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sn#095853 filedate 1974-04-10 generic text, type C, neo UTF8
COMMENT ⊗ VALID 00002 PAGES
C REC PAGE DESCRIPTION
C00001 00001
C00002 00002 GEOMED'S BODY INTERSECTION ALGORITHM.
C00005 ENDMK
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GEOMED'S BODY INTERSECTION ALGORITHM.
The body intersection algorithm consists of two parts: first
there is a geometric part in which all the faces and edges are
compared for potential intersections and second there is a
topological part in which the result polyhedral body is copied off of
the arguments and all the pointer are made right.
After all the face piercing vertices have been made,
the new result body
MKSURF
OTHERV
ETRACE
COMMENT FIXUP1
FIXUP1 places vertex and wing pointers in all the non-surface
edges. A non-surface edge is distinguished by its non-zero ALT link,
and the new vertices are provided with a A as a first edge, PED, if
it be lacking.;
A←BODY0;
WHILE (A←PED(A)) ≠ BODY0 DO IF (E←ALT(A))≠0 THEN
BEGIN
PVT(A) ←V← ALT(PVT(E)); IF PED(V)=0 THEN PED(V)←A;
NVT(A) ←V← ALT(NVT(E)); IF PED(V)=0 THEN PED(V)←A;
NCW(A) ← ALT(NCW(E));
PCW(A) ← ALT(PCW(E));
NCCW(A) ← ALT(NCCW(E));
PCCW(A) ← ALT(PCCW(E));
END;
COMMENT FIXUP2
FIXUP2 wigns together the surface vertex tridedral corners.
Since it is our good luck that all surface vertices are necessarily
trihedral, the edges can be passed to the WING primitive for oriented
linking, in any order. The two surface wings of a surface vertex
were stored in the NED and PED links by MKSURF; the inward wing can
be retrieve as the PED(ALT(U)). Surface vertices are distinguished by
their ALT vertex having his SURBIT on.;
COMMENT FIXUP3
FIXUP3 replaces the alein faces of the result body with
native faces; this is done by scaning the edge ring of the body,
testing the two faces of each edge to see if they belong to the
result body, and if a face doesn't belong it is replaced by a new
one. Face replacement, as ususal, requires clocking around a face
perimeter and changing the appropriate face link in each edge.