~F8DATA STRUCTURE: THE RINGS, TREES, LISTS AND ARRAYS.

	Besides the  tree  of rings,   there  is the  tree of  nested
polygons.   The  nested  polygon tree  is implemented  with  the four
links named ENDO, EXO,  NGON and  PGON.  The EXO of a polygon  points
at its  surronding polygon. The  ENDO of a  polygon points at  one of
the  polygons that may be enclaved within  the given polygon; and the
NGON and PGON links  form a ring of  polygons that have the  same EXO
polygon.

	The time line  lists run thru arc and polygon  nodes.  In the
simple  case,    the  time  line  links  of  a  polygon  point  to  a
corresponding polygon  in the  image previous  (NTIME) or  subsequent
(PTIME)  of the  current polygon. The  correspondence being  that the
time polygon  is  exactly  the  same intensity  at  nearly  the  same
location, orientation, and size as the  given polygon. In the case of
polygon  fusion, the  time line link  of a  polygon points  to a time
polygon of which the  given polygon becomes a  part.  In the  case of
polygon fission, the  time line link of a polygon  points to only one
the pieces into which the given polygon splits.

	The  time  line   links  of   an  arc  vector   point  to   a
corresponding arc vector in  the image previous or subsequent  of the
current  arc vector. The  polygons of arc  vectors mated  in time are
also mated in time; because  after polygon time line links have  been
made, one polygon  is temporarily translated, rotated  and dilated so
as  to have the same  lamina inertia tensor as its  mate; that is the
locus of  the arc  vectors of  one polygon  are temporarily  altered;
then  the corresponding  arc vectors  are found  and their  time line
linkages are made.

	The empty node list is maintained in the CCW link  positions;
the last empty  node contains a  zero link. All nodes  are explicitly
made  from  and killed  to the  empty  node list  by  the subroutines
MKNODE and KLNODE.

	The arc ring of  a polygon is just like a  vector ring except
that  the pointer to  it is  stored in the  ALT link of  the polygon,
while the polygon has both a ring of vectors and a ring of arcs.

	The  fusion shape ring of a intensity  level runs thru the CW
and CCW links  of shape nodes and  is pointed at  by the ALT link  of
the level.  Fusion shape  nodes are the shapes generated to represent
pairs of polygons unmated in time.
~I1973,800;F8- 10 -