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Re: 3D vector rotation to the Z axis [message #74027 is a reply to message #73974]

Thu, 16 December 2010 11:48

James[2]
Messages: 44
Registered: November 2009

Member

Do you have to use a rotation matrix? Quaternions (http://
en.wikipedia.org/wiki/Quaternion) are more numerically stable, and
there is an easy-to-use library by Craig Markwardt at
http://www.physics.wisc.edu/~craigm/idl/math.html. I was just using
them to draw some rotating polyhedra in IDL, and they work great!
Your code would look like this:

;define the axis of rotation
rotaxis = crossp (input, [0,0,1])

;find the angle of rotation
rotangle = transpose(input) # [0,0,1]

;make the quaternion
q = qtcompose(rotaxis, rotangle)

;do it
rotated = qtvrot(input, q)

;there is even a routine to create a rotation matrix from the
quaternion:
rmatx = qtmat(q)

by the way, I like the # operator. It lets you treat 1D arrays as
column vectors; then defining a matrix is just concatenating a group
of column vectors across the second dimension. Matrix-by-vector
multiplication works like you expect.

- James

On Dec 14, 12:28 pm, David Fanning <n...@dfanning.com> wrote:
> MartyL writes:
>> Any help appreciated.
>
> I don't have any suggestions. You obviously know more
> about this than I do. Just an observation. I know any
> time I try to translate matrix operations out of a book
> into IDL my head swells up to about three times its
> normal size trying to keep columns and rows straight.
>
> That said, using the # operator, rather than the ## operator
> always throws me back from discovering the solution at LEAST
> two days, sometimes more. :-(
>
> 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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[Message index]

		Re: 3D vector rotation to the Z axis By: David Fanning on Tue, 14 December 2010 12:28
		Re: 3D vector rotation to the Z axis By: MartyL on Tue, 14 December 2010 12:20
		Re: 3D vector rotation to the Z axis By: penteado on Mon, 13 December 2010 19:23
		Re: 3D vector rotation to the Z axis By: James[2] on Thu, 16 December 2010 11:48
		Re: 3D vector rotation to the Z axis By: MartyL on Mon, 03 January 2011 17:17

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