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

	CRE inputs an image into an array  called TVBUF; it makes the
node structures, some  of which are temporary; and it outputs a final
version  of  the  structure  representing  a  film  of   images.  The
temporary structures  are relevant to understanding  the process; but
only  the  final  structure is  relevant  to using  CRE  output.   In
summary, the important structures are:

	FOUR RINGS.
		1. Image ring of the film.
		2. Level ring of an image.
		3. Polygon ring of a level.
		4. Vector ring of a polygon.

	TWO TREES.
		1. The tree of rings.
		2. The tree of nested polygons.

	TWO LISTS.
		1. Time line lists.
		2. The empty node list.

	TEMPORARY STRUCTURES.
		1. Arc rings of polygons.
		2. Fusion shape rings of levels.

	FIVE ARRAYS.
		1. TVBUF - 216 rows, 288 columns of  6 bit bytes.
		2. PAC   - 216 rows, 288 columns of  1 bit bytes.
		3. VSEG  - 216 rows, 289 columns of  1 bit bytes.
		4. HSEG  - 217 rows, 288 columns of  1 bit bytes.
		5. SKY   - 216 rows, 289 columns of 18 bit bytes.

	There is one  film node.  The film  is composed of a  ring of
images.   Each image is composed of a ring  of levels.  Each level is
composed of a ring  of polygons. Each polygon  is composed of a  ring
of vectors.  The ring structures  are implemented with the four links
named  DAD, SON,   CW and  CCW.  The  rings are headless  only in the
sense that all the elements of a ring are brothers;  a pointer to the
head of a ring is stored  in the DAD link of each element. The DAD of
the film node is  NIL; and NIL  is an 18-bit zero.  The final SON  of
all vector nodes  is also NIL. The DAD  and SON links form a  tree of
rings.

~I1973,800;F8- 9 -