COMMENT ⊗   VALID 00003 PAGES
C REC  PAGE   DESCRIPTION
C00001 00001
C00002 00002	4.3	Representation of a Geometric Mental Universe.
C00005 00003	MEMORY - TOP LEVEL GEM STRUCTURE.
C00008 ENDMK
C⊗;
4.3	Representation of a Geometric Mental Universe.

	At the top of the data structure is a  single  universe  node
from  which  everything  else can be reached.   Immediately below the
universe node is a ring  of  world  models.   A  robot  dealing  with
physical world sensor input, such as video data, has one of its world
models dedicated to simulating  the  immediate  here  and  now;  this
mental  world  is  called the reality world model. In addition to the
reality world, a robot may have  fantasy  world  models  for  problem
solving, planning or for recalling platonic object prototypes. In the
following, a two world mental universe will be the most common,  with
the  reality world being referred to as a "map" and the fantasy world
being referred to as a "dictionary".

	Geometric world models have four  basic  kinds  of  nodes:
body, face, edge and vertex. The face, edge and vertex nodes are used
to form polyhedrons which may be attached to body nodes.  Body  nodes
in  turn  are  connected  to  each other in rings and trees to form a
world model. Additional kinds of nodes  discribe  cameras  and  light
sources  as  well  as  temporary  data  such  as shadows, spines, and
trajectories.

	The image data structure  presented  in  this  section  is  a
computer's  internal  notation  for  what  is  vulgarly called a line
drawing; the common term is misleading because it  does  not  suggest
the  equally  important  space between the lines; terms closer to the
idea would be "mosaic drawing" or "stained glass window drawing".

The  data  structure  has  main  levels:  TV  raster,  video
intensity contour, arc contour, and region-edge.
MEMORY - TOP LEVEL GEM STRUCTURE.

	The top level  of the GEM  data structure is  constructed out
of  six kinds of  nodes: UNIVERSE,  WORLD, IMAGE, WINDOW,  CAMERA and
LAMP; by way  of contrast the polyhedron  data structure consists  of
BODY,  FACE, EDGE,  and VERTEX  nodes;  and the  auxillary nodes  are
classified  as EMPTY, FRAME, XNODE, YNODE  and ZNODE. Initially there
is no image  node and  only one Universe,  World, Window, Camera  and
Lamp Node. The approximate interconnections of the nodes is:

			  ←←  UNIVERSE  →→ 
			/	  ↓	    \
	 	      /	     empty nodes      \
		worlds				displays
		  ↓				  ↓
		  ↓   				windows
		  ↓				  ↓
lamps	←←←←	WORLD	 →→→→	cameras  ←←←←	WINDOW
  ↓		  ↓		  ↓		
LAMP		  ↓		CAMERA	
		  ↓	      /	       \
	 	  ↓	synthetic	perceived
		  ↓	 images		 images
		  ↓	    ↓		    ↓
		  ↓	    ↓		    ↓
		  ↓	    ↓		    ↓
	       3-D bodies	2-D  bodies
		    ↓                 ↓
		 faces, edges and vertices.


	Now for  the casual  definitions of  the SIX  top nodes.  The
Universe node is unique  and all nodes are connected to it so that it
serves as an OBLIST node. The GEM universe is the mental  universe or
universe of  discourse for geometric modeling.  Immediately below the
universe node  is a ring of world nodes and a ring or displays (and a
list of empty  nodes). A world node  is for representing one  physics
like  world at a  particular moment  in time;  three kinds  of worlds
might include a  perceived here-and-now  world or map;  a desired  or
goal world; and  a world of  prototype platonic forms,  or dictionary
world.  The  world points  immediately  at a  ring  of  light sources
(lamps), a ring of physical camera models, and a ring of bodies.