~F8DATA STRUCTURE: BASICS.

	The two generic data structures of CRE  are arrays and nodes;
there are  five kinds of arrays  and eight kinds of  nodes.  The node
structures to be discussed are implemented as seven word  fixed sized
blocks  in   a  fashion   usual  to   graphics  and   simulation;  an
introduction  to this  technology  can be  found in  Knuth [4].   The
language  of implementation  is  PDP-10  machine code  via  the  FAIL
assembler.

	The  whole nodal structure  in CRE  represents a  sequence in
time of video  intensity contour maps.   Such contour  maps are  like
topographical elevation  contour maps, in  that no two  contour lines
should  every cross and in  that all the contour  lines should close.
Consequently,  the loops  of  contours  enclose  regions;  and  these
regions  overlap  in  a  nested  fashion forming  a  tree  like  data
structure.

	As  the  general  examples  of  contoured  images  on page  5
illustrate, a notion that is  emphatically not in CRE,  is that  of a
schematic line drawing.   Although the CRE output can  be viewed as a
collection of lines  on a display  screen,  people  expecting a  line
drawing  rendition   of  the   given  television   picture  will   be
disappointed.    A CRE  picture  is a  simple  transformation  of the
photometry,   geometry  and topology  of  the original  video  image;
whereas  the typical  line  drawing  from a  human  illustrator is  a
representation  of the scene without  photometric information. On the
other  hand,   the  work  of  an  artist  such as  Peter  Max;  or  a
paint-by-the-numbers grid  does resembly CRE output.   This is not an
idle coincidance  but rather  a  consequence of  whether or  not  the
artist is trying to represent photometric data by quantum lines.

	The explanation of  CRE node structures will be  presented in
three  parts: first,   the  several  kinds of  nodes will  be briefly
explained; second,   the  sub  structures such  as rings,  trees  and
lists  will be  discribed; and  third,   the node  formats and  their
contents  will be  explained in  detail.   Following that will  be an
explanation of  the five  arrays in CRE.  The reader  is warned  that
this  whole sub section  (on data  structure) is an  elaborate shaggy
dog story of naming names and  defining things; all the action is  to
be found in the following sub section (on the algorithm).
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