sorry, no math help here, but why are you using disks?  wouldn't spheres work?
rotation wouldn't matter then would it?

Mike Weber wrote:

> I have a question for a problem relating to 3D rotations.
> I'm working on a bspline plug-in dll for Moray.  Its very close to being
> ready for release once I get this small problem fixed.
>
> So here it is:
>
> I'm creating a tube like object for the spline by using a series of 'discs'
> along the path of the spline and orientating its surface or normal to point
> to the next joint or point along the spline.  Using the following math and
> trig functions that Sean Worle provided:
>
>   rz = atan2(dx, dy);
>   rx = atan2(sqrt(dx*dx + dy*dy), dz);
>
> atan2 = arctangent of y/x (in radians)
>
> where dx, dy, dz are the differences between the current point and the next
> point in the spline.
> rz and rx are the amount to rotate the disc in the z and x axis.  The disc
> is created in the X-Y plane.
>
> The problem is when I create a b-spline object along the Z axis, which looks
> like a straigt cylinder (or tube) - it is fine.  But if one of the points
> moves in the x direction, then the disc is rotated by 90 degrees along the Z
> axis which is not good.  But it does this because when using atan2 and the
> dx goes negative it returns -90.
>
> I certainly would appreciate any help.  I can provide pictures if needed.
>
> --
> Mike
>
> wk: mik### [at] pyxiscom  www.pyxis.com
> hm: mwe### [at] sciticom  www.geocities.com/mikepweber

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