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C00002 00002	~[C<NαOCCULTλ30P40
C00003 00003	[4.0	Introduction to Hidden Line Elimination.]
C00007 00004	[4.1	Hiding a Topologically Coherent Object.]
C00009 00005	[4.2	Hidden Line Eliminators based on Face-Vertex and Edge-Edge Comparing.]
C00013 00006	[4.3	2-D Partition Sort of Faces, Edges and Vertices.]
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~[C;<N;αOCCULT;λ30;P40;
I425,0;JCFA                SECTION 4.
~JCFD HIDDEN LINE ELIMINATION FOR COMPUTER VISION.

~JUFA
4.0	Introduction to hidden line elimination.
4.1	Hiding a Topologically Coherent Object.
4.2	Two hidden line eliminators that almost work.
4.3	2-D Partition Sort of Faces, Edges and Vertices.
4.4	Propagating Underfaces.
4.5	Shadows.
4.6	Photometric Modeling and Video Generation.
4.7	Performance.
~I950,0;V950,1260;JUFA

[4.0	Introduction to Hidden Line Elimination.]

	Hidden line elimination  refers to the process  of simulating
the  appearance  of  opaque  three  dimensional  objects. The  phrase
{hidden line elimination} dates  from when the problem only  involved
deleting  the undesired,  that is  the  {hidden} lines,  from a  line
drawing; today  the phrase persists but connotes the wider problem of
synthesizing  realistic  images  using  a  computer.   The  following
discussion  of hidden  line elimination  in the  context  of computer
vision using GEOMED and winged edged polyhdera, will revolve around a
particular hidden line eliminator named OCCULT.  The word {occult} as
a verb literally means to hide.

	In  computer graphics the results  of hidden line elimination
are intended  for humans,  while  in computer  vision the  output  is
intended  for further  machine processing.  That is  for vision,  the
output  of the hidden line  eliminator must be  in a form appropriate
for some kind of image comparing process.  The idea of using a hidden
line  eliminator in  a vision  system is  not new  (although somewhat
rare) and has  appeared in  two other  vision systems,  one by  Larry
Roberts and one by Gill Falk. In fact, the present system is a direct
heir of  the work of Roberts  and Falk; the last  version of the Falk
system contained one of the early versions of OCCULT (as installed by
Richard Orban).

	Hidden  line  elimination,  that  is image  synthesis,  is  a
perenial  research  problem  because  it  can  not  be isolated  from
physical modeling  and because  by its  very nature  its results  are
quite  visible. Hidden  line elimination  is the  visible tip  of the
iceberg of physical simulation, weaknesses of the underlying model or
improvements in modeling show up first in passing through the process
of  image synthesis.  The present  day  collection of  techniques are
still quite lacking in realism, economy and flexibility.
[4.1	Hiding a Topologically Coherent Object.]

	In a context of polyhedral models with surface topology that
can be rapidly traveled and modified;
one very simple technique ends up doing most of the actual hiding

	OCCULT marks  the faces, edges  and vertices of  a polyhedral
scene as being, either visible  or hidden with  respect to  a simulated camera.
Edges that were at first  partially visible are split into pieces  so
that each piece is either fully visible or fully hidden.

	OCCULT  has two  parts: a  recursive outer
part that splits an image into halves, until there are only afew edges in it or it
has  become too  small;  the  inner  part is  the  real  hidden  line
eliminator.  The  inner  part of  OCCULT  is  based  on the  idea  of
comparing each edge with  all the other edges  and hiding as much  as
possible when a pair of intersecting edges is discovered.
[4.2	Hidden Line Eliminators based on Face-Vertex and Edge-Edge Comparing.]

	Two very simple kinds of hidden line eliminators that almost work
are based on comparing edges with edges or faces with vertices.
[4.3	2-D Partition Sort of Faces, Edges and Vertices.]

[4.4	Propagating Underfaces and Shadows.]

[4.5	Photometric Modeling and Video Generation.]

[4.6	Performance.]