On Feb 2, 2011, at 12:28 PM, Bryan Hanson wrote:
Thanks Dan for pointing that out. My question really arose from the
need to draw splines between arbitrary 3d pairs of points, so I
posted a new question to the list addressing that more
specifically. While the issue of 3d splines must have been dealt
with in graphics/animation oriented programs/languages, I think I
may have to "grow my own" and I'll need the suggestions offered by
you and Petr. Thanks, Bryan
I fit crossed cubic regression splines using the rms package, which
might provide this functionality. However, I get the idea you want an
exact fit rather than a fit that is constrained to a particular
functional form with parameters determined by a minimization metric
involving a large number of points.
--
David
On Feb 2, 2011, at 11:55 AM, Daniel Nordlund wrote:
-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org
]
On Behalf Of Bryan Hanson
Sent: Wednesday, February 02, 2011 5:23 AM
To: Petr Savicky
Cc: r-help@r-project.org
Subject: Re: [R] Function to locate points in 3d octants or points
on
twoaxes
Thanks Petr, the sign function will be of help. I was not aware of
it. Bryan
On Feb 2, 2011, at 3:21 AM, Petr Savicky wrote:
On Tue, Feb 01, 2011 at 08:30:22PM -0500, Bryan Hanson wrote:
[Sorry, resending with a proper subject line!]
Hi Guru's...
I have a set of points that may lie along any of the x, y and z
axes
in a Cartesian coordinate system. I am hoping that a function
exists
which will determine if any two selected points are on different
axes,
i.e, if the one of the points is on x and the other on y or z, not
elsewhere on the x axis. Put another way, I need to determine
if the
triangle formed by the two points and the origin lies in the xy,
xz
or
yz planes. This might be as simple as testing if any particular
value
is zero, i.e. if the x coordinate is zero, then the points must
be on
the z and y axes and the triangle in the yz plane. But, I'm
looking
for a fairly general solution, one that also returns the
appropriate
plane as the answer. Very closely related to this, I could use a
function that determines which of the 8 octants a point lies in.
Seems
like the cross product might be part of this, but I'm a little
rusty
on how to apply it.
I hope this is clear enough, and someone has a suggestion to
point me
in the right direction. Before writing my own klunky version, I
thought I'd ask.
For a general solution you also need to consider how you want to
deal with "boundary" conditions. For example, if the x and y
coordinates are zero for both points then the points lie in both
the xz and yz planes. And if one of the coordinates of a point is
zero, then how do you decide which quadrant it is in?
Dan
Daniel Nordlund
Bothell, WA USA
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______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
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David Winsemius, MD
West Hartford, CT
______________________________________________
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