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<html>
<head>
<title>
CCVT_REFLECT - Constrained CVT Algorithm
</title>
</head>
<body bgcolor="#EEEEEE" link="#CC0000" alink="#FF3300" vlink="#000055">
<h1 align = "center">
CCVT_REFLECT <br> Constrained CVT Algorithm
</h1>
<hr>
<p>
<b>CCVT_REFLECT</b>
is a MATLAB program which
allows the user to specify certain parameters, and then
creates a Centroidal Voronoi Tessellation
of points in a region, using a special reflection technique
to try to attract some points to move to the boundary.
</p>
<p>
The standard CVT algorithm will tend to place points uniformly
inside the region, but will never place points <i>on</i> the boundary.
However, in many cases, it would be very desirable to smoothly
modify the set of points so that some of them fall on the boundary.
This is the case, for instance, when the points are to be used to
triangulate the region and define a finite element grid.
</p>
<p>
The method for doing this relies on the observation that the
generator points become uniformly distributed because they
"push" each other away, by grabbing sample points. The generator
points do not approach the boundary too closely, because there
are no sample points on the other side of the boundary. In essence,
the boundary also "pushes" the generator points away.
By making "reflected" sample points whenever a generator is near
the boundary, we essentially neutralize the boundary effect, allowing
the generator points in the interior to push a layer of generators
onto the boundary. In most cases, once a point hits the boundary,
it will not leave, although it may continue to adjust its position
on the boundary itself.
</p>
<p>
<i>
This program is a "work in progress". Currently, only a simple
2D box region has been examined. The next step is to work on
more general 2D regions; then to make the natural extension to
3D or arbitrary dimension.
</i>
</p>
<p>
<b>CCVT_REFLECT</b> is an experimental code; so far, the experiment
is not doing well. I haven't figured out yet how to make the
points behave as well as they do for <b>CCVT_BOX</b>. The idea is,
though, that the method of pulling points to the boundary seems
fairly natural and flexible to me, so maybe I just need to
find the right way to implement it.
</p>
<h3 align = "center">
Licensing:
</h3>
<p>
The computer code and data files described and made available on this web page
are distributed under
<a href = "../../txt/gnu_lgpl.txt">the GNU LGPL license.</a>
</p>
<h3 align = "center">
Languages:
</h3>
<p>
<b>CCVT_REFLECT</b> is available in
<a href = "../../m_src/ccvt_reflect/ccvt_reflect.html">a MATLAB version</a>.
</p>
<h3 align = "center">
Related Data and Programs:
</h3>
<p>
<a href = "../../m_src/ccvt_box/ccvt_box.html">
CCVT_BOX</a>,
a MATLAB program which
solves a similar problem on a box.
</p>
<p>
<a href = "../../m_src/cvt_1d_lloyd/cvt_1d_lloyd.html">
CVT_1D_LLOYD</a>,
a MATLAB program which
computes an N-point Centroidal Voronoi Tessellation (CVT)
within the interval [0,1], under a uniform density.
</p>
<p>
<a href = "../../m_src/cvt_1d_sampling/cvt_1d_sampling.html">
CVT_1D_SAMPLING</a>,
a MATLAB program which
computes an N-point Centroidal Voronoi Tessellation (CVT)
within the interval [0,1], under a uniform density,
using sampling to estimate the Voronoi regions.
</p>
<h3 align = "center">
Reference:
</h3>
<p>
<ol>
<li>
Franz Aurenhammer,<br>
Voronoi diagrams -
a study of a fundamental geometric data structure,<br>
ACM Computing Surveys,<br>
Volume 23, Number 3, September 1991, pages 345-405.
</li>
<li>
John Burkardt, Max Gunzburger, Janet Peterson, Rebecca Brannon,<br>
User Manual and Supporting Information for Library of Codes
for Centroidal Voronoi Placement and Associated Zeroth,
First, and Second Moment Determination,<br>
Sandia National Laboratories Technical Report SAND2002-0099,<br>
February 2002.
</li>
<li>
Qiang Du, Vance Faber, Max Gunzburger,<br>
Centroidal Voronoi Tessellations: Applications and Algorithms,<br>
SIAM Review,<br>
Volume 41, Number 4, December 1999, pages 637-676.
</li>
<li>
Qiang Du, Max Gunzburger, Lili Ju,<br>
Meshfree, Probabilistic Determination of Point Sets and Support
Regions for Meshfree Computing,<br>
Computer Methods in Applied Mechanics in Engineering,<br>
Volume 191, 2002, pages 1349-1366.
</li>
<li>
Lili Ju, Qiang Du, Max Gunzburger,<br>
Probabilistic methods for centroidal Voronoi tessellations
and their parallel implementations,<br>
Parallel Computing,<br>
Volume 28, 2002, pages 1477-1500.
</li>
</ol>
</p>
<h3 align = "center">
Source Code:
</h3>
<p>
<ul>
<li>
<a href = "ccvt_reflect.m">ccvt_reflect.m</a>
gets information from the user, creates the CVT using
reflection, and saves the animation.
</li>
<li>
<a href = "ch_cap.m">ch_cap.m</a>
capitalizes a single character.
</li>
<li>
<a href = "cvt_energy.m">cvt_energy.m</a>
computes the CVT energy of a dataset.
</li>
<li>
<a href = "cvt_sample.m">cvt_sample.m</a>
returns sample points.
</li>
<li>
<a href = "cvt_write.m">cvt_write.m</a>
writes a CVT dataset to a file.
</li>
<li>
<a href = "data_read.m">data_read.m</a>
reads data from a file.
</li>
<li>
<a href = "file_exist.m">file_exist.m</a>
is true if a file exists.
</li>
<li>
<a href = "file_delete.m">file_delete.m</a>
deletes a file.
</li>
<li>
<a href = "find_closest.m">find_closest.m</a>
finds the Voronoi cell generator closest to a point X.
</li>
<li>
<a href = "get_seed.m">get_seed.m</a>
returns a seed for the random number generator.
</li>
<li>
<a href = "halham_leap_check.m">halham_leap_check.m</a>
checks LEAP for a Halton or Hammersley sequence.
</li>
<li>
<a href = "halham_n_check.m">halham_n_check.m</a>
checks N for a Halton or Hammersley sequence.
</li>
<li>
<a href = "halham_dim_num_check.m">halham_dim_num_check.m</a>
checks NDIM for a Halton or Hammersley sequence.
</li>
<li>
<a href = "halham_seed_check.m">halham_seed_check.m</a>
checks SEED for a Halton or Hammersley sequence.
</li>
<li>
<a href = "halham_step_check.m">halham_step_check.m</a>
checks STEP for a Halton or Hammersley sequence.
</li>
<li>
<a href = "halton_base_check.m">halton_base_check.m</a>
checks BASE for a Halton sequence.
</li>
<li>
<a href = "i4_to_halton_sequence.m">i4_to_halton_sequence.m</a>
next N elements of an NDIM-dimensional Halton sequence
</li>
<li>
<a href = "mpb.m">mpb.m</a>
replaces points near the boundary by their projections on
the boundary.
</li>
<li>
<a href = "points_eps.m">points_eps.m</a>
make an EPS plot of the generators.
</li>
<li>
<a href = "prime.m">prime.m</a>
returns the N-th prime number.
</li>
<li>
<a href = "r8_huge.m">r8_huge.m</a>
returns a "huge" R8.
</li>
<li>
<a href = "r8mat_transpose_print.m">r8mat_transpose_print.m</a>
prints the transpose of an R8MAT, with an optional title.
</li>
<li>
<a href = "r8mat_transpose_print_some.m">
r8mat_transpose_print_some.m</a>
prints the transpose of some of an R8MAT, with an optional title.
</li>
<li>
<a href = "r8mat_uniform_01.m">r8mat_uniform_01.m</a>
returns a unit pseudorandom R8MAT.
</li>
<li>
<a href = "random_initialize.m">random_initialize.m</a>
initializes the MATLAB random number seed.
</li>
<li>
<a href = "s_eqi.m">s_eqi.m</a>
is TRUE if two strings are equal, ignoring case.
</li>
<li>
<a href = "s_len_trim.m">s_len_trim.m</a>
returns the length of a string to the last nonblank.
</li>
<li>
<a href = "timestamp.m">timestamp.m</a>
prints the current YMDHMS date as a timestamp.
</li>
<li>
<a href = "tuple_next_fast.m">tuple_next_fast.m</a>
computes the next element of a tuple space, "fast".
</li>
</ul>
</p>
<p>
You can go up one level to <a href = "../m_src.html">
the MATLAB source codes</a>.
</p>
<hr>
<i>
Last revised on 08 November 2006.
</i>
<!-- John Burkardt -->
</body>
</html>