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	<p><a name="TOP"><b>Up:</b></a> <a
	href="http://www.qhull.org">Home page</a> for Qhull
	(http://www.qhull.org)<br>
	<b>Up:</b> <a href="index.htm#TOC">Qhull manual</a>: Table of Contents<br>
	<b>To:</b> <a href="qh-quick.htm#programs">Programs</a>
	<a href="qh-quick.htm#options">Options</a>
	<a href="qh-opto.htm#output">Output</a>
	<a href="qh-optf.htm#format">Formats</a>
	<a href="qh-optg.htm#geomview">Geomview</a>
	<a href="qh-optp.htm#print">Print</a>
	<a href="qh-optq.htm#qhull">Qhull</a>
	<a href="qh-optc.htm#prec">Precision</a>
	<a href="qh-optt.htm#trace">Trace</a>
	<a href="../src/libqhull_r/index.htm">Functions</a><br>
	<b>To:</b> <a href="#TOC">FAQ: Table of Contents</a> (please
	wait while loading) <br>

	<hr>
	<!-- Main text of document -->
	<h1><a
	href="http://www.geom.uiuc.edu/graphics/pix/Special_Topics/Computational_Geometry/4dcube.html"><IMG
	align=middle alt ="[4-d cube]"
	height=100 src="qh--4d.gif" width=100 ></a> Frequently Asked Questions about Qhull</h1><!--
	<p><i>This is the most recent FAQ. It was updated Sept. 13, 1999.</i>
	-->
	<p>If your question does not appear here, see: </p>

	<ul>
	<li><a href="http://www.qhull.org/news">News</a> about Qhull
	<li><a href="index.htm#TOC">Qhull manual:</a> table of contents
	<li><a href="../README.txt">Installation</a> instructions for Qhull and rbox

	<li><a href="mailto:qhull@qhull.org">Send e-mail</a> to
	qhull@qhull.org
	<li><a href="mailto:qhull_bug@qhull.org">Report bugs</a>
	to qhull_bug@qhull.org </li>
	</ul>

	<p>Qhull is a general dimension code for computing convex hulls,
	Delaunay triangulations, halfspace intersections about a point,
	Voronoi diagrams, furthest-site Delaunay triangulations, and
	furthest-site Voronoi diagrams. These structures have
	applications in science, engineering, statistics, and
	mathematics. For a detailed introduction, see O'Rourke [<a
	href="index.htm#orou94" >'94</a>], <i>Computational Geometry in C</i>.
	</p>

	<p>There are separate programs for each application of
	Qhull. These programs disable experimental and inappropriate
	options. If you prefer, you may use Qhull directly. All programs
	run the same code.

	<p>Version 3.1 added triangulated output ('<a href="qh-optq.htm#Qt">Qt</a>').
	It should be used for Delaunay triangulations instead of
	using joggled input ('<a href="qh-optq.htm#QJn">QJ</a>').

	<p><i>Brad Barber, Arlington MA,
	2010/01/09 <!--
	--> </i></p>

	<p><b>Copyright © 1998-2015 C.B. Barber</b></p>

	<hr>

	<h2><a href="#TOP">»</a><a name="TOC">FAQ: Table of Contents </a></h2>

	<p>Within each category, the most recently asked questions are
	first.
	<ul>
	<li>Startup questions <ul>
	<li><a href="#console">How</a> do I run Qhull from Windows?
	<li><a href="#input">How</a> do I enter points for Qhull?
	<li><a href="#learn">How</a> do I learn to use Qhull?</li>
	</ul>
	<li>Convex hull questions<ul>
	<li><a href="#area">How</a> do I report just the area and volume of a
	convex hull?
	<li><a href="#extra">Why</a> are there extra points in a 4-d or higher
	convex hull?
	<li><a href="#dup">How</a> do I report duplicate
	vertices? </li>
	</ul>
	<li>Delaunay triangulation questions<ul>
	<li><a href="#flat">How</a> do I get rid of nearly flat Delaunay
	triangles?
	<li><a href="#vclosest">How</a> do I find the Delaunay triangle or Voronoi
	region that is closest to a point?

	<li><a href="#mesh">How</a> do I compute the Delaunay triangulation of a
	non-convex object?

	<li><a href="#mesh">How</a> do I mesh a volume from a set of triangulated
	surface points?

	<li><a href="#constrained">Can</a> Qhull produce a triangular mesh for an
	object?

	<li><a href="#dridges">For</a> 3-d Delaunay triangulations, how do I
	report the triangles of each tetrahedron?

	<li><a href="#3dd">How</a> do I construct a 3-d Delaunay triangulation?
	<li><a href="#2d">How</a> do I get the triangles for a 2-d Delaunay
	triangulation and the vertices of its Voronoi diagram?
	<li><a href="#big">Can </a>Qhull triangulate a
	hundred 16-d points?</li>
	</ul>

	<li>Voronoi diagram questions<ul>
	<li>See also "Delaunay diagram questions". Qhull computes the Voronoi diagram from the Delaunay triagulation.
	<li><a href="#volume">How</a> do I compute the volume of a Voronoi region?
	<li><a href="#maxsphere">How</a> do I get the radii of the empty
	spheres for each Voronoi vertex?

	<li><a href="#square">What</a> is the Voronoi diagram of a square?

	<li><a href="#vsphere">How</a> do I construct the Voronoi diagram of
	cospherical points?
	<li><a href="#rays">Can</a> Qhull compute the unbounded rays of the
	Voronoi diagram?
	</ul>
	<li>Approximation questions<ul>
	<li><a href="#simplex">How</a> do I approximate data
	with a simplex?</li>
	</ul>
	<li>Halfspace questions<ul>
	<li><a href="#halfspace">How</a> do I compute the
	intersection of halfspaces with Qhull?</li>
	</ul>
	<li><a name="library">Qhull library</a> questions<ul>
	<li><a href="#math">Is</a> Qhull available for Mathematica, Matlab, or
	Maple?

	<li><a href="#ridges">Why</a> are there too few ridges?
	<li><a href="#call">Can</a> Qhull use coordinates without placing them in
	a data file?
	<li><a href="#size">How</a> large are Qhull's data structures?
	<li><a href="#inc">Can</a> Qhull construct convex hulls and Delaunay
	triangulations one point at a time?
	<li><a href="#ridges2">How</a> do I visit the ridges of a Delaunay
	triangulation?
	<li><a href="#listd">How</a> do I visit the Delaunay facets?
	<LI><a
	href="#outside">When</a> is a point outside or inside a facet?
	<li><a href="#closest">How</a> do I find the facet that is closest to a
	point?
	<li><a href="#vclosest">How</a> do I find the Delaunay triangle or Voronoi
	region that is closest to a point?
	<li><a href="#vertices">How</a> do I list the vertices?
	<li><a href="#test">How</a> do I test code that uses the Qhull library?
	<li><a href="#orient">When</a> I compute a plane
	equation from a facet, I sometimes get an
	outward-pointing normal and sometimes an
	inward-pointing normal</li>
	</ul>
	</li>
	</ul>

	<hr>

	<h2><a href="#TOC">»</a><a name="startup">Startup</a> questions</h2>

	<h4><a href="#TOC">»</a><a name="console">How</a> do I run Qhull
	from Windows?</h4><blockquote>

	<p>Qhull is a console program. You will first need a command window
	(i.e., a "command prompt"). You can double click on
	'eg\Qhull-go.bat'. </p>

	<blockquote><ul>
	<li>Type 'qconvex', 'qdelaunay', 'qhalf', 'qvoronoi,
	'qhull', and 'rbox' for a synopsis of each program.

	<li>Type 'rbox c D2 \| qconvex s i' to compute the
	convex hull of a square.

	<li>Type 'rbox c D2 \| qconvex s i TO results.txt' to
	write the results to the file 'results.txt'. A summary is still printed on
	the the console.

	<li>Type 'rbox c D2' to see the input format for
	qconvex.

	<li>Type 'qconvex < data.txt s i TO results.txt' to
	read input data from 'data.txt'.

	<li>If you want to enter data by hand, type 'qconvex s i TO
	results.txt' to read input data from the console. Type in
	the numbers and end with a ctrl-D. </li>
	</ul></blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="input">How</a> do I enter
	points for Qhull?</h4><blockquote>

	<p>Qhull takes its data from standard input. For example, create
	a file named 'data.txt' with the following contents: </p>

	<blockquote>
	<pre>
	2 #sample 2-d input
	5 #number of points
	1 2 #coordinates of points
	-1.1 3
	3 2.2
	4 5
	-10 -10
	</pre>
	</blockquote>

	<p>Then call qconvex with 'qconvex < data.txt'. It will print a
	summary of the convex hull. Use 'qconvex < data.txt o' to print
	the vertices and edges. See also <a href="index.htm#input">input
	format</a>. </p>

	<p>You can generate sample data with rbox, e.g., 'rbox 10'
	generates 10 random points in 3-d. Use a pipe ('\|') to run rbox
	and qhull together, e.g., </p>

	<blockquote>
	<p>rbox c \| qconvex o </p>
	</blockquote>

	<p>computes the convex hull of a cube. </p>

	</blockquote><h4><a href="#TOC">»</a><a name="learn">How</a> do I learn to
	use Qhull?</h4><blockquote>

	<p>First read: </p>

	<ul>
	<li><a href="index.htm">Introduction</a> to Qhull
	<li><a href="index.htm#when">When</a> to use Qhull
	<li><a href="qconvex.htm">qconvex</a> -- convex hull
	<li><a href="qdelaun.htm">qdelaunay</a> -- Delaunay triangulation
	<li><a href="qhalf.htm">qhalf</a> -- half-space intersection about a point

	<li><a href="qvoronoi.htm">qvoronoi</a> -- Voronoi diagram
	<li><a href="rbox.htm">Rbox</a>, for sample inputs
	<li><a href="qh-eg.htm">Examples</a> of Qhull</li>
	</ul>

	<p>Look at Qhull's on-line documentation: </p>

	<ul>
	<li>'qconvex' gives a synopsis of qconvex and its options

	<li>'rbox' lists all of the options for generating point
	sets
	<li>'qconvex - \| more' lists the options for qconvex
	<li>'qconvex .' gives a concise list of options
	<li>'qdelaunay', 'qhalf', 'qvoronoi', and 'qhull' also have a synopsis and option list</li>
	</ul>

	<p>Then try out the Qhull programs on small examples. </p>

	<ul>
	<li>'rbox c' lists the vertices of a cube
	<li>'rbox c \| qconvex' is the convex hull of a cube
	<li>'rbox c \| qconvex o' lists the vertices and facets of
	a cube
	<li>'rbox c \| qconvex Qt o' triangulates the cube
	<li>'rbox c \| qconvex QJ o' joggles the input and
	triangulates the cube
	<li>'rbox c D2 \| qconvex' generates the convex hull of a
	square
	<li>'rbox c D4 \| qconvex' generates the convex hull of a
	hypercube
	<li>'rbox 6 s D2 \| qconvex p Fx' lists 6 random points in
	a circle and lists the vertices of their convex hull in order
	<li>'rbox c D2 c G2 \| qdelaunay' computes the Delaunay
	triangulation of two embedded squares. It merges the cospherical facets.
	<li>'rbox c D2 c G2 \| qdelaunay Qt' computes the Delaunay
	triangulation of two embedded squares. It triangulates the cospherical facets.
	<li>'rbox c D2 c G2 \| qvoronoi o' computes the
	corresponding Voronoi vertices and regions.
	<li>'rbox c D2 c G2 \| qvoronio Fv' shows the Voronoi diagram
	for the previous example. Each line is one edge of the
	diagram. The first number is 4, the next two numbers list
	a pair of input sites, and the last two numbers list the
	corresponding pair of Voronoi vertices. </li>
	</ul>

	<p>Install <a href="http://www.geomview.org">Geomview</a>
	if you are running SGI Irix, Solaris, SunOS, Linux, HP, IBM
	RS/6000, DEC Alpha, or Next. You can then visualize the output of
	Qhull. Qhull comes with Geomview <a href="qh-eg.htm">examples</a>.
	</p>

	<p>Then try Qhull with a small example of your application. Work
	out the results by hand. Then experiment with Qhull's options to
	find the ones that you need. </p>

	<p>You will need to decide how Qhull should handle precision
	problems. It can triangulate the output ('<a
	href="qh-optq.htm#Qt" >Qt</a>'), joggle the input ('<a
	href="qh-optq.htm#QJn" >QJ</a>'), or merge facets (the default). </p>

	<ul>
	<li>With joggle, Qhull produces simplicial (i.e.,
	triangular) output by joggling the input. After joggle,
	no points are cocircular or cospherical.
	<li>With facet merging, Qhull produces a better
	approximation and does not modify the input.
	<li>With triangulated output, Qhull merges facets and triangulates
	the result.</li>
	<li>See <a href="qh-impre.htm#joggle">Merged facets or joggled input</a>. </li>
	</ul>

	</blockquote>
	<h2><a href="#TOC">»</a><a name="convex">Convex hull questions</a></h2>

	<h4><a href="#TOC">»</a><a name="area">How</a> do I report just the area
	and volume of a convex hull?</h4><blockquote>

	Use option 'FS'. For example,

	<blockquote><pre>
	C:\qhull>rbox 10 \| qconvex FS
	0
	2 2.192915621644613 0.2027867899638665

	C:\qhull>rbox 10 \| qconvex FA

	Convex hull of 10 points in 3-d:

	Number of vertices: 10
	Number of facets: 16

	Statistics for: RBOX 10 \| QCONVEX FA

	Number of points processed: 10
	Number of hyperplanes created: 28
	Number of distance tests for qhull: 44
	CPU seconds to compute hull (after input): 0
	Total facet area: 2.1929156
	Total volume: 0.20278679
	</pre></blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="extra">Why</a> are there extra
	points in a 4-d or higher convex hull?</h4><blockquote>

	<p>You may see extra points if you use options '<a
	href="qh-opto.htm#i" >i</a>' or '<a href="qh-optf.htm#Ft">Ft</a>'
	without using triangulated output ('<a href="qh-optq.htm#Qt">Qt</a>').
	The extra points occur when a facet is non-simplicial (i.e., a
	facet with more than <i>d</i> vertices). For example, Qhull
	reports the following for one facet of the convex hull of a hypercube.
	Option 'Pd0:0.5' returns the facet along the positive-x axis: </p>

	<blockquote>
	<pre>
	rbox c D4 \| qconvex i Pd0:0.5
	12
	17 13 14 15
	17 13 12 14
	17 11 13 15
	17 14 11 15
	17 10 11 14
	17 14 12 8
	17 12 13 8
	17 10 14 8
	17 11 10 8
	17 13 9 8
	17 9 11 8
	17 11 9 13
	</pre>
	</blockquote>

	<p>The 4-d hypercube has 16 vertices; so point "17" was
	added by qconvex. Qhull adds the point in order to report a
	simplicial decomposition of the facet. The point corresponds to
	the "centrum" which Qhull computes to test for
	convexity. </p>

	<p>Triangulate the output ('<a href="qh-optq.htm#Qt">Qt</a>') to avoid the extra points.
	Since the hypercube is 4-d, each simplicial facet is a tetrahedron.
	<blockquote>
	<pre>
	C:\qhull3.1>rbox c D4 \| qconvex i Pd0:0.5 Qt
	9
	9 13 14 15
	12 9 13 14
	9 11 13 15
	11 9 14 15
	9 10 11 14
	12 9 14 8
	9 12 13 8
	9 10 14 8
	10 9 11 8
	</pre>
	</blockquote>

	<p>Use the '<a href="qh-optf.htm#Fv">Fv</a>' option to print the
	vertices of simplicial and non-simplicial facets. For example,
	here is the same hypercube facet with option 'Fv' instead of 'i':
	</p>

	<blockquote>
	<pre>
	C:\qhull>rbox c D4 \| qconvex Pd0:0.5 Fv
	1
	8 9 10 12 11 13 14 15 8
	</pre>
	</blockquote>

	<p>The coordinates of the extra point are printed with the '<A
	href="qh-optf.htm#Ft" >Ft</a>' option. </p>

	<blockquote>
	<pre>
	rbox c D4 \| qconvex Pd0:0.5 Ft
	4
	17 12 3
	-0.5 -0.5 -0.5 -0.5
	-0.5 -0.5 -0.5 0.5
	-0.5 -0.5 0.5 -0.5
	-0.5 -0.5 0.5 0.5
	-0.5 0.5 -0.5 -0.5
	-0.5 0.5 -0.5 0.5
	-0.5 0.5 0.5 -0.5
	-0.5 0.5 0.5 0.5
	0.5 -0.5 -0.5 -0.5
	0.5 -0.5 -0.5 0.5
	0.5 -0.5 0.5 -0.5
	0.5 -0.5 0.5 0.5
	0.5 0.5 -0.5 -0.5
	0.5 0.5 -0.5 0.5
	0.5 0.5 0.5 -0.5
	0.5 0.5 0.5 0.5
	0.5 0 0 0
	4 16 13 14 15
	4 16 13 12 14
	4 16 11 13 15
	4 16 14 11 15
	4 16 10 11 14
	4 16 14 12 8
	4 16 12 13 8
	4 16 10 14 8
	4 16 11 10 8
	4 16 13 9 8
	4 16 9 11 8
	4 16 11 9 13
	</pre>
	</blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="dup">How</a> do I report
	duplicate vertices?</h4><blockquote>

	<p>There's no direct way. You can use option
	'<a href="qh-optf.htm#FP">FP</a>' to
	report the distance to the nearest vertex for coplanar input
	points. Select the minimum distance for a duplicated vertex, and
	locate all input sites less than this distance. </p>

	<p>For Delaunay triangulations, all coplanar points are nearly
	incident to a vertex. If you want a report of coincident input
	sites, do not use option '<a href="qh-optq.htm#QJn">QJ</a>'. By
	adding a small random quantity to each input coordinate, it
	prevents coincident input sites. </p>

	</blockquote>
	<h2><a href="#TOC">»</a><a name="delaunay">Delaunay triangulation questions</a></h2>

	<h4><a href="#TOC">»</a><a name="flat">How</a> do I get rid of
	nearly flat Delaunay triangles?</h4><blockquote>

	<p>Nearly flat triangles occur when boundary points are nearly
	collinear or coplanar. They also occur for nearly coincident
	points. Both events can easily occur when using joggle. For example
	(rbox 10 W0 D2 \| qdelaunay QJ Fa) lists the areas of the Delaunay
	triangles of 10 points on the boundary of a square. Some of
	these triangles are nearly flat. This occurs when one point
	is joggled inside of two other points. In this case, nearly flat
	triangles do not occur with triangulated output (rbox 10 W0 D2 \| qdelaunay Qt Fa).


	<p>Another example, (rbox c P0 P0 D2 \| qdelaunay QJ Fa), computes the
	areas of the Delaunay triangles for the unit square and two
	instances of the origin. Four of the triangles have an area
	of 0.25 while two have an area of 2.0e-11. The later are due to
	the duplicated origin. With triangulated output (rbox c P0 P0 D2 \| qdelaunay Qt Fa)
	there are four triangles of equal area.

	<p>Nearly flat triangles also occur without using joggle. For
	example, (rbox c P0 P0,0.4999999999 \| qdelaunay Fa), computes
	the areas of the Delaunay triangles for the unit square,
	a nearly collinear point, and the origin. One triangle has an
	area of 3.3e-11.

	<p>Unfortunately, none of Qhull's merging options remove nearly
	flat Delaunay triangles due to nearly collinear or coplanar boundary
	points.
	The merging options concern the empty circumsphere
	property of Delaunay triangles. This is independent of the area of
	the Delaunay triangles. Qhull does handle nearly coincident points.

	<p>If you are calling Qhull from a program, you can merge slivers into an adjacent facet.
	In d dimensions with simplicial facets (e.g., from 'Qt'), each facet has
	d+1 neighbors. Each neighbor shares d vertices of the facet's d+1 vertices. Let the
	other vertex be the <i>opposite</i> vertex. For each neighboring facet, if its circumsphere
	includes the opposite.vertex, the two facets can be merged. [M. Treacy]

	<p>You can handle collinear or coplanar boundary points by
	enclosing the points in a box. For example,
	(rbox c P0 P0,0.4999999999 c G1 \| qdelaunay Fa), surrounds the
	previous points with [(1,1), (1,-1), (-1,-1), (-1, 1)].
	Its Delaunay triangulation does not include a
	nearly flat triangle. The box also simplifies the graphical
	output from Qhull.

	<p>Without joggle, Qhull lists coincident points as "coplanar"
	points. For example, (rbox c P0 P0 D2 \| qdelaunay Fa), ignores
	the duplicated origin and lists four triangles of size 0.25.
	Use 'Fc' to list the coincident points (e.g.,
	rbox c P0 P0 D2 \| qdelaunay Fc).

	<p>There is no easy way to determine coincident points with joggle.
	Joggle removes all coincident, cocircular, and cospherical points
	before running Qhull. Instead use facet merging (the default)
	or triangulated output ('<a href="qh-optq.htm#Qt">Qt</a>').

	</blockquote><h4><a href="#TOC">»</a><a name="mesh">How</a> do I compute
	the Delaunay triangulation of a non-convex object?</h4><blockquote>

	<p>A similar question is
	"How do I mesh a volume from a set of triangulated surface points?"

	<p>This is an instance of the constrained Delaunay Triangulation
	problem. Qhull does not handle constraints. The boundary of the
	Delaunay triangulation is always convex. But if the input set
	contains enough points, the triangulation will include the
	boundary. The number of points needed depends on the input.

	<p>Shewchuk has developed a theory of constrained Delaunay triangulations.
	See his
	<a href="http://www.cs.cmu.edu/~jrs/jrspapers.html#cdt">paper</a> at the
	1998 Computational Geometry Conference. Using these ideas, constraints
	could be added to Qhull. They would have many applications.

	<p>There is a large literature on mesh generation and many commercial
	offerings. For pointers see
	<a href="http://www.imr.sandia.gov/papers/topics.html">Owen's International Meshing Roundtable</a>
	and <a href="http://www.robertschneiders.de/meshgeneration/meshgeneration.html">Schneiders'
	Finite Element Mesh Generation page</a>.</p>

	</blockquote><h4><a href="#TOC">»</a><a name="constrained">Can</a> Qhull
	produce a triangular mesh for an object?</h4><blockquote>

	<p>Yes for convex objects, no for non-convex objects. For
	non-convex objects, it triangulates the concavities. Unless the
	object has many points on its surface, triangles may cross the
	surface. </p>

	</blockquote><h4><a href="#TOC">»</a><a name="dridges">For</a> 3-d Delaunay
	triangulations, how do I report the triangles of each
	tetrahedron?</h4><blockquote>

	<p>For points in general position, a 3-d Delaunay triangulation
	generates tetrahedron. Each face of a tetrahedron is a triangle.
	For example, the 3-d Delaunay triangulation of random points on
	the surface of a cube, is a cellular structure of tetrahedron. </p>

	<p>Use triangulated output ('qdelaunay Qt i') or joggled input ('qdelaunay QJ i')
	to generate the Delaunay triangulation.
	Option 'i' reports each tetrahedron. The triangles are
	every combination of 3 vertices. Each triangle is a
	"ridge" of the Delaunay triangulation. </p>

	<p>For example, </p>

	<pre>
	rbox 10 \| qdelaunay Qt i
	14
	9 5 8 7
	0 9 8 7
	5 3 8 7
	3 0 8 7
	5 4 8 1
	4 6 8 1
	2 9 5 8
	4 2 5 8
	4 2 9 5
	6 2 4 8
	9 2 0 8
	2 6 0 8
	2 4 9 1
	2 6 4 1
	</pre>

	<p>is the Delaunay triangulation of 10 random points. Ridge 9-5-8
	occurs twice. Once for tetrahedron 9 5 8 7 and the other for
	tetrahedron 2 9 5 8. </p>

	<p>You can also use the Qhull library to generate the triangles.
	See "<a href="#ridges2">How</a> do I visit the ridges of a
	Delaunay triangulation?"</p>

	</blockquote><h4><a href="#TOC">»</a><a name="3dd">How</a> do I construct a
	3-d Delaunay triangulation?</h4><blockquote>

	<p>For 3-d Delaunay triangulations with cospherical input sites,
	use triangulated output ('<a href="qh-optq.htm#Qt">Qt</a>') or
	joggled input ('<a href="qh-optq.htm#QJn">QJ</a>'). Otherwise
	option 'i' will
	triangulate non-simplicial facets by adding a point to the facet.

	<p>If you want non-simplicial output for cospherical sites, use
	option
	'<a href="qh-optf.htm#Fv">Fv</a>' or '<a href="qh-opto.htm#o">o</a>'.
	For option 'o', ignore the last coordinate. It is the lifted
	coordinate for the corresponding convex hull in 4-d.

	<p>The following example is a cube
	inside a tetrahedron. The 8-vertex facet is the cube. Ignore the
	last coordinates. </p>

	<blockquote>
	<pre>
	C:\qhull>rbox r y c G0.1 \| qdelaunay Fv
	4
	12 20 44
	0.5 0 0 0.3055555555555555
	0 0.5 0 0.3055555555555555
	0 0 0.5 0.3055555555555555
	-0.5 -0.5 -0.5 0.9999999999999999
	-0.1 -0.1 -0.1 -6.938893903907228e-018
	-0.1 -0.1 0.1 -6.938893903907228e-018
	-0.1 0.1 -0.1 -6.938893903907228e-018
	-0.1 0.1 0.1 -6.938893903907228e-018
	0.1 -0.1 -0.1 -6.938893903907228e-018
	0.1 -0.1 0.1 -6.938893903907228e-018
	0.1 0.1 -0.1 -6.938893903907228e-018
	0.1 0.1 0.1 -6.938893903907228e-018
	4 2 11 1 0
	4 10 1 0 3
	4 11 10 1 0
	4 2 9 0 3
	4 9 11 2 0
	4 7 2 1 3
	4 11 7 2 1
	4 8 10 0 3
	4 9 8 0 3
	5 8 9 10 11 0
	4 10 6 1 3
	4 6 7 1 3
	5 6 8 10 4 3
	5 6 7 10 11 1
	4 5 9 2 3
	4 7 5 2 3
	5 5 8 9 4 3
	5 5 6 7 4 3
	8 5 6 8 7 9 10 11 4
	5 5 7 9 11 2
	</pre>
	</blockquote>

	<p>If you want simplicial output use options
	'<a href="qh-optq.htm#Qt">Qt</a> <A
	href="qh-optf.htm#Ft" >i</a>' or
	'<a href="qh-optq.htm#QJn">QJ</a> <A
	href="qh-optf.htm#Ft" >i</a>', e.g.,
	</p>

	<blockquote>
	<pre>
	rbox r y c G0.1 \| qdelaunay Qt i
	31
	2 11 1 0
	11 10 1 0
	9 11 2 0
	11 7 2 1
	8 10 0 3
	9 8 0 3
	10 6 1 3
	6 7 1 3
	5 9 2 3
	7 5 2 3
	9 8 10 11
	8 10 11 0
	9 8 11 0
	6 8 10 4
	8 6 10 3
	6 8 4 3
	6 7 10 11
	10 6 11 1
	6 7 11 1
	8 5 4 3
	5 8 9 3
	5 6 4 3
	6 5 7 3
	5 9 10 11
	8 5 9 10
	7 5 10 11
	5 6 7 10
	8 5 10 4
	5 6 10 4
	5 9 11 2
	7 5 11 2
	</pre>
	</blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="2d">How</a> do I get the
	triangles for a 2-d Delaunay triangulation and the vertices of
	its Voronoi diagram?</h4><blockquote>

	<p>To compute the Delaunay triangles indexed by the indices of
	the input sites, use </p>

	<blockquote>
	<p>rbox 10 D2 \| qdelaunay Qt i </p>
	</blockquote>

	<p>To compute the Voronoi vertices and the Voronoi region for
	each input site, use </p>

	<blockquote>
	<p>rbox 10 D2 \| qvoronoi o </p>
	</blockquote>

	<p>To compute each edge ("ridge") of the Voronoi
	diagram for each pair of adjacent input sites, use</p>

	<blockquote>
	<p>rbox 10 D2 \| qvoronoi Fv </p>
	</blockquote>

	<p>To compute the area and volume of the Voronoi region for input site 5 (site 0 is the first one),
	use </p>

	<blockquote>
	<p>rbox 10 D2 \| qvoronoi QV5 p \| qconvex s FS </p>
	</blockquote>

	<p>To compute the lines ("hyperplanes") that define the
	Voronoi region for input site 5, use </p>

	<blockquote>
	<p>rbox 10 D2 \| qvoronoi QV5 p \| qconvex n </p>
	</blockquote>
	or
	<blockquote>
	<p>rbox 10 D2 \| qvoronoi QV5 Fi Fo</p>
	</blockquote>

	<p>To list the extreme points of the input sites use </p>

	<blockquote>
	<p>rbox 10 D2 \| qdelaunay Fx </p>
	</blockquote>

	<p>You will get the same point ids with </p>

	<blockquote>
	<p>rbox 10 D2 \| qconvex Fx </p>
	</blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="big">Can </a>Qhull triangulate
	a hundred 16-d points?</h4><blockquote>

	<p>No. This is an immense structure. A triangulation of 19, 16-d
	points has 43 simplices. If you add one point at a time, the
	triangulation increased as follows: 43, 189, 523, 1289, 2830,
	6071, 11410, 20487. The last triangulation for 26 points used 13
	megabytes of memory. When Qhull uses virtual memory, it becomes
	too slow to use. </p>

	</blockquote>
	<h2><a href="#TOC">»</a><a name="voronoi">Voronoi
	diagram questions</a></h2>

	<h4><a href="#TOC">»</a><a name="volume">How</a> do I compute the volume of a Voronoi region?</h4><blockquote>

	<p>For each Voronoi region, compute the convex hull of the region's Voronoi vertices. The volume of each convex hull is the volume
	of the corresponding Vornoi region.</p>

	<p>For example, to compute the volume of the bounded Voronoi region about [0,0,0]: output the origin's Voronoi vertices and
	compute the volume of their convex hull. The last number from option '<a href="qh-optf.htm#FS">FS</a>' is the volume.</p>
	<blockquote><pre>
	rbox P0 10 \| qvoronoi QV0 p \| qhull FS
	0
	2 1.448134756744281 0.1067973560800857
	</pre></blockquote>

	<p>For another example, see <a href="#2d">How</a> do I get the triangles for a 2-d Delaunay
	triangulation and the vertices of its Voronoi diagram?</p>

	<p>This approach is slow if you are using the command line. A faster approcach is to call Qhull from
	a program. The fastest method is Clarkson's <a href="http://www.netlib.org/voronoi/hull.html">hull</a> program.
	It computes the volume for all Voronoi regions.</p>

	<p>An unbounded Voronoi region does not have a volume.</p>

	</blockquote><h4><a href="#TOC">»</a><a name="maxsphere">How</a> do I get the radii of the empty
	spheres for each Voronoi vertex?</h4><blockquote>

	Use option '<a href="qh-optf.htm#Fi">Fi</a>' to list each bisector (i.e. Delaunay ridge). Then compute the
	minimum distance for each Voronoi vertex.

	<p>There's other ways to get the same information. Let me know if you
	find a better method.

	</blockquote><h4><a href="#TOC">»</a><a name="square">What</a> is the Voronoi diagram
	of a square?</h4><blockquote>

	<p>
	Consider a square,
	<blockquote><pre>
	C:\qhull>rbox c D2
	2 RBOX c D2
	4
	-0.5 -0.5
	-0.5 0.5
	0.5 -0.5
	0.5 0.5
	</pre></blockquote>

	<p>There's two ways to compute the Voronoi diagram: with facet merging
	or with joggle. With facet merging, the
	result is:

	<blockquote><pre>
	C:\qhull>rbox c D2 \| qvoronoi Qz

	Voronoi diagram by the convex hull of 5 points in 3-d:

	Number of Voronoi regions and at-infinity: 5
	Number of Voronoi vertices: 1
	Number of facets in hull: 5

	Statistics for: RBOX c D2 \| QVORONOI Qz

	Number of points processed: 5
	Number of hyperplanes created: 7
	Number of distance tests for qhull: 8
	Number of merged facets: 1
	Number of distance tests for merging: 29
	CPU seconds to compute hull (after input): 0

	C:\qhull>rbox c D2 \| qvoronoi Qz o
	2
	2 5 1
	-10.101 -10.101
	0 0
	2 0 1
	2 0 1
	2 0 1
	2 0 1
	0

	C:\qhull>rbox c D2 \| qvoronoi Qz Fv
	4
	4 0 1 0 1
	4 0 2 0 1
	4 1 3 0 1
	4 2 3 0 1
	</pre></blockquote>

	<p>There is one Voronoi vertex at the origin and rays from the origin
	along each of the coordinate axes.
	The last line '4 2 3 0 1' means that there is
	a ray that bisects input points #2 and #3 from infinity (vertex 0) to
	the origin (vertex 1).
	Option 'Qz' adds an artificial point since the input is cocircular.
	Coordinates -10.101 indicate the
	vertex at infinity.

	<p>With triangulated output, the Voronoi vertex is
	duplicated:

	<blockquote><pre>
	C:\qhull3.1>rbox c D2 \| qvoronoi Qt Qz

	Voronoi diagram by the convex hull of 5 points in 3-d:

	Number of Voronoi regions and at-infinity: 5
	Number of Voronoi vertices: 2
	Number of triangulated facets: 1

	Statistics for: RBOX c D2 \| QVORONOI Qt Qz

	Number of points processed: 5
	Number of hyperplanes created: 7
	Number of facets in hull: 6
	Number of distance tests for qhull: 8
	Number of distance tests for merging: 33
	Number of distance tests for checking: 30
	Number of merged facets: 1
	CPU seconds to compute hull (after input): 0.05

	C:\qhull3.1>rbox c D2 \| qvoronoi Qt Qz o
	2
	3 5 1
	-10.101 -10.101
	0 0
	0 0
	3 2 0 1
	2 1 0
	2 2 0
	3 2 0 1
	0

	C:\qhull3.1>rbox c D2 \| qvoronoi Qt Qz Fv
	4
	4 0 2 0 2
	4 0 1 0 1
	4 1 3 0 1
	4 2 3 0 2
	</pre></blockquote>


	<p>With joggle, the input is no longer cocircular and the Voronoi vertex is
	split into two:

	<blockquote><pre>
	C:\qhull>rbox c D2 \| qvoronoi Qt Qz

	C:\qhull>rbox c D2 \| qvoronoi QJ o
	2
	3 4 1
	-10.101 -10.101
	-4.71511718558304e-012 -1.775812830118184e-011
	9.020340030474472e-012 -4.02267108512433e-012
	2 0 1
	3 2 1 0
	3 2 0 1
	2 2 0

	C:\qhull>rbox c D2 \| qvoronoi QJ Fv
	5
	4 0 2 0 1
	4 0 1 0 1
	4 1 2 1 2
	4 1 3 0 2
	4 2 3 0 2
	</pre></blockquote>

	<p>Note that the Voronoi diagram includes the same rays as
	before plus a short edge between the two vertices.</p>


	</blockquote><h4><a href="#TOC">»</a><a name="vsphere">How</a> do I construct
	the Voronoi diagram of cospherical points?</h4><blockquote>

	<p>Three-d terrain data can be approximated with cospherical
	points. The Delaunay triangulation of cospherical points is the
	same as their convex hull. If the points lie on the unit sphere,
	the facet normals are the Voronoi vertices [via S. Fortune]. </p>

	<p>For example, consider the points {[1,0,0], [-1,0,0], [0,1,0],
	...}. Their convex hull is: </p>

	<pre>
	rbox d G1 \| qconvex o
	3
	6 8 12
	0 0 -1
	0 0 1
	0 -1 0
	0 1 0
	-1 0 0
	1 0 0
	3 3 1 4
	3 1 3 5
	3 0 3 4
	3 3 0 5
	3 2 1 5
	3 1 2 4
	3 2 0 4
	3 0 2 5
	</pre>

	<p>The facet normals are: </p>

	<pre>
	rbox d G1 \| qconvex n
	4
	8
	-0.5773502691896258 0.5773502691896258 0.5773502691896258 -0.5773502691896258
	0.5773502691896258 0.5773502691896258 0.5773502691896258 -0.5773502691896258
	-0.5773502691896258 0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	0.5773502691896258 0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	0.5773502691896258 -0.5773502691896258 0.5773502691896258 -0.5773502691896258
	-0.5773502691896258 -0.5773502691896258 0.5773502691896258 -0.5773502691896258
	-0.5773502691896258 -0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	0.5773502691896258 -0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	</pre>

	<p>If you drop the offset from each line (the last number), each
	line is the Voronoi vertex for the corresponding facet. The
	neighboring facets for each point define the Voronoi region for
	each point. For example: </p>

	<pre>
	rbox d G1 \| qconvex FN
	6
	4 7 3 2 6
	4 5 0 1 4
	4 7 4 5 6
	4 3 1 0 2
	4 6 2 0 5
	4 7 3 1 4
	</pre>

	<p>The Voronoi vertices {7, 3, 2, 6} define the Voronoi region
	for point 0. Point 0 is [0,0,-1]. Its Voronoi vertices are </p>

	<pre>
	-0.5773502691896258 0.5773502691896258 -0.5773502691896258
	0.5773502691896258 0.5773502691896258 -0.5773502691896258
	-0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	0.5773502691896258 -0.5773502691896258 -0.5773502691896258
	</pre>

	<p>In this case, the Voronoi vertices are oriented, but in
	general they are unordered. </p>

	<p>By taking the dual of the Delaunay triangulation, you can
	construct the Voronoi diagram. For cospherical points, the convex
	hull vertices for each facet, define the input sites for each
	Voronoi vertex. In 3-d, the input sites are oriented. For
	example: </p>

	<pre>
	rbox d G1 \| qconvex i
	8
	3 1 4
	1 3 5
	0 3 4
	3 0 5
	2 1 5
	1 2 4
	2 0 4
	0 2 5
	</pre>

	<p>The convex hull vertices for facet 0 are {3, 1, 4}. So Voronoi
	vertex 0 (i.e., [-0.577, 0.577, 0.577]) is the Voronoi vertex for
	input sites {3, 1, 4} (i.e., {[0,1,0], [0,0,1], [-1,0,0]}). </p>

	</blockquote><h4><a href="#TOC">»</a><a name="rays">Can</a> Qhull compute the
	unbounded rays of the Voronoi diagram?</h4><blockquote>

	<p>Use '<a href="qh-optf.htm#Fo2">Fo</a>' to compute the separating
	hyperplanes for unbounded Voronoi regions. The corresponding ray
	goes to infinity from the Voronoi vertices. If you enclose the
	input sites in a large enough box, the outermost bounded regions
	will represent the unbounded regions of the original points. </p>

	<p>If you do not box the input sites, you can identify the
	unbounded regions. They list '0' as a vertex. Vertex 0 represents
	"infinity". Each unbounded ray includes vertex 0 in
	option '<a href="qh-optf.htm#Fv2">Fv</a>. See <A
	href="qvoronoi.htm#graphics" >Voronoi graphics</a> and <A
	href="qvoronoi.htm#notes" >Voronoi notes</a>. </p>

	</blockquote>
	<h2><a href="#TOC">»</a>Approximation questions</h2>

	<h4><a href="#TOC">»</a><a name="simplex">How</a> do I
	approximate data with a simplex</h4><blockquote>

	<p>Qhull may be used to help select a simplex that approximates a
	data set. It will take experimentation. Geomview will help to
	visualize the results. This task may be difficult to do in 5-d
	and higher. Use rbox options 'x' and 'y' to produce random
	distributions within a simplex. Your methods work if you can
	recover the simplex. </p>

	<p>Use Qhull's precision options to get a first approximation to
	the hull, say with 10 to 50 facets. For example, try 'C0.05' to
	remove small facets after constructing the hull. Use 'W0.05' to
	ignore points within 0.05 of a facet. Use 'PA5' to print the five
	largest facets by area. </p>

	<p>Then use other methods to fit a simplex to this data. Remove
	outlying vertices with few nearby points. Look for large facets
	in different quadrants. You can use option 'Pd0d1d2' to print all
	the facets in a quadrant. </p>

	<p>In 4-d and higher, use the outer planes (option 'Fo' or
	'facet->maxoutside') since the hyperplane of an approximate
	facet may be below many of the input points. </p>

	<p>For example, consider fitting a cube to 1000 uniformly random
	points in the unit cube. In this case, the first try was good: </p>

	<blockquote>
	<pre>
	rbox 1000 \| qconvex W0.05 C0.05 PA6 Fo
	4
	6
	0.35715408374381 0.08706467018177928 -0.9299788727015564 -0.5985514741284483
	0.995841591359023 -0.02512604712761577 0.08756829720435189 -0.5258834069202866
	0.02448099521570909 -0.02685210459017302 0.9993396046151313 -0.5158104982631999
	-0.9990223929415094 -0.01261133513150079 0.04236994958247349 -0.509218270408407
	-0.0128069014364698 -0.9998380680115362 0.01264203427283151 -0.5002512653670584
	0.01120895057872914 0.01803671994177704 -0.9997744926535512 -0.5056824072956361
	</pre>
	</blockquote>

	</blockquote>
	<h2><a href="#TOC">»</a>Halfspace questions</h2>

	<h4><a href="#TOC">»</a><a name="halfspace">How</a> do I compute the
	intersection of halfspaces with Qhull?</h4><blockquote>

	<p>Qhull computes the halfspace intersection about a point. The
	point must be inside all of the halfspaces. Given a point, a
	duality turns a halfspace intersection problem into a convex
	hull problem.

	<p>Use linear programming if you
	do not know a point in the interior of the halfspaces.
	See the <a href="qhalf.htm#notes">notes</a> for qhalf. You will need
	a linear programming code. This may require a fair amount of work to
	implement.</p>



	</blockquote>
	<h2><a href="#TOC">»</a><a name="library">Qhull library
	questions</a></h2>

	<h4><a href="#TOC">»</a><a name="math">Is</a> Qhull available for Mathematica, Matlab, or Maple?</h4><blockquote>

	<p><b>MATLAB</b>

	<p>Z. You of <a href="http://www.mathworks.com">MathWorks</a> added qhull to MATLAB 6.
	See functions <a href="http://www.mathworks.com/help/techdoc/ref/convhulln.html"
	>convhulln</a>,
	<a href="http://www.mathworks.com/help/techdoc/ref/delaunayn.html"
	>delaunayn</a>,
	<a href="http://www.mathworks.com/help/techdoc/ref/griddata3.html"
	>griddata3</a>,
	<a href="http://www.mathworks.com/help/techdoc/ref/griddatan.html"
	>griddatan</a>,
	<a href="http://www.mathworks.com/help/techdoc/ref/tsearch.html"
	>tsearch</a>,
	<a href="http://www.mathworks.com/help/techdoc/ref/tsearchn.html"
	>tsearchn</a>, and
	<a href="http://www.mathworks.com/help/techdoc/ref/voronoin.html"
	>voronoin</a>. V. Brumberg update MATLAB R14 for Qhull 2003.1 and triangulated output.

	<p>Engwirda wrote <a href="http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do?objectId=10307&objectType=file">mesh2d</a> for unstructured mesh generation in MATLAB.
	It is based on the iterative method of Persson and generally results in better quality meshes than delaunay refinement.


	<p><b>Mathematica and Maple</b>

	<p>See <a href="http://library.wolfram.com/infocenter/MathSource/1160/"
	>qh-math</a>
	for a Delaunay interface to Mathematica. It includes projects for CodeWarrior
	on the Macintosh and Visual C++ on Win32 PCs.

	<p>See Mathematica ('<a
	href="qh-opto.htm#m">m</a>') and Maple ('<a
	href="qh-optf.htm#FM">FM</a>') output options.

	<p></p>
	</blockquote><h4><a href="#TOC">»</a><a name="ridges">Why</a> are there too few ridges?</h4><blockquote>

	The following sample code may produce fewer ridges than expected:

	<blockquote><pre>
	facetT *facetp;
	ridgeT ridge, *ridgep;

	FORALLfacets {
	printf("facet f%d\n", facet->id);
	FOREACHridge_(facet->ridges) {
	printf(" ridge r%d between f%d and f%d\n", ridge->id, ridge->top->id, ridge->bottom->id);
	}
	}
	</pre></blockquote>

	<p> Qhull does not create ridges for simplicial facets.
	Instead it computes ridges from facet->neighbors. To make ridges for a
	simplicial facet, use qh_makeridges() in merge.c. Usefacet->visit_id to visit
	each ridge once (instead of twice). For example,

	<blockquote><pre>
	facetT facet, neighbor;
	ridgeT ridge, *ridgep;

	qh visit_id++;
	FORALLfacets {
	printf("facet f%d\n", facet->id);
	qh_makeridges(facet);
	facet->visitId= qh visit_id;
	FOREACHridge_(facet->ridges) {
	neighbor= otherfacet_(ridge, visible);
	if (neighbor->visitid != qh visit_id)
	printf(" ridge r%d between f%d and f%d\n", ridge->id, ridge->top->id, ridge->bottom->id);
	}
	}
	</pre></blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="call">Can</a> Qhull use coordinates without placing
	them in a data file?</h4><blockquote>

	<p>You may call Qhull from a program. Please use the reentrant Qhull library (libqhullstatic_r.a, libqhull_r.so, or qhull_r.dll).

	See user_eg.c and "Qhull-template" in user_r.c for examples..

	See <a href="qh-code.htm">Qhull internals</a> for an introduction to Qhull's reentrant library and its C++ interface.

	<p>Hint: Start with a small example for which you know the
	answer.</p>

	</blockquote><h4><a href="#TOC">»</a><a name="size">How</a> large are Qhull's data structures?</h4><blockquote>

	<p>Qhull uses a general-dimension data structure.
	The size depends on the dimension. Use option 'Ts' to print
	out the memory statistics [e.g., 'rbox D2 10 \| qconvex Ts'].


	</blockquote><h4><a href="#TOC">»</a><a name="inc">Can</a> Qhull construct
	convex hulls and Delaunay triangulations one point at a time?</h4><blockquote>

	<p>The Qhull library may be used to construct convex hulls and
	Delaunay triangulations one point at a time. It may not be used
	for deleting points or moving points. </p>

	<p>Qhull is designed for batch processing. Neither Clarkson's
	randomized incremental algorithm nor Qhull are designed for
	on-line operation. For many applications, it is better to
	reconstruct the convex hull or Delaunay triangulation from
	scratch for each new point. </p>

	<p>With random point sets and on-line processing, Clarkson's
	algorithm should run faster than Qhull. Clarkson uses the
	intermediate facets to reject new, interior points, while Qhull,
	when used on-line, visits every facet to reject such points. If
	used on-line for n points, Clarkson may take O(n) times as much
	memory as the average off-line case, while Qhull's space
	requirement does not change. </p>

	<p>If you triangulate the output before adding all the points
	(option 'Qt' and procedure qh_triangulate), you must set
	option '<a href="qh-optq.htm#Q11">Q11</a>'. It duplicates the
	normals of triangulated facets and recomputes the centrums.
	This should be avoided for regular use since triangulated facets
	are not clearly convex with their neighbors. It appears to
	work most of the time, but fails for cases that Qhull normally
	handles well [see the test call to qh_triangulate in qh_addpoint].

	</blockquote><h4><a href="#TOC">»</a><a name="ridges2">How</a> do I visit the
	ridges of a Delaunay triangulation?</h4><blockquote>

	<p>To visit the ridges of a Delaunay triangulation, visit each
	facet. Each ridge will appear twice since it belongs to two
	facets. In pseudo-code: </p>

	<pre>
	for each facet of the triangulation
	if the facet is Delaunay (i.e., part of the lower convex hull)
	for each ridge of the facet
	if the ridge's neighboring facet has not been visited
	... process a ridge of the Delaunay triangulation ...
	</pre>

	<p>In undebugged, C code: </p>

	<pre>
	qh visit_id++;
	FORALLfacets_(facetlist)
	if (!facet->upperdelaunay) {
	facet->visitid= qh visit_id;
	qh_makeridges(facet);
	FOREACHridge_(facet->ridges) {
	neighbor= otherfacet_(ridge, facet);
	if (neighbor->visitid != qh visit_id) {
	/* Print ridge here with facet-id and neighbor-id */
	/fprintf(fp, "f%d\tf%d\t",facet->id,neighbor->ID);/
	FOREACHvertex_(ridge->vertices)
	fprintf(fp,"%d ",qh_pointid (vertex->point) );
	qh_printfacetNvertex_simplicial (fp, facet, format);
	fprintf(fp," ");
	if(neighbor->upperdelaunay)
	fprintf(fp," -1 -1 -1 -1 ");
	else
	qh_printfacetNvertex_simplicial (fp, neighbor, format);
	fprintf(fp,"\n");
	}
	}
	}
	}
	</pre>

	<p>Qhull should be redesigned as a class library, or at least as
	an API. It currently provides everything needed, but the
	programmer has to do a lot of work. Hopefully someone will write
	C++ wrapper classes or a Python module for Qhull. </p>

	</blockquote><h4><a href="#TOC">»</a><a name="listd">How</a> do I visit the
	Delaunay regions?</h4><blockquote>

	<p>Qhull constructs a Delaunay triangulation by lifting the
	input sites to a paraboloid. The Delaunay triangulation
	corresponds to the lower convex hull of the lifted points. To
	visit each facet of the lower convex hull, use: </p>

	<pre>
	facetT *facet;

	...
	FORALLfacets {
	if (!facet->upperdelaunay) {
	... only facets for Delaunay regions ...
	}
	}
	</pre>

	</blockquote><h4><a href="#TOC">»</a><a name="outside">When</a> is a point
	outside or inside a facet?</h4><blockquote>

	<p>A point is outside of a facet if it is clearly outside the
	facet's outer plane. The outer plane is defined by an offset
	(facet->maxoutside) from the facet's hyperplane. </p>

	<pre>
	facetT *facet;
	pointT *point;
	realT dist;

	...
	qh_distplane(point, facet, &dist);
	if (dist > facet->maxoutside + 2 * qh DISTround) {
	/* point is clearly outside of facet */
	}
	</pre>

	<p>A point is inside of a facet if it is clearly inside the
	facet's inner plane. The inner plane is computed as the maximum
	distance of a vertex to the facet. It may be computed for an
	individual facet, or you may use the maximum over all facets. For
	example: </p>

	<pre>
	facetT *facet;
	pointT *point;
	realT dist;

	...
	qh_distplane(point, facet, &dist);
	if (dist < qh min_vertex - 2 * qh DISTround) {
	/* point is clearly inside of facet */
	}
	</pre>

	<p>Both tests include two qh.DISTrounds because the computation
	of the furthest point from a facet may be off by qh.DISTround and
	the computation of the current distance to the facet may be off
	by qh.DISTround. </p>

	</blockquote><h4><a href="#TOC">»</a><a name="closest">How</a> do I find the
	facet that is closest to a point?</h4><blockquote>

	<p>Use qh_findbestfacet(). For example, </p>

	<pre>
	coordT point[ DIM ];
	boolT isoutside;
	realT bestdist;
	facetT *facet;

	... set coordinates for point ...

	facet= qh_findbestfacet (point, qh_ALL, &bestdist, &isoutside);

	/* 'facet' is the closest facet to 'point' */
	</pre>

	<p>qh_findbestfacet() performs a directed search for the facet
	furthest below the point. If the point lies inside this facet,
	qh_findbestfacet() performs an exhaustive search of all facets.
	An exhaustive search may be needed because a facet on the far
	side of a lens-shaped distribution may be closer to a point than
	all of the facet's neighbors. The exhaustive search may be
	skipped for spherical distributions. </p>

	<p>Also see, "<a href="#vclosest">How</a> do I find the
	Delaunay triangle that is closest to a
	point?" </p>

	</blockquote><h4><a href="#TOC">»</a><a name="vclosest">How</a> do I find the
	Delaunay triangle or Voronoi region that is closest to a point?</h4><blockquote>

	<p>A Delaunay triangulation subdivides the plane, or in general
	dimension, subdivides space. Given a point, how do you determine
	the subdivision containing the point? Or, given a set of points,
	how do you determine the subdivision containing each point of the set?
	Efficiency is important -- an exhaustive search of the subdivision
	is too slow.

	<p>First compute the Delaunay triangle with qh_new_qhull() in user_r.c or Qhull::runQhull().
	Lift the point to the paraboloid by summing the squares of the
	coordinates. Use qh_findbestfacet() [poly2.c] to find the closest Delaunay
	triangle. Determine the closest vertex to find the corresponding
	Voronoi region. Do not use options
	'<a href="qh-optq.htm#Qbb">Qbb</a>', '<a href="qh-optq.htm#QbB">QbB</a>',
	'<a href="qh-optq.htm#Qbk">Qbk:n</a>', or '<A
	href="qh-optq.htm#QBk" >QBk:n</a>' since these scale the last
	coordinate. Optimizations of qh_findbestfacet() should
	be possible for Delaunay triangulations.</p>

	<p>You first need to lift the point to the paraboloid (i.e., the
	last coordinate is the sum of the squares of the point's coordinates).
	The
	routine, qh_setdelaunay() [geom2.c], lifts an array of points to the
	paraboloid. The following excerpt is from findclosest() in
	user_eg.c. </p>

	<pre>
	coordT point[ DIM + 1]; /* one extra coordinate for lifting the point */
	boolT isoutside;
	realT bestdist;
	facetT *facet;

	... set coordinates for point[] ...

	qh_setdelaunay (DIM+1, 1, point);
	facet= qh_findbestfacet (point, qh_ALL, &bestdist, &isoutside);
	/* 'facet' is the closest Delaunay triangle to 'point' */
	</pre>

	<p>The returned facet either contains the point or it is the
	closest Delaunay triangle along the convex hull of the input set.

	<p>Point location is an active research area in Computational
	Geometry. For a practical approach, see Mucke, et al, "Fast randomized
	point location without preprocessing in two- and
	three-dimensional Delaunay triangulations," <i>Computational
	Geometry '96</i>, p. 274-283, May 1996.
	For an introduction to planar point location see [O'Rourke '93].
	Also see, "<A
	href="#closest" >How</a> do I find the facet that is closest to a
	point?" </p>

	<p>To locate the closest Voronoi region, determine the closest
	vertex of the closest Delaunay triangle. </p>

	<pre>
	realT dist, bestdist= REALmax;
	vertexT bestvertex= NULL, vertex, **vertexp;

	/* 'facet' is the closest Delaunay triangle to 'point' */

	FOREACHvertex_( facet->vertices ) {
	dist= qh_pointdist( point, vertex->point, DIM );
	if (dist < bestdist) {
	bestdist= dist;
	bestvertex= vertex;
	}
	}
	/* 'bestvertex' represents the Voronoi region closest to 'point'. The corresponding
	input site is 'bestvertex->point' */
	</pre>

	</blockquote><h4><a href="#TOC">»</a><a name="vertices">How</a> do I list the
	vertices?</h4><blockquote>

	<p>To list the vertices (i.e., extreme points) of the convex hull
	use </p>

	<blockquote>
	<pre>
	vertexT *vertex;

	FORALLvertices {
	...
	// vertex->point is the coordinates of the vertex
	// qh_pointid(vertex->point) is the point ID of the vertex
	...
	}
	</pre>
	</blockquote>

	</blockquote><h4><a href="#TOC">»</a><a name="test">How</a> do I test code
	that uses the Qhull library?</h4><blockquote>

	<p>Compare the output from your program with the output from the
	Qhull program. Use option 'T1' or 'T4' to trace what Qhull is
	doing. Prepare a <i>small</i> example for which you know the
	output. Run the example through the Qhull program and your code.
	Compare the trace outputs. If you do everything right, the two
	trace outputs should be almost the same. The trace output will
	also guide you to the functions that you need to review. </p>

	</blockquote><h4><a href="#TOC">»</a><a name="orient">When</a> I compute a
	plane equation from a facet, I sometimes get an outward-pointing
	normal and sometimes an inward-pointing normal</h4><blockquote>

	<p>Qhull orients simplicial facets, and prints oriented output
	for 'i', 'Ft', and other options. The orientation depends on <i>both</i>
	the vertex order and the flag facet->toporient.</p>

	<p>Qhull does not orient
	non-simplicial facets. Instead it orients the facet's ridges. These are
	printed with the 'Qt' and 'Ft' option. The facet's hyperplane is oriented. </p>

	</blockquote>
	<hr><!-- Navigation links -->

	<p><a><b>Up:</b> </a><a
	href="http://www.qhull.org">Home page for Qhull</a><br>
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	<p><a href="http://www.geom.uiuc.edu/"><IMG align=middle
	height=40 src="qh--geom.gif" width=40 ></a><i>The Geometry Center
	Home Page </i></p>

	<p>Comments to: <a href=mailto:qhull@qhull.org>qhull@qhull.org</a>
	</a><br>
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