~F8DATA STRUCTURE: KINDS OF NODES.

	The are  eight kinds of  CRE nodes: Vector,   Arc,   Polygon,
Shape,  Image, Level, Film and Empty.

1. At  the very top of all the structure is  the film node,  the film
node is unique  and serves as  an OBLIST from  which all other  nodes
may  be reached.   The  film node  embodies the  idea of  a piece  of
celluloid  film or  a length  of magnetic  video tape.   A film  is a
sequence of images  taken by the same  camera of the same  scene with
only a small amount of action between images.

2. An  image node represents the  familiar two dimensional  idea of a
photograph or an oil  painting or to be  exact a digital video  image
of 216 rows by 288  columns of numbers ranging from 0 for  dark to 63
for bright.   The image is formed by a  thin lens and is projected on
a flat image  plane. The idea  of an image  is so  common that it  is
easy to overlook the wonder of  sun light scattering off of surfaces,
refracting  thru a lens, and forming a  complex pattern called a real
image.

3.  Below the image node are the intensity  contour levels. A contour
level is a binary image  that results from thresholding a gray scaled
image. So an image  is composed of  levels, and in  turn, a level  is
composed of polygons.

4.   A  Polygon node  represents the  idea  of a  contour loop  which
always  closes upon  itself and  does not cross  itself or  any other
contour. Contour loops are approximated by a ring of  vectors; hence,
the term "polygon".  The contour polygons always have  at least three
sides and are simply connected.

5. Shape nodes contain data about one or two  polygons. The data in a
shape node  is not a positive representation  of the notion of shape;
but is  rather the parameters  of allignment  and normalization  that
must be compensated before shapes can be compared.

6. Vector nodes contain the locus  of an image vertex; however  since
vectors always  belong to a  polygon and  always have two  neighbors;
their  counterclockwise  neighbor is  considered  to  determine their
vector direction.

7.  Arc  nodes are  vectors that  are made by  the polygon  smoothing
routine; one  arc typically replace  several vectors. When  both arcs
and vectors  are being discussed; vectors are strictly horizontal and
vertical, whereas arcs may point in any direction.

8.   Empty  nodes are  an artifact  of the  fixed  node size  dynamic
storage  allocation mechanism  used  in CRE.   Entities  are  made by
taking empty nodes  from an  AVAIL list  and entities  are killed  by
returning  their  node  to  the  AVAIL  list;  there  is  no  garbage
collector,  but there is a space compactor.
~I1973,800;F8- 7 -