pete Ford has come up with a new, more rational,
ship-shape format.  It features:

    1. Keyword oriented system, to allow for more flexibility and future 
       extension.

    2. Additional gun sites allowed on ships; main gun is relocatable and up 
       to three extra gun sites are allowed on each side of the ship for wide 
       angle guns.

    3. Additional marking light positions allowed - up to three for each 
       color (red = left, blue = right as before).

    4. Alternative missile racks - positions where missiles launch from. Up 
       to four racks can be given, and mini missiles will launch from separate
       racks if available.

    5. Engine is relocatable, although it still fires along the x-axis of the 
       ship as defined.

    6. Compatibility with the 3.1 ship-shape format for exisiting 3.1 clients 
       joining '3.2' servers and 3.2 clients joining 3.1 servers.

    The new ship shape format is:

    xpilot*shipShape: (keyword: x1,y1 ...) (keyword: x1, y1) etc.

    Keywords can appear any number of times in the shape string; the 
    behaviour is dependent on the keyword.
    If a keyword is not given then a default will be applied: if the shape is 
    not defined then the default shape is used.
    Allowed keywords are:

    shape:  defines points for ship shape. Up to 24 points will be stored, 
            any more than 24 are ignored. Points should appear as x,y pairs, 
            each pair separated with at least one space character. 
            Multiple 'shape:' keywords have a cumulative effect; adding extra 
            points to the list, up to the limit of 24. There is no requirement
            to give the number of points to expect - this is worked out after
            the whole string is parsed.
            The existing restrictions on separation of points apply.
            If no 'shape:' keyword present then the contents of the string 
            will be ignored and the default ship used.

    mainGun: defines the firing position for the main gun. Only one x,y pair 
             is read; further 'mainGun:' keywords will be ignored.
             Default is the foremost point in the 'shape:' list.

    leftGun: defines a firing position for an additional left-side gun. Up to 
             three of these can be specified, any more will be ignored.
             The left-hand part of any wide-angle shots will be fired from
             these points if they exist.
             Default is to have no left guns: only a main gun.

    rightGun: defines a firing position for an additional left-side gun. Up to
              three of these can be specified, any more will be ignored.
              The left-hand part of any wide-angle shots will be fired from
              these points if they exist. There is no need for left and right
              guns to balance (or even be on the correct side of the ship!)
              Default is to have no right guns: only a main gun.

    leftRearGun: defines a firing position for an additional left-side rear gun.
                 Up to three of these can be specified, any more will be ignored.
                 If you have an even number of rear gun items the rear shots
                 are distributed among left and right rear gun positions.
                 If you have an odd number of rear gun items one rear shot is
                 fired from the usual rear position and the others are distributed.
                 Default is to have no left rear guns: only a main rear gun.

    rightRearGun: defines a firing position for an additional right-side rear gun.
                  Up to three of these can be specified, any more will be ignored.
                  If you have an even number of rear gun items the rear shots
                  are distributed among left and right rear gun positions.
                  If you have an odd number of rear gun items one rear shot is
                  fired from the usual rear position and the others are distributed.
                  Default is to have no right rear guns: only a main rear gun.

    leftLight: defines a position for the left (red) marking light. Up to 
               three of these can be given.
               Default is one light on the left-most point on the ship.

    rightLight: defines a position for the right (blue) marking light. Up to 
                three of these can be given.
                Default is one light on the right-most point on the ship.

    engine: defines the point from which the engine thrust originates. Only 
            one of these points will be defined; multiple 'engine:' keywords 
            will override previous positions.
            Default is on the ship x-axis at the rear-most x-value defined.

    missileRack: defines a missile launch point. More than one of these will
                 mean that missiles are launched from each rack in turn. This 
                 works best with mini missiles (especially torpedos).
                 Default is one rack at the main gun position.

    Because the ship shape string would become extremely long in the new
    format short aliases for the keywords in the shipshape string are
    provided.  The list of keywords together with their aliases is:

        Full version   Short Version   Meaning
        ------------   -------------   -------
        shape:         SH:             ship shape points
        mainGun:       MG:             Main gun position
        leftGun:       LG:             Left wide gun position(s)
        rightGun:      RG:             Right wide gun position(s)
        leftLight:     LL:             Left (red) light position(s)
        rightLight:    RL:             Right (blue) light positions(s)
        engine:        EN:             Engine thrust point
        missileRack:   MR:             Missile launch point(s)
        name:          NM:             The name of the ship

    The points given for weapons, lights etc. need not be points defined in 
    the ship shape definition.

    Example:
    This is my latest version of my own ship-shape (called Enigma); note that 
    it should be read as one continuous string without line breaks.

    xpilot*shipShape: (shape: 15,0 -2,4 -8,13 0,13 0,14 -15,14 -12,3 -15,3 
    -15,-3 -12,-3 -15,-14 0,-14 0,-13 -8,-13 -2,-4) (missileRack: 0,14 0,-14) 
    (leftGun: 0,14 -5,14) (rightGun: 0,-14 -5,-14) (leftLight: -15,14 0,0) 
    (rightLight: -15,-14 0,0) (engine: -15,0)

    I've tried to make the format easy to extend; extensions can be added in 
    the parsing routines in math.c quite easily. Of course, the playing code 
    will have to be changed to accommodate any extension.
    My own ideas for improvement are to include a 'name:' string for the name 
    of the ship (possibly to allow selection from a set of shapes while 
    playing...) and positions for rear guns.

    Known Problems:

    1. At the moment, mini smart and heat missiles still spread from the
       missile racks, so the effect is lost a little. It is probably possible
       to determine whether spread is required, but I haven't done that yet.

    2. Wide angle guns all fire from additional gun positions if possible; 
       they probably should use the main gun more. Also they spread in the 
       same way as the existing program - of course setting Z3 on the weapons 
       mods will fire in parallel lines of shots; Z3 was could almost have 
       been designed for this patch!

    Thanks to those people who've made suggestions already (Bjoern & Bert).
    Any comments/suggestions/bugs/bug-fixes should be sent to me:

    p.s.ford@durham.ac.uk

    The latest improvements are:

    1. Code to restrict lights,guns etc. to within the ship-shape design.
       It's not perfect, but it tends to allow invalid positions rather than
       disallow valid ones. Thanks to Bert for a very nice diagram of the
       problem!

    Pete Ford (Enigma)

New ship shapes should obey a new size restriction.  This is because some
ship-shape-artists made ships very small to have a slight advantage in
manouvring in tight places.  Also, those small ships are a little less
easy to spot.  Therefore the following size restriction is now enforced:
the width and height of the ship added together should be at least 38.
To give everybody some time to convert their ships to the new format
and to adapt it to meet this restriction it is not enforced right away, but
rather a warning is printed that it will be enforced in a future version.

The left and right rear-guns were added by Michael Gruber.