COMMENT ⊗   VALID 00002 PAGES
C REC  PAGE   DESCRIPTION
C00001 00001
C00002 00002	TOP LEVEL GEM STRUCTURE
C00005 ENDMK
C⊗;
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
		  ↓	    ↓		    ↓
		  ↓	    ↓		    ↓
		  ↓	    ↓		    ↓
		3D bodies	"2D" 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.