"Greg M. Johnson" wrote:
> 
> The rub comes in determining the angle.  I have vectors that can be pointing in
> any which way.  I am not making a merry go round, but having particles
> (spacecraft, birds)  interact with hundreds of other particles via forces which
> are in 3d.  I want the "orientation" to be correct but with minimum or no
> "roll".
> 
> A fish or bird in a swarming school might "point" its nose in any direction, but
> I'm sure that it maximizes the amount of time its feet are below its body:
> minimum roll.  The plain ol' reorient macro gives a roll as one reorients around
> a circle.  My hunch is that I could just change a sign or two in one of the
> matrices and be all right.
> 
> Nieminen Juha wrote:
> 
	Provided you stay in a x-z plane, and provided velocity is normalized,
you can replace your call to reorient by:
matrix <velocity.x, 0, -velocity.y,
            0,      1,       0,
        velocity.y, 0,  velocity.x,
            0,      0,       0>

	If you don't stay in the x-z plane, there is no real way to do it, you
can still try this if you don't go on too steep slopes:
#local u = vnormalize (<v.x, 0, v.z>);
matrix <v.x, v.y*u.z-v.z*u.y, u.x,
        v.y, v.z*u.x-v.x*u.z, u.y,
        v.z, v.x*u.y-v.y*u.x, u.z,
         0,         0,         0>

	(There might be a sign problem in the middle column wich would cause a
"mirror effect", and i might have mixed lines and columns wich would
cause very strange behavior...)

		Jerome

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