poly_hull

poly_hull calculates the convex hull in the x/y plane.

rvIdx = poly_hull(x, y)

rvIdx = poly_hull(x, y, opt)

Return Value

rvIdx are the hull indices. In case of an error rvIdx is a string vector with two elements. The first element identifies the error, the second element is the error text.

Parameters

x

x is a vector with x coordinates. The vector must have the same number of elements as the vector y.

y

y is a vector with y coordinates. The vector must have the same number of elements as the vector x. At least 4 elements are necessary.

opt

If opt is set to 1, points on the edge of the hull will not be ignored. If set to 0 (default value) points on the edge of the hull will be ignored.

Example

* poly_hull([0,1,1,0,0], [0,0,1,1,0])
    1.0000
    2.0000
    3.0000
    4.0000
    1.0000

* poly_hull(rand(1,1000), rand(1,1000))
  443.0000
  970.0000
  939.0000
  591.0000
  526.0000
  776.0000
  584.0000
   92.0000
  536.0000
  877.0000
  580.0000
  238.0000
   50.0000
  875.0000
  338.0000
  230.0000
   44.0000
  288.0000
  243.0000
  443.0000

* poly_hull([1,1,1],[1,1,1])
2,x-poly-error
Singular Input Data (e.g. Points form a line)

idx = poly_hull(x = rand(1,1000), y = rand(1,1000))
if (type(idx) == "string") {
   MessageBoxError("Error: " + idx[1] + "\n\n + idx[2]);
   return 0;
}
h = plot(x[idx], y[idx])
h2 = plot(x, y, h[2])
XYShowMarker(h2[3], TRUE)
XYShowLine(h2[3], FALSE)
LayerSetAxisMinMaxDelta(h2[2], "X", -0.1, 1.1, 0.1)
LayerSetAxisMinMaxDelta(h2[2], "Y", -0.1, 1.1, 0.1)
LayerShowGrid(h2[2], "X", FALSE, FALSE)
LayerShowGrid(h2[2], "Y", FALSE, FALSE)
LayerSetAxisTitle(h2[2], "X", "")
LayerSetAxisTitle(h2[2], "Y", "")
PageReplot(h2[1])

See also

poly_triangulate

This function uses the qhull algorithm see:

Barber, C.B., Dobkin, D.P., and Huhdanpaa, H.T., “The Quickhull algorithm for convex hulls,” ACM Trans. on Mathematical Software, Dec 1996. http://www.qhull.org/.

                   Qhull, Copyright (c) 1993-2002

       The National Science and Technology Research Center for
        Computation and Visualization of Geometric Structures
                        (The Geometry Center)
                       University of Minnesota
                            400 Lind Hall
                        207 Church Street S.E.
                      Minneapolis, MN 55455  USA

                       email: qhull@geom.umn.edu

This software includes Qhull from The Geometry Center.  Qhull is
copyrighted as noted above.  Qhull is free software and may be obtained
via http from www.geom.umn.edu.  It may be freely copied, modified,
and redistributed under the following conditions:

1. All copyright notices must remain intact in all files.

2. A copy of this text file must be distributed along with any copies
   of Qhull that you redistribute; this includes copies that you have
   modified, or copies of programs or other software products that
   include Qhull.

3. If you modify Qhull, you must include a notice giving the
   name of the person performing the modification, the date of
   modification, and the reason for such modification.

4. When distributing modified versions of Qhull, or other software
   products that include Qhull, you must provide notice that the original
   source code may be obtained as noted above.

5. There is no warranty or other guarantee of fitness for Qhull, it is
   provided solely "as is".  Bug reports or fixes may be sent to
   qhull_bug@geom.umn.edu; the authors may or may not act on them as
   they desire.

_id-749714