# [R-sig-Geo] R-sig-Geo Digest, Vol 43, Issue 14

Tim Keitt tkeitt at gmail.com
Fri Mar 16 16:55:06 CET 2007

```Ya, there can be pretty arbitrary holes. The situation is a large
hexagonal lattice where some fraction of cells have q-values < 0.05.
Now draw lines around those cells following the hexagonal edges, but
omitting any interior arcs. I can send you an example solution. The
images are bit big for the list.

THK

On 3/16/07, Nicholas Lewin-Koh <nikko at hailmail.net> wrote:
> Hi Tim,
> You could compute the convex hull first, and then iterate from points
> on the convex hull. That should be much faster already, especially since
> hexagons are convex and the perimeter will be locally convex around all
> the
> points touching the convex hull. You could do a variation
> of the "monotone pieces" algorithm that is used in computational
> geometry.
> But this is a simpler problem. Are there cases with interior holes?
>
> I have been meaning to write something like this for hexbin for a while.
> There
> are many cases where it would be nice to find approximations to the
> density contours
> and a quick and dirty way is to threshold the hexagon counts, find the
> hull and
> smooth the perimeter.
>
> Nicholas
>
> On Fri, 16 Mar 2007 08:34:20 -0500, "Tim Keitt" <tkeitt at gmail.com> said:
> > Hi Nic,
> >
> > The convex hull would be fast and easy to compute (there's existing
> > code in R). I want the ordinary hull which is the set of arcs forming
> > the perimeters (inside and out). My crude and very slow solution was
> > to convert all the polygons (in this case hexagons on a lattice) into
> > their constituent arcs and then for each arc count how many times it
> > occurs in the set (requires slightly fuzzy matching of points). Arcs
> > that occur more than once are removed. The remaining arcs form the
> > hull. Runs in about 20 minutes with a  few hundred hexagons.
> > Sufficient for the moment.
> >
> > THK
> >
> > On 3/16/07, Nicholas Lewin-Koh <nikko at hailmail.net> wrote:
> > > Hi Tim,
> > > I am not quite sure what you are getting at here. Do you want to
> > > intersect
> > > polygons and then select the set of lines that form the outer perimeter?
> > > Do you wan the convex hull of a set of polygons. I guess I have been out
> > > of the
> > > GIS world to long. It seems to me that this would be something easy to
> > > solve,
> > > just tedious iteration of the polygon coordinates and some
> > > triangulation.
> > >
> > > Nicholas
> > >
> > > > Date: Thu, 15 Mar 2007 10:49:23 -0500
> > > > From: "Tim Keitt" <tkeitt at gmail.com>
> > > > Subject: [R-sig-Geo] polygons to arcs?
> > > > To: r-sig-geo at stat.math.ethz.ch
> > > > Message-ID:
> > > >       <6262c54c0703150849qe60ab14nfef1eb3bf73dfb5d at mail.gmail.com>
> > > > Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> > > >
> > > > Is there an 'sp' function that takes a polygon as its argument and
> > > > returns a set of line objects corresponding to the arcs in the
> > > > polygon?
> > > >
> > > > Or better yet, a function that given a set of polygons, returns the
> > > > hull? (ie the set of singleton arcs after applying the polys to arcs
> > > > function)
> > > >
> > > > THK
> > > >
> > > > --
> > > > Timothy H. Keitt, University of Texas at Austin
> > > > Contact info and schedule at http://www.keittlab.org/tkeitt/
> > > > Reprints at http://www.keittlab.org/tkeitt/papers/
> > > > ODF attachment? See http://www.openoffice.org/
> > > >
> > > >
> > > >
> > >
> >
> >
> > --
> > Timothy H. Keitt, University of Texas at Austin
> > Contact info and schedule at http://www.keittlab.org/tkeitt/
> > Reprints at http://www.keittlab.org/tkeitt/papers/
> > ODF attachment? See http://www.openoffice.org/
>

--
Timothy H. Keitt, University of Texas at Austin
Contact info and schedule at http://www.keittlab.org/tkeitt/
Reprints at http://www.keittlab.org/tkeitt/papers/
ODF attachment? See http://www.openoffice.org/

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