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.

Now that you have stated your problem with more precision, I can help.

As a rule, I design objects so that x is right, y is up, and z is
forward, and what I consider to be the "center" is at the origin.

If I want to have it face a given direction, with its top as close to
up as possible, and located at a given place, I do the following:

// sample code
// assumes that Direction= the direction the object needs to face
//              Sky = the direction that's up in your world
//              Location = where it needs to go

#local vZ=vnormalize(Direction);
#local vX=vnormalize(vcross(Sky,vZ));
#local vY=vnormalize(vcross(vZ,vX));

object { MyObject Matrix(vX,vY,vZ,Location) }

// end of sample code


Hope this helps,
John
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ICQ: 46085459

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