| Re: Calculate convex hull of scattered data? [message #3766 is a reply to message #3760] |
Mon, 13 March 1995 09:13  |
sterner
Messages: 106
Registered: February 1991
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Senior Member |
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art.croucher@jhuapl.edu writes:
> I'm trying to calculate the convex hull which encloses a scattered data
> set. The JHU/APL CONVEXHULL routine didn't work, presumably because
> the data set isn't a polygon. Does anyone have a routine that will
> calculate either a convex hull or a polygon suitable for input to
> CONVEXHULL?
Hi Art,
I wanted to do the same thing but wasn't sure how. Your message
inspired me to think about it again, it's very easy. (I don't have
a need right now but I'm sure one will come up).
Random x,y points can be preprocessed to put them in a form
that the convexhull routine can handle. Here is an example
that first generates 100 random points, then preprocesses the
points, and finally finds the convexhull. I don't know how
far you can puch this as far as number of points goes, 1000
works ok, I'm waiting for 10,000 as I write this (several minutes
so far).
------------------------------------------------------------ -----
;------ Generate some random scatterplot data -----
a = randomu(k,100)*360 ; Random angle from 0 to 360 deg.
r = randomu(k,100) ; Random radius from 0 to 1.
polrec,r,a,/deg,x,y ; Convert to rectangular.
plot,x,y,psym=2 ; Plot.
;------ Preprocess scatterplot data --------
xm = mean(x) ; Find mean of x amd y.
ym = mean(y)
dx = x-xm ; Remove means.
dy = y-ym
recpol,dx,dy,r,a ; Conert to polar form.
is = sort(a) ; Sort on angle.
a = a(is)
r = r(is)
polrec,r,a,x2,y2 ; Convert sorted values back to rectangular
x2 = x2+xm ; Restore means.
y2 = y2+ym
;------ Find the convex hull ---------
convexhull, x2, y2, xh, yh ; Find convexhull.
oplot, xh, yh ; Plot convexhull (not closed).
------------------------------------------------------------ -----
This example assumes you have the JHU/APL/S1R IDL library:
(polrec, recpol, and convexhull)
ftp fermi.jhuapl.edu
login: anonymous
password: your email address
cd pub/idl
get README
bye
The text file README describes what is in the libraries, how to
get them, and how to set them up.
Or see the web page: ftp://fermi.jhuapl.edu/www/s1r/idl/s1rlib/local_idl.html
------------------------------------------------------------ -----
By the way, 10,000 points did work. It took a few minutes on my
HP 7/35 so I wouldn't want to do too many.
Ray Sterner sterner@tesla.jhuapl.edu
The Johns Hopkins University North latitude 39.16 degrees.
Applied Physics Laboratory West longitude 76.90 degrees.
Laurel, MD 20723-6099
WWW Home page: ftp://fermi.jhuapl.edu/www/s1r/people/res/res.html
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