Finding a 3D convex hull in R

-----Original Message-----
From: Prof Brian D Ripley [mailto:ripley at stats.ox.ac.uk]
Sent: Saturday, September 22, 2001 2:53 AM
To: Greg Jefferis
Cc: r-help at stat.math.ethz.ch
Subject: Re: [R] Finding a 3D convex hull in R


On Fri, 21 Sep 2001, Greg Jefferis wrote:

Dear List Members,

I'm presently carrying out morphological analysis of a data 

set of neuronal

structures.  These are essentially 3D binary trees.  In due 

course I will be

trying to use discriminant analysis or other methods to 

classify these

neurons based on morphological variables such as total tree 

length, segment

number etc.

I would like to calculate a 3D convex hull for a set of 

X,Y,Z data points

which define the tips of my neurons.  I have found the R 

function chull

which finds a 2D convex hull, but not have found any 3D routines.  I
wondered if there were any packages / publicly available 

code I could use.

Having found the convex hull, it would be handy if there 

were also functions

to find the centroid, volume etc.  I've only been using R 

for 10 days and am

not much of a programmer, so I would rather not have to 

write code to

interface to external C routines etc if possible.

3D convex hull programs are very much more specialized than 2D ones.
When I added chull I looked at the possibility of allowing kD, k > 2,
but it was hard to find suitable code, let alone interface to it.
Known algorithms are far too complicated to implement 
tolerably at R level.

netlib.bell-labs.com/netlib/voronoi/hull.html

is one place I considered starting, but note the portability comments.

Very many thanks for any suggestions,

Greg.

P.S. I'm running R 1.3.1 on MacOS 9.1

Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch