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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