This is a "rough" document, but hopefully if you are interested in
adding a new paint operator to xpaint, it should make a good 
staring point.

The interface to add new functions is as follows, all operations have
two functions named (where OP == Function name, ## == concatination):
	void *	OP ## Add(Widget w)
	void	OP ## Remove(Widget w, void *local)

Thus for the Pencil operation you have a PencilAdd() and PencilRemove()
function.  Then what is needed is add a two lines to Operation.c, one
of GENERATE_OP( OP ), and a entry in the iconList[] table for the icon
that is displayed for the operation.

Your Add method is expected to return a pointer to whatever local 
instance information that your operator might need to keep track of,
which is the argument into the Remove method.  Also, usually I
use this data pointer are the information to pass into the event
methods as well.

Events handlers are added using the:
	OpAddEventHandler(Widget, surface, mask, flag, function, data)
and OpRemoveEventHandler() is used to remove them.  The Widget
argument should be the same ass the Widget passed into the add
function.  The surface argument is weather you want events on either
the Window or the Pixmap, or both (you still are required to update
the Window if you arn't zoomed).

The first thing your operator should do before it changes the drawable
is call UndoStart{Point,Rectangle}, which will cause a new undo
buffer to be allocated for your operator.  

Operators are passed an info structure, important parts are:
	isFat		Boolean
		-- the XtWindow() is zoomed, you should only
		   draw on the window if you know how, when 
		   this is set.
	surface		OpSurface == enum { opPixmap, opWindow }
		-- what surface you are being called with
		   either the backing store pixmap or the
		   actuall window
	drawable	Drawable (either the window or pixmap)
		-- what the drawable ID is, this in only valid
		   after the call to UndoStart()
	filled_gc	GC
		-- the graphic context you should use for
		   all your drawing operations.
	x, y		int
		-- The x and y position of the event,
		   these may be in the zoomed coordinate
		   on the Window, while they will always
		   be the real x, y for the Pixmap.
	realX, realY	int
		-- The real x and y position of the event, if 
		   gridding is on x,y is modified by the gridding 
		   factor.  This only should be used in rare cases 
		   where it is non-sensical to pay attention to the 
		   grid factor (ie flood fill).
	zoom 		int
		-- the current zoom factor, if isFat is True.

The XtNcompress value for the paint widet controlles wheather
or not motion events are compressed into a single event if
possible. Thus:
	XtVaSetValues(w, XtNcompress, False, NULL);
says that your operator is interested in all motion events
(ie. the pencil, or paint brush).

Generally, if you look at the operators that I have
provided there is probably an example of every attrocity that
I might be able to think of using the above information.