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<title>merge.c -- facet merge operations</title>
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<h2>merge.c -- facet merge operations</h2>
<blockquote>
<p>Qhull handles precision problems by merged facets or joggled input.
Except for redundant vertices, it corrects a problem by
merging two facets. When done, all facets are clearly
convex. See <a href="../../html/qh-impre.htm">Imprecision in Qhull</a>
for further information. </p>
<p>Users may joggle the input ('<a href="../../html/qh-optq.htm#QJn">QJn</a>')
instead of merging facets. </p>
<p>Qhull detects and corrects the following problems: </p>
<ul>
<li><b>More than two facets meeting at a ridge. </b>When
Qhull creates facets, it creates an even number
of facets for each ridge. A convex hull always
has two facets for each ridge. More than two
facets may be created if non-adjacent facets
share a vertex. This is called a <em>duplicate
ridge</em>. In 2-d, a duplicate ridge would
create a loop of facets. </li>
</ul>
<ul>
<li><b>A facet contained in another facet. </b>Facet
merging may leave all vertices of one facet as a
subset of the vertices of another facet. This is
called a <em>redundant facet</em>. </li>
</ul>
<ul>
<li><b>A facet with fewer than three neighbors. </b>Facet
merging may leave a facet with one or two
neighbors. This is called a <em>degenerate facet</em>.
</li>
</ul>
<ul>
<li><b>A facet with flipped orientation. </b>A
facet's hyperplane may define a halfspace that
does not include the interior point.This is
called a <em>flipped facet</em>. </li>
</ul>
<ul>
<li><strong>A coplanar horizon facet.</strong> A
newly processed point may be coplanar with an
horizon facet. Qhull creates a new facet without
a hyperplane. It links new facets for the same
horizon facet together. This is called a <em>samecycle</em>.
The new facet or samecycle is merged into the
horizon facet. </li>
</ul>
<ul>
<li><b>Concave facets. </b>A facet's centrum may be
above a neighboring facet. If so, the facets meet
at a concave angle. </li>
</ul>
<ul>
<li><b>Coplanar facets. </b>A facet's centrum may be
coplanar with a neighboring facet (i.e., it is
neither clearly below nor clearly above the
facet's hyperplane). Qhull removes coplanar
facets in independent sets sorted by angle.</li>
</ul>
<ul>
<li><b>Redundant vertex. </b>A vertex may have fewer
than three neighboring facets. If so, it is
redundant and may be renamed to an adjacent
vertex without changing the topological
structure.This is called a <em>redundant vertex</em>.
</li>
</ul>
</blockquote>
<p><b>Copyright &copy; 1995-2011 C.B. Barber</b></p>
<hr>
<p><a href="#TOP">&#187;</a> <a href="qh-geom.htm#TOC">Geom</a>
<a name="TOC">&#149;</a> <a href="qh-globa.htm#TOC">Global</a>
&#149; <a href="qh-io.htm#TOC">Io</a> &#149; <a href="qh-mem.htm#TOC">Mem</a>
&#149; <b>Merge</b> &#149; <a href="qh-poly.htm#TOC">Poly</a>
&#149; <a href="qh-qhull.htm#TOC">Qhull</a> &#149; <a href="qh-set.htm#TOC">Set</a>
&#149; <a href="qh-stat.htm#TOC">Stat</a> &#149; <a href="qh-user.htm#TOC">User</a>
</p>
<h3>Index to <a href="merge.c">merge.c</a> and
<a href="merge.h">merge.h</a></h3>
<ul>
<li><a href="#mtype">merge.h data types, macros, and
global sets</a> </li>
<li><a href="#mconst">merge.h constants</a> </li>
</ul>
<ul>
<li><a href="#mtop">top-level merge functions</a> </li>
<li><a href="#mset">functions for identifying merges</a></li>
<li><a href="#mbest">functions for determining the
best merge</a> </li>
<li><a href="#mmerge">functions for merging facets</a>
</li>
<li><a href="#mcycle">functions for merging a cycle
of facets</a> </li>
<li><a href="#mrename">functions for renaming a
vertex</a> </li>
<li><a href="#mvertex">functions for identifying
vertices for renaming</a> </li>
<li><a href="#mcheck">functions for check and trace</a> </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mtype">merge.h data
types, macros, and global sets</a></h3>
<ul>
<li><a href="merge.h#mergeT">mergeT</a> structure to
identify a merge of two facets</li>
<li><a href="merge.h#FOREACHmerge_">FOREACHmerge_</a>
assign 'merge' to each merge in merges </li>
<li><a href="libqhull.h#qh-set">qh global sets</a>
qh.facet_mergeset contains non-convex merges
while qh.degen_mergeset contains degenerate and
redundant facets</li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mconst">merge.h
constants</a></h3>
<ul>
<li><a href="libqhull.h#qh-prec">qh precision constants</a>
precision constants for Qhull </li>
<li><a href="merge.h#MRG">MRG...</a> indicates the
type of a merge (mergeT-&gt;type)</li>
<li><a href="merge.h#qh_ANGLEredundant">qh_ANGLEredundant</a>
indicates redundant merge in mergeT-&gt;angle </li>
<li><a href="merge.h#qh_ANGLEdegen">qh_ANGLEdegen</a>
indicates degenerate facet in mergeT-&gt;angle </li>
<li><a href="merge.h#qh_ANGLEconcave">qh_ANGLEconcave</a>
offset to indicate concave facets in
mergeT-&gt;angle </li>
<li><a href="merge.h#qh_MERGEapex">qh_MERGEapex</a>
flag for qh_mergefacet() to indicate an apex
merge </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mtop">top-level merge
functions</a></h3>
<ul>
<li><a href="merge.c#all_merges">qh_all_merges</a>
merge all non-convex facets </li>
<li><a href="merge.c#checkzero">qh_checkzero</a>
check that facets are clearly convex </li>
<li><a href="merge.c#flippedmerges">qh_flippedmerges</a>
merge flipped facets into best neighbor </li>
<li><a href="merge.c#forcedmerges">qh_forcedmerges</a>
merge all duplicate ridges </li>
<li><a href="merge.c#merge_degenredundant">qh_merge_degenredundant</a>
merge degenerate and redundant facets </li>
<li><a href="merge.c#merge_nonconvex">qh_merge_nonconvex</a>
merge a non-convex ridge </li>
<li><a href="merge.c#premerge">qh_premerge</a>
pre-merge non-convex facets </li>
<li><a href="merge.c#postmerge">qh_postmerge</a>
post-merge nonconvex facets as defined by
maxcentrum/maxangle </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mset">functions for
identifying merges</a></h3>
<ul>
<li><a href="merge.c#appendmergeset">qh_appendmergeset</a>
appends an entry to qh.facet_mergeset</li>
<li><a href="merge.c#compareangle">qh_compareangle</a>
used by qsort() to order merges </li>
<li><a href="merge.c#comparemerge">qh_comparemerge</a>
used by qsort() to order merges </li>
<li><a href="merge.c#degen_redundant_facet">qh_degen_redundant_facet</a>
check for a degenerate and redundant facet</li>
<li><a href="merge.c#degen_redundant_neighbors">qh_degen_redundant_neighbors</a>
append degenerate and redundant neighbors to
qh.degen_mergeset </li>
<li><a href="merge.c#getmergeset_initial">qh_getmergeset_initial</a>
build initial qh.facet_mergeset </li>
<li><a href="merge.c#getmergeset">qh_getmergeset</a>
update qh.facet_mergeset </li>
<li><a href="merge.c#mark_dupridges">qh_mark_dupridges</a>
add duplicated ridges to qh.facet_mergeset</li>
<li><a href="merge.c#maydropneighbor">qh_maydropneighbor</a>
drop neighbor relationship if no ridge between
facet and neighbor </li>
<li><a href="merge.c#test_appendmerge">qh_test_appendmerge</a>
test a pair of facets for convexity and append to
qh.facet_mergeset if non-convex </li>
<li><a href="merge.c#test_vneighbors">qh_test_vneighbors</a>
test vertex neighbors for convexity </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mbest">functions for
determining the best merge</a></h3>
<ul>
<li><a href="merge.c#findbest_test">qh_findbest_test</a>
test neighbor for best merge </li>
<li><a href="merge.c#findbestneighbor">qh_findbestneighbor</a>
finds best neighbor of a facet for merging (i.e.,
closest hyperplane) </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mmerge">functions for
merging facets</a></h3>
<ul>
<li><a href="merge.c#copynonconvex">qh_copynonconvex</a>
copy non-convex flag to another ridge for the
same neighbor </li>
<li><a href="merge.c#makeridges">qh_makeridges</a>
creates explicit ridges between simplicial facets
</li>
<li><a href="merge.c#mergefacet">qh_mergefacet</a>
merges one facet into another facet</li>
<li><a href="merge.c#mergeneighbors">qh_mergeneighbors</a>
merges the neighbors of two facets </li>
<li><a href="merge.c#mergeridges">qh_mergeridges</a>
merges the ridge sets of two facets </li>
<li><a href="merge.c#mergesimplex">qh_mergesimplex</a>
merge a simplicial facet into another simplicial
facet </li>
<li><a href="merge.c#mergevertex_del">qh_mergevertex_del</a>
delete a vertex due to merging one facet into
another facet </li>
<li><a href="merge.c#mergevertex_neighbors">qh_mergevertex_neighbors</a>
merge the vertex neighbors of two facets </li>
<li><a href="merge.c#mergevertices">qh_mergevertices</a>
merge the vertex sets of two facets </li>
<li><a href="merge.c#newvertices">qh_newvertices</a>
register all vertices as new vertices </li>
<li><a href="merge.c#updatetested">qh_updatetested</a>
clear tested flags and centrums involved in a
merge </li>
<li><a href="merge.c#willdelete">qh_willdelete</a>
moves facet to qh.visible_list; sets replacement
or NULL </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mcycle">functions for
merging a cycle of facets</a></h3>
<p>If a point is coplanar with an horizon facet, the
corresponding new facets are linked together (a <em>samecycle</em>)
for merging.</p>
<ul>
<li><a href="merge.c#basevertices">qh_basevertices</a>
return temporary set of base vertices for a
samecycle </li>
<li><a href="merge.c#mergecycle">qh_mergecycle</a>
merge a samecycle into a horizon facet </li>
<li><a href="merge.c#mergecycle_all">qh_mergecycle_all</a>
merge all samecycles into horizon facets</li>
<li><a href="merge.c#mergecycle_facets">qh_mergecycle_facets</a>
finish merge of samecycle </li>
<li><a href="merge.c#mergecycle_neighbors">qh_mergecycle_neighbors</a>
merge neighbor sets for samecycle </li>
<li><a href="merge.c#mergecycle_ridges">qh_mergecycle_ridges</a>
merge ridge sets for samecycle </li>
<li><a href="merge.c#mergecycle_vneighbors">qh_mergecycle_vneighbors</a>
merge vertex neighbor sets for samecycle </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mrename">functions
for renaming a vertex</a></h3>
<ul>
<li><a href="merge.c#comparevisit">qh_comparevisit</a>
used by qsort() to order vertices by visitid</li>
<li><a href="merge.c#reducevertices">qh_reducevertices</a>
reduce vertex sets </li>
<li><a href="merge.c#redundant_vertex">qh_redundant_vertex</a>
returns true if detect and rename redundant
vertex </li>
<li><a href="merge.c#rename_sharedvertex">qh_rename_sharedvertex</a>
detect and rename a shared vertex </li>
<li><a href="merge.c#renameridgevertex">qh_renameridgevertex</a>
rename oldvertex to newvertex in a ridge </li>
<li><a href="merge.c#renamevertex">qh_renamevertex</a>
rename oldvertex to newvertex in ridges </li>
<li><a href="merge.c#remove_extravertices">qh_remove_extravertices</a>
remove extra vertices in non-simplicial facets </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mvertex">functions
for identifying vertices for renaming</a></h3>
<ul>
<li><a href="merge.c#find_newvertex">qh_find_newvertex</a>
locate new vertex for renaming old vertex </li>
<li><a href="merge.c#hashridge">qh_hashridge</a> add
ridge to hashtable </li>
<li><a href="merge.c#hashridge_find">qh_hashridge_find</a>
returns matching ridge in hashtable</li>
<li><a href="merge.c#neighbor_intersections">qh_neighbor_intersections</a>
return intersection of vertex sets for
neighboring facets </li>
<li><a href="merge.c#vertexridges">qh_vertexridges</a>
return temporary set of ridges adjacent to a
vertex </li>
<li><a href="merge.c#vertexridges_facet">qh_vertexridges_facet</a>
add adjacent ridges for a vertex in facet </li>
</ul>
<h3><a href="qh-merge.htm#TOC">&#187;</a><a name="mcheck">functions for check and
trace</a></h3>
<ul>
<li><a href="merge.c#checkconnect">qh_checkconnect</a>
check that new facets are connected </li>
<li><a href="merge.c#tracemerge">qh_tracemerge</a>
print trace message after merge </li>
<li><a href="merge.c#tracemerging">qh_tracemerging</a>
print trace message during post-merging </li>
</ul>
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