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<title>qhalf -- halfspace intersection about a point</title>
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<h1><a
href="http://www.geom.uiuc.edu/graphics/pix/Special_Topics/Computational_Geometry/half.html"><img
src="qh--half.gif" alt="[halfspace]" align="middle" width="100"
height="100"></a>qhalf -- halfspace intersection about a point</h1>
<p>The intersection of a set of halfspaces is a polytope. The
polytope may be unbounded. See Preparata &amp; Shamos [<a
href="index.htm#pre-sha85">'85</a>] for a discussion. In low
dimensions, halfspace intersection may be used for linear
programming. </p>
<blockquote>
<dl compact>
<dt><b>Example:</b> rbox c | qconvex <a href="qh-optf.htm#FQ">FQ</a> <a href="qh-optf.htm#FV">FV</a>
<a href="qh-opto.htm#n">n</a> | qhalf <a
href="qh-optf.htm#Fp">Fp</a></dt>
<dd>Print the intersection of the facets of a cube. <tt>rbox c</tt>
generates the vertices of a cube. <tt>qconvex FV n</tt> returns of average
of the cube's vertices (in this case, the origin) and the halfspaces
that define the cube. <tt>qhalf Fp</tt> computes the intersection of
the halfspaces about the origin. The intersection is the vertices
of the original cube.</dd>
<dt><p><b>Example:</b> rbox c d G0.55 | qconvex <a href="qh-optf.htm#FQ">FQ</a> <a href="qh-optf.htm#FV">FV</a>
<a href="qh-opto.htm#n">n</a> | qhalf <a
href="qh-optf.htm#Fp">Fp</a></dt>
<dd>Print the intersection of the facets of a cube and a diamond. There
are 24 facets and 14 intersection points. Four facets define each diamond
vertex. Six facets define each cube vertex.
</dd>
<dt><p><b>Example:</b> rbox c d G0.55 | qconvex <a href="qh-optf.htm#FQ">FQ</a> <a href="qh-optf.htm#FV">FV</a>
<a href="qh-opto.htm#n">n</a> | qhalf <a
href="qh-optf.htm#Fp">Fp</a>
<a href="qh-optq.htm#Qt">Qt</a></dt>
<dd>Same as above except triangulate before computing
the intersection points. Three facets define each intersection
point. There are two duplicates of the diamond and four duplicates of the cube.
</dd>
</dl>
</blockquote>
<p>Qhull computes a halfspace intersection by the geometric
duality between points and halfspaces.
See <a href="qh-eg.htm#half">halfspace examples</a>,
<a href="#notes">qhalf notes</a>, and
option 'p' of <a href="#outputs">qhalf outputs</a>. </p>
<p>By default, halfspace intersections may be defined by more than
<i>d</i> halfspaces. See the previous cube and diamond example.
This is the expected output for halfspace intersection.
<p>You can try triangulated output and joggled input. It demonstrates
that triangulated output is more accurate than joggled input.
<p>If you use '<a href="qh-optq.htm#Qt">Qt</a>' (triangulated output), all
halfspace intersections are simplicial (e.g., three halfspaces per
intersection in 3-d). In 3-d, if more than three halfspaces intersect
at the same point, triangulated output will produce
duplicate intersections, one for each additional halfspace. See the previous
cube and diamond example.</p>
<p>If you use '<a href="qh-optq.htm#QJn">QJ</a>' (joggled input), all halfspace
intersections are simplicial. This may lead to nearly identical
intersections. For example, replace 'Qt' with 'QJ' above and
compare the duplicated intersections.
See <a
href="qh-impre.htm#joggle">Merged facets or joggled input</a>. </p>
<p>The 'qhalf' program is equivalent to
'<a href=qhull.htm#outputs>qhull H</a>' in 2-d to 4-d, and
'<a href=qhull.htm#outputs>qhull H</a> <a href=qh-optq.htm#Qx>Qx</a>'
in 5-d and higher. It disables the following Qhull
<a href=qh-quick.htm#options>options</a>: <i>d n v Qbb QbB Qf Qg Qm
Qr QR Qv Qx Qz TR E V Fa FA FC FD FS Ft FV Gt Q0,etc</i>.
<p><b>Copyright &copy; 1995-2012 C.B. Barber</b></p>
<hr>
<h3><a href="#TOP">&#187;</a><a name="synopsis">qhalf synopsis</a></h3>
<pre>
qhalf- halfspace intersection about a point.
input (stdin): [dim, 1, interior point]
dim+1, n
halfspace coefficients + offset
comments start with a non-numeric character
options (qhalf.htm):
Hn,n - specify coordinates of interior point
Qt - triangulated output
QJ - joggle input instead of merging facets
Tv - verify result: structure, convexity, and redundancy
. - concise list of all options
- - one-line description of all options
output options (subset):
s - summary of results (default)
Fp - intersection coordinates
Fv - non-redundant halfspaces incident to each intersection
Fx - non-redundant halfspaces
o - OFF file format (dual convex hull)
G - Geomview output (dual convex hull)
m - Mathematica output (dual convex hull)
QVn - print intersections for halfspace n, -n if not
TO file - output results to file, may be enclosed in single quotes
examples:
rbox d | qconvex n | qhalf s H0,0,0 Fp
rbox c | qconvex FV n | qhalf s i
rbox c | qconvex FV n | qhalf s o
</pre>
<h3><a href="#TOP">&#187;</a><a name="input">qhalf input</a></h3>
<blockquote>
<p>The input data on <tt>stdin</tt> consists of:</p>
<ul>
<li>[optional] interior point
<ul>
<li>dimension
<li>1
<li>coordinates of interior point
</ul>
<li>dimension + 1
<li>number of halfspaces</li>
<li>halfspace coefficients followed by offset</li>
</ul>
<p>Use I/O redirection (e.g., qhalf &lt; data.txt), a pipe (e.g., rbox c | qconvex FV n | qhalf),
or the '<a href=qh-optt.htm#TI>TI</a>' option (e.g., qhalf TI data.txt).
<p>Qhull needs an interior point to compute the halfspace
intersection. An interior point is clearly inside all of the halfspaces.
The interior point may be in the input. If not, option
'Hn,n' defines the interior point as
[n,n,0,...] where <em>0</em> is the default coordinate (e.g.,
'H0' is the origin). Use linear programming if you do not know
the interior point (see <a href="#notes">halfspace notes</a>),</p>
<p>The input to qhalf is a set of halfspaces. Each halfspace is defined by
<em>d</em> coefficients followed by a signed offset. This defines
a linear inequality. The coefficients define a vector that is
normal to the halfspace. The vector may have any length. If it
has length one, the offset is the distance from the origin to the
halfspace's boundary. The distance from the interior point to each
halfspace is negative.</p>
<p>The halfspace format is the same as Qhull's output options '<a
href="qh-opto.htm#n">n</a>', '<a href="qh-optf.htm#Fo">Fo</a>',
and '<a href="qh-optf.htm#Fi">Fi</a>'. Use option '<a
href="qh-optf.htm#Fd">Fd</a>' to use cdd format for the
halfspaces.</p>
<p>For example, here is the input for computing the intersection
of halfplanes that form a cube.</p>
<blockquote>
<p>rbox c | qconvex FQ FV n TO data </p>
<pre>
RBOX c | QCONVEX FQ FV n
3 1
0 0 0
4
6
0 0 -1 -0.5
0 -1 0 -0.5
1 0 0 -0.5
-1 0 0 -0.5
0 1 0 -0.5
0 0 1 -0.5
</pre>
<p>qhalf s Fp &lt; data </p>
<pre>
Halfspace intersection by the convex hull of 6 points in 3-d:
Number of halfspaces: 6
Number of non-redundant halfspaces: 6
Number of intersection points: 8
Statistics for: RBOX c | QCONVEX FQ FV n | QHALF s Fp
Number of points processed: 6
Number of hyperplanes created: 11
Number of distance tests for qhull: 11
Number of merged facets: 1
Number of distance tests for merging: 45
CPU seconds to compute hull (after input): 0
3
3
8
-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
</pre>
</blockquote>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="outputs">qhalf outputs</a></h3>
<blockquote>
<p>The following options control the output for halfspace
intersection.</p>
<blockquote>
<dl compact>
<dt>&nbsp;</dt>
<dd><b>Intersections</b></dd>
<dt><a href="qh-optf.htm#FN">FN</a></dt>
<dd>list intersection points for each non-redundant
halfspace. The first line
is the number of non-redundant halfspaces. Each remaining
lines starts with the number of intersection points. For the cube
example, each halfspace has four intersection points.</dd>
<dt><a href="qh-optf.htm#Fn">Fn</a></dt>
<dd>list neighboring intersections for each intersection point. The first line
is the number of intersection points. Each remaining line
starts with the number of neighboring intersections. For the cube
example, each intersection point has three neighboring intersections.
In 3-d, a non-simplicial intersection has more than three neighboring
intersections. Use option '<a href="qh-optq.htm#QJn">QJ</a>' to
avoid non-simplicial intersections.
</dd>
<dt><a href="qh-optf.htm#Fp">Fp</a></dt>
<dd>print intersection coordinates. The first line is the dimension and the
second line is the number of intersection points. The following lines are the
coordinates of each intersection.</dd>
<dt><a href="qh-optf.htm#FI">FI</a></dt>
<dd>list intersection IDs. The first line is the number of
intersections. The IDs follow, one per line.</dd>
<dt>&nbsp;</dt>
<dt>&nbsp;</dt>
<dd><b>Halfspaces</b></dd>
<dt><a href="qh-optf.htm#Fx">Fx</a></dt>
<dd>list non-redundant halfspaces. The first line is the number of
non-redundant halfspaces. The other lines list one halfspace per line.
A halfspace is <i>non-redundant</i> if it
defines a facet of the intersection. Redundant halfspaces are ignored. For
the cube example, all of the halfspaces are non-redundant.
</dd>
<dt><a href="qh-optf.htm#Fv">Fv</a></dt>
<dd>list non-redundant halfspaces incident to each intersection point.
The first line is the number of
non-redundant halfspaces. Each remaining line starts with the number
of non-redundant halfspaces. For the
cube example, each intersection is incident to three halfspaces.</dd>
<dt><a href="qh-opto.htm#i">i</a></dt>
<dd>list non-redundant halfspaces incident to each intersection point. The first
line is the number of intersection points. Each remaining line
lists the incident, non-redundant halfspaces. For the
cube example, each intersection is incident to three halfspaces.
</dd>
<dt><a href="qh-optf.htm#Fc">Fc</a></dt>
<dd>list coplanar halfspaces for each intersection point. The first line is
the number of intersection points. Each remaining line starts with
the number of coplanar halfspaces. A coplanar halfspace is listed for
one intersection point even though it is coplanar to multiple intersection
points.</dd>
<dt><a href="qh-optq.htm#Qc">Qi</a> <a href="qh-optf.htm#Fc">Fc</a></dt>
<dd>list redundant halfspaces for each intersection point. The first line is
the number of intersection points. Each remaining line starts with
the number of redundant halfspaces. Use options '<a href="qh-optq.htm#Qc">Qc</a> Qi Fc' to list
coplanar and redundant halfspaces.</dd>
<dt>&nbsp;</dt>
<dt>&nbsp;</dt>
<dd><b>General</b></dd>
<dt><a href="qh-opto.htm#s">s</a></dt>
<dd>print summary for the halfspace intersection. Use '<a
href="qh-optf.htm#Fs">Fs</a>' if you need numeric data.</dd>
<dt><a href="qh-opto.htm#o">o</a></dt>
<dd>print vertices and facets of the dual convex hull. The
first line is the dimension. The second line is the number of
vertices, facets, and ridges. The vertex
coordinates are next, followed by the facets, one per line.</dd>
<dt><a href="qh-opto.htm#p">p</a></dt>
<dd>print vertex coordinates of the dual convex hull. Each vertex corresponds
to a non-redundant halfspace. Its coordinates are the negative of the hyperplane's coefficients
divided by the offset plus the inner product of the coefficients and
the interior point (-c/(b+a.p).
Options 'p <a href="qh-optq.htm#Qc">Qc</a>' includes coplanar halfspaces.
Options 'p <a href="qh-optq.htm#Qi">Qi</a>' includes redundant halfspaces.</dd>
<dt><a href="qh-opto.htm#m">m</a></dt>
<dd>Mathematica output for the dual convex hull in 2-d or 3-d.</dd>
<dt><a href="qh-optf.htm#FM">FM</a></dt>
<dd>Maple output for the dual convex hull in 2-d or 3-d.</dd>
<dt><a href="qh-optg.htm#G">G</a></dt>
<dd>Geomview output for the dual convex hull in 2-d, 3-d, or 4-d.</dd>
</dl>
</blockquote>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="controls">qhalf controls</a></h3>
<blockquote>
<p>These options provide additional control:</p>
<blockquote>
<dl compact>
<dt><a href="qh-optq.htm#Qt">Qt</a></dt>
<dd>triangulated output. If a 3-d intersection is defined by more than
three hyperplanes, Qhull produces duplicate intersections -- one for
each extra hyperplane.</dd>
<dt><a href="qh-optq.htm#QJn">QJ</a></dt>
<dd>joggle the input instead of merging facets. In 3-d, this guarantees that
each intersection is defined by three hyperplanes.</dd>
<dt><a href="qh-opto.htm#f">f </a></dt>
<dd>facet dump. Print the data structure for each intersection (i.e.,
facet)</dd>
<dt><a href="qh-optt.htm#TFn">TFn</a></dt>
<dd>report summary after constructing <em>n</em>
intersections</dd>
<dt><a href="qh-optq.htm#QVn">QVn</a></dt>
<dd>select intersection points for halfspace <em>n</em>
(marked 'good')</dd>
<dt><a href="qh-optq.htm#QGn">QGn</a></dt>
<dd>select intersection points that are visible to halfspace <em>n</em>
(marked 'good'). Use <em>-n</em> for the remainder.</dd>
<dt><a href="qh-optq.htm#Qb0">Qbk:0Bk:0</a></dt>
<dd>remove the k-th coordinate from the input. This computes the
halfspace intersection in one lower dimension.</dd>
<dt><a href="qh-optt.htm#Tv">Tv</a></dt>
<dd>verify result</dd>
<dt><a href="qh-optt.htm#TO">TI file</a></dt>
<dd>input data from file. The filename may not use spaces or quotes.</dd>
<dt><a href="qh-optt.htm#TO">TO file</a></dt>
<dd>output results to file. Use single quotes if the filename
contains spaces (e.g., <tt>TO 'file with spaces.txt'</tt></dd>
<dt><a href="qh-optq.htm#Qs">Qs</a></dt>
<dd>search all points for the initial simplex. If Qhull can
not construct an initial simplex, it reports a
descriptive message. Usually, the point set is degenerate and one
or more dimensions should be removed ('<a href="qh-optq.htm#Qb0">Qbk:0Bk:0</a>').
If not, use option 'Qs'. It performs an exhaustive search for the
best initial simplex. This is expensive is high dimensions.</dd>
</dl>
</blockquote>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="graphics">qhalf graphics</a></h3>
<blockquote>
<p>To view the results with Geomview, compute the convex hull of
the intersection points ('qhull FQ H0 Fp | qhull G'). See <a
href="qh-eg.htm#half">Halfspace examples</a>.</p>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="notes">qhalf notes</a></h3>
<blockquote>
<p>See <a href="qh-impre.htm#halfspace">halfspace intersection</a> for precision issues related to qhalf.</p>
<p>If you do not know an interior point for the halfspaces, use
linear programming to find one. Assume, <em>n</em> halfspaces
defined by: <em>aj*x1+bj*x2+cj*x3+dj&gt;=0, j=1..n</em>. Perform
the following linear program: </p>
<blockquote>
<p>max(x5) aj*x1+bj*x2+cj*x3+dj*x4-x5&gt;=0, j=1..n</p>
</blockquote>
<p>Then, if <em>[x1,x2,x3,x4,x5]</em> is an optimal solution with
<em>x4,x5&gt;0</em> we get: </p>
<blockquote>
<p>aj*(x1/x4)+bj*(x2/x4)+cj*(x3/x4)+dj&gt;=(x5/x4)&gt;0,
j=1..n </p>
</blockquote>
<p>and conclude that the point <em>[x1/x4,x2/x4,x3/x4]</em> is in
the interior of all the halfspaces. Note that <em>x5</em> is
optimal, so this point is &quot;way in&quot; the interior (good
for precision errors).</p>
<p>After finding an interior point, the rest of the intersection
algorithm is from Preparata &amp; Shamos [<a
href="index.htm#pre-sha85">'85</a>, p. 316, &quot;A simple case
...&quot;]. Translate the halfspaces so that the interior point
is the origin. Calculate the dual polytope. The dual polytope is
the convex hull of the vertices dual to the original faces in
regard to the unit sphere (i.e., halfspaces at distance <em>d</em>
from the origin are dual to vertices at distance <em>1/d</em>).
Then calculate the resulting polytope, which is the dual of the
dual polytope, and translate the origin back to the interior
point [S. Spitz and S. Teller].</p>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="conventions">qhalf
conventions</a></h3>
<blockquote>
<p>The following terminology is used for halfspace intersection
in Qhull. This is the hardest structure to understand. The
underlying structure is a convex hull with one vertex per
non-redundant halfspace. See <a href="#conventions">convex hull
conventions</a> and <a href="index.htm#structure">Qhull's data structures</a>.</p>
<ul>
<li><em>interior point</em> - a point in the intersection of
the halfspaces. Qhull needs an interior point to compute
the intersection. See <a href="#input">halfspace input</a>.</li>
<li><em>halfspace</em> - <em>d</em> coordinates for the
normal and a signed offset. The distance to an interior
point is negative.</li>
<li><em>non-redundant halfspace</em> - a halfspace that
defines an output facet</li>
<li><em>vertex</em> - a dual vertex in the convex hull
corresponding to a non-redundant halfspace</li>
<li><em>coplanar point</em> - the dual point corresponding to
a similar halfspace</li>
<li><em>interior point</em> - the dual point corresponding to
a redundant halfspace</li>
<li><em>intersection point</em>- the intersection of <em>d</em>
or more non-redundant halfspaces</li>
<li><em>facet</em> - a dual facet in the convex hull
corresponding to an intersection point</li>
<li><em>non-simplicial facet</em> - more than <em>d</em>
halfspaces intersect at a point</li>
<li><em>good facet</em> - an intersection point that
satisfies restriction '<a href="qh-optq.htm#QVn">QVn</a>',
etc.</li>
</ul>
</blockquote>
<h3><a href="#TOP">&#187;</a><a name="options">qhalf options</a></h3>
<pre>
qhalf- compute the intersection of halfspaces about a point
http://www.qhull.org
input (stdin):
optional interior point: dimension, 1, coordinates
first lines: dimension+1 and number of halfspaces
other lines: halfspace coefficients followed by offset
comments: start with a non-numeric character
options:
Hn,n - specify coordinates of interior point
Qt - triangulated ouput
QJ - joggle input instead of merging facets
Qc - keep coplanar halfspaces
Qi - keep other redundant halfspaces
Qhull control options:
QJn - randomly joggle input in range [-n,n]
Qbk:0Bk:0 - remove k-th coordinate from input
Qs - search all halfspaces for the initial simplex
QGn - print intersection if redundant to halfspace n, -n for not
QVn - print intersections for halfspace n, -n if not
Trace options:
T4 - trace at level n, 4=all, 5=mem/gauss, -1= events
Tc - check frequently during execution
Ts - print statistics
Tv - verify result: structure, convexity, and redundancy
Tz - send all output to stdout
TFn - report summary when n or more facets created
TI file - input data from file, no spaces or single quotes
TO file - output results to file, may be enclosed in single quotes
TPn - turn on tracing when halfspace n added to intersection
TMn - turn on tracing at merge n
TWn - trace merge facets when width > n
TVn - stop qhull after adding halfspace n, -n for before (see TCn)
TCn - stop qhull after building cone for halfspace n (see TVn)
Precision options:
Cn - radius of centrum (roundoff added). Merge facets if non-convex
An - cosine of maximum angle. Merge facets if cosine > n or non-convex
C-0 roundoff, A-0.99/C-0.01 pre-merge, A0.99/C0.01 post-merge
Rn - randomly perturb computations by a factor of [1-n,1+n]
Un - max distance below plane for a new, coplanar halfspace
Wn - min facet width for outside halfspace (before roundoff)
Output formats (may be combined; if none, produces a summary to stdout):
f - facet dump
G - Geomview output (dual convex hull)
i - non-redundant halfspaces incident to each intersection
m - Mathematica output (dual convex hull)
o - OFF format (dual convex hull: dimension, points, and facets)
p - vertex coordinates of dual convex hull (coplanars if 'Qc' or 'Qi')
s - summary (stderr)
More formats:
Fc - count plus redundant halfspaces for each intersection
- Qc (default) for coplanar and Qi for other redundant
Fd - use cdd format for input (homogeneous with offset first)
FF - facet dump without ridges
FI - ID of each intersection
Fm - merge count for each intersection (511 max)
FM - Maple output (dual convex hull)
Fn - count plus neighboring intersections for each intersection
FN - count plus intersections for each non-redundant halfspace
FO - options and precision constants
Fp - dim, count, and intersection coordinates
FP - nearest halfspace and distance for each redundant halfspace
FQ - command used for qhalf
Fs - summary: #int (8), dim, #halfspaces, #non-redundant, #intersections
for output: #non-redundant, #intersections, #coplanar
halfspaces, #non-simplicial intersections
#real (2), max outer plane, min vertex
Fv - count plus non-redundant halfspaces for each intersection
Fx - non-redundant halfspaces
Geomview output (2-d, 3-d and 4-d; dual convex hull)
Ga - all points (i.e., transformed halfspaces) as dots
Gp - coplanar points and vertices as radii
Gv - vertices (i.e., non-redundant halfspaces) as spheres
Gi - inner planes (i.e., halfspace intersections) only
Gn - no planes
Go - outer planes only
Gc - centrums
Gh - hyperplane intersections
Gr - ridges
GDn - drop dimension n in 3-d and 4-d output
Print options:
PAn - keep n largest facets (i.e., intersections) by area
Pdk:n- drop facet if normal[k] &lt;= n (default 0.0)
PDk:n- drop facet if normal[k] >= n
Pg - print good facets (needs 'QGn' or 'QVn')
PFn - keep facets whose area is at least n
PG - print neighbors of good facets
PMn - keep n facets with most merges
Po - force output. If error, output neighborhood of facet
Pp - do not report precision problems
. - list of all options
- - one line descriptions of all options
</pre>
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<p><b>Up:</b> <a href="http://www.qhull.org">Home page</a> for Qhull<br>
<b>Up:</b> <a href="index.htm#TOC">Qhull manual</a>: Table of Contents<br>
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<p><a href="http://www.geom.uiuc.edu/"><img src="qh--geom.gif"
align="middle" width="40" height="40"></a><i>The Geometry Center
Home Page </i></p>
<p>Comments to: <a href=mailto:qhull@qhull.org>qhull@qhull.org</a>
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Created: Sept. 25, 1995 --- <!-- hhmts start --> Last modified: see top <!-- hhmts end --> </p>
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