| Re: speher keyword of qhull [message #51173] |
Wed, 08 November 2006 08:34  |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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Andy Heaps writes:
> an interesting question for me as I was puzzling over the
> same point last week. I don't have any real answers but the following
> should maybe provide some illumination on the subject:
Yikes! Here is a project for someone. (Not me. I'm still
recovering from serving as a radio reporter doing election
coverage last night. You have NO idea how hard it is to
write a coherent story at 2:30 AM when you have a deadline
in 5 minutes. Most nerve-wracking experience of my life!)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: speher keyword of qhull [message #51174 is a reply to message #51173] |
Wed, 08 November 2006 08:12  |
Andy Heaps
Messages: 22
Registered: March 2006
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Junior Member |
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Hello Sebinjapan,
an interesting question for me as I was puzzling over the
same point last week. I don't have any real answers but the following
should maybe provide some illumination on the subject:
I have a strange grid of points in longitude and latitude on a globe:
LATS FLOAT = Array[722, 511]
LONGS FLOAT = Array[722, 511]
TEMPERATURE FLOAT = Array[722,511]
The longitudes (LONGS) go from -179.997 to 179.999 and the latitudes
(LATS) go from -77.0105 to 89.8854. TEMPERATURE is the ocean surface
temperature on this grid.
The location of the longitudes and latitudes are very irregular so
triangulation data is needed to pass to the CONTOUR command. The
TRIANGULATE procedure works and CONTOUR produces a pretty decent plot.
There is a wrapping problem, however, at the dateline seen as a
line from the north to south poles. Adding the /SPHERE to the
TRANGULATE call produces:
% TRIANGULATE: Points are co-linear, no solution.
No hope here then so I then used QHULL on the suggestion of a colleague:
If I use QHULL, longs, lats, tri, /DELAUNAY, SPHERE =1
I get:
% QHULL: Expression must be named variable in this context: <INT
( 1)>.
Okay, let's used a named variable:
QHULL, longs, lats, tri, SPHERE = s
HELP, s, tri
S DOUBLE = Array[3, 368943]
TRI LONG = Array[3, 733642]
Using the triangulation data TRI with CONTOUR produces a perfect plot
with no dateline missing data.
So we're back to your original question - what is returned by
SPHERE=s? THE IDL reference guide for version 6.0 QHULL says:
DELAUNAY Performs a Delaunay triangulation and returns the vertex
indicies of the resulting polyhedra; otherwise, the convex hull of the
data are returned.
SPHERE Computes the Delaunay triangulation of the points which lie on
the surface of a sphere. The V0 argument contains the longitude, in
degrees, and V1 contains the latitude, in degrees, of each point.
It looks like the IDL documentation could be better just here. Does
SPHERE=s imply a Delaunay triangulation and that s is the convex hull
of the data? Printing off some values of s I get:
PRINT, s(*, 0:3)
PRINT, 'min, max of s are', MIN(s), MAX(s)
0.066654513 0.21466261 -0.97441118
0.064778714 0.21523610 -0.97441118
0.062897982 0.21579320 -0.97441118
0.061012460 0.21633386 -0.97441118
min, max of s are -0.99999048 0.99999800
Is this the sort of thing you're trying to work out as well?
Cheers
Andy
David Fanning wrote:
> sebinjapan writes:
>
>> Does anyone knows was is returned in the sphere variable when using:
>> qhull, v0, v1, tr, /delaunay, sphere = s
>
> As I read the documentation, it seems to me SPHERE is
> an input variable, with a value of 0 or 1. :-)
>
> Cheers,
>
> David
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| Re: speher keyword of qhull [message #51181 is a reply to message #51174] |
Wed, 08 November 2006 06:47 |
David Fanning
Messages: 11724
Registered: August 2001
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Senior Member |
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sebinjapan writes:
> Does anyone knows was is returned in the sphere variable when using:
> qhull, v0, v1, tr, /delaunay, sphere = s
As I read the documentation, it seems to me SPHERE is
an input variable, with a value of 0 or 1. :-)
Cheers,
David
--
David Fanning, Ph.D.
Fanning Software Consulting, Inc.
Coyote's Guide to IDL Programming: http://www.dfanning.com/
Sepore ma de ni thui. ("Perhaps thou speakest truth.")
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| Re: speher keyword of qhull [message #51267 is a reply to message #51174] |
Fri, 10 November 2006 09:55 |
Haje Korth
Messages: 651
Registered: May 1997
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Senior Member |
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Andy,
yes I hope one of these days, ITTVIS provide a detailed tutorial on
qhull/griddata, describing the right combination of keywords to get a
particular job done. This should benefit anyone involved in Earth
and/or space sciences, which is a pretty broad audience.
I got bit for the colinear-points problem with triangulate a few years
ago because I wanted to interpolate data on a sphere which came on a
regular grid. On regular grids the probability that three point lie
along a great circle is pretty large, thus resulting in the
colinear-point error message in triangulate.
Haje
Andy Heaps wrote:
> Hello Sebinjapan,
> an interesting question for me as I was puzzling over the
> same point last week. I don't have any real answers but the following
> should maybe provide some illumination on the subject:
>
> I have a strange grid of points in longitude and latitude on a globe:
> LATS FLOAT = Array[722, 511]
> LONGS FLOAT = Array[722, 511]
> TEMPERATURE FLOAT = Array[722,511]
> The longitudes (LONGS) go from -179.997 to 179.999 and the latitudes
> (LATS) go from -77.0105 to 89.8854. TEMPERATURE is the ocean surface
> temperature on this grid.
>
> The location of the longitudes and latitudes are very irregular so
> triangulation data is needed to pass to the CONTOUR command. The
> TRIANGULATE procedure works and CONTOUR produces a pretty decent plot.
> There is a wrapping problem, however, at the dateline seen as a
> line from the north to south poles. Adding the /SPHERE to the
> TRANGULATE call produces:
> % TRIANGULATE: Points are co-linear, no solution.
>
> No hope here then so I then used QHULL on the suggestion of a colleague:
> If I use QHULL, longs, lats, tri, /DELAUNAY, SPHERE =1
> I get:
> % QHULL: Expression must be named variable in this context: <INT
> ( 1)>.
>
> Okay, let's used a named variable:
> QHULL, longs, lats, tri, SPHERE = s
> HELP, s, tri
> S DOUBLE = Array[3, 368943]
> TRI LONG = Array[3, 733642]
>
> Using the triangulation data TRI with CONTOUR produces a perfect plot
> with no dateline missing data.
>
> So we're back to your original question - what is returned by
> SPHERE=s? THE IDL reference guide for version 6.0 QHULL says:
> DELAUNAY Performs a Delaunay triangulation and returns the vertex
> indicies of the resulting polyhedra; otherwise, the convex hull of the
> data are returned.
> SPHERE Computes the Delaunay triangulation of the points which lie on
> the surface of a sphere. The V0 argument contains the longitude, in
> degrees, and V1 contains the latitude, in degrees, of each point.
>
> It looks like the IDL documentation could be better just here. Does
> SPHERE=s imply a Delaunay triangulation and that s is the convex hull
> of the data? Printing off some values of s I get:
> PRINT, s(*, 0:3)
> PRINT, 'min, max of s are', MIN(s), MAX(s)
> 0.066654513 0.21466261 -0.97441118
> 0.064778714 0.21523610 -0.97441118
> 0.062897982 0.21579320 -0.97441118
> 0.061012460 0.21633386 -0.97441118
> min, max of s are -0.99999048 0.99999800
>
>
> Is this the sort of thing you're trying to work out as well?
>
>
> Cheers
> Andy
>
>
>
>
> David Fanning wrote:
>> sebinjapan writes:
>>
>>> Does anyone knows was is returned in the sphere variable when using:
>>> qhull, v0, v1, tr, /delaunay, sphere = s
>>
>> As I read the documentation, it seems to me SPHERE is
>> an input variable, with a value of 0 or 1. :-)
>>
>> Cheers,
>>
>> David
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| Re: speher keyword of qhull [message #51285 is a reply to message #51173] |
Thu, 09 November 2006 07:23 |
Andy Heaps
Messages: 22
Registered: March 2006
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Junior Member |
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ITT-Vis help desk have resolved the query:
The SPHERE keyword to QHULL will return the Delaunay triangulation of
the points which lie on the surface of a sphere. Consider the
following example:
;***************************************************
> pro test
>
> seed = 1
> n = 50
>
> lon = randomu(seed, n) * 360. - 180.
> lat = randomu(seed, n) * 180. - 90.
> f = sin(lon * !dtor)^2 * cos(lat * !dtor)
>
> qhull, lon, lat, qtr, $
> SPHERE = q_sphere, /DELAUNAY
>
> qpolys = lonarr(4,(size(qtr, /DIM))[1])
> qpolys[0,*] = 3
> qpolys[1:3,*] = qtr
>
> q_poly = obj_new('idlgrpolygon', $
> q_sphere, COLOR = [0,255,0], $
> POLYGONS = qpolys, STYLE = 2)
>
> xobjview, q_poly, XSIZE = 400, YSIZE = 400, $
> XOFF = 410, YOFF = 450, $
> TITLE = 'QHULL triangulation'
>
> end
> ;***************************************************
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