Prevent Tangent Constraint from flipping?

[volxen]

Hi,
I tangent-constrained some circles to a curve. The problem is that whenever the tangent changes its side (always points to the bigger angle ...) my circles flip around about 180 degrees. This causes trouble of course when I want to loft these circles... Fixing the rotation once is not a solution because I

[failure]

Keep the pivot point on the curve.
Also consider using animation path instead.

[volxen]

Maybe I didn

[failure]

Use tangent constraint together with geometry constraint that will force the circle to always pivots on the path so tangent constraint can do it's job properly.

[joojaa]

Curves base their tangents on curvature, and they do not have set tangents in all conditions its just a fact if life. This menas in parctice that acurve has no sense of up and might therefore flip it at will. The same applies to path anim too.

The solution is to make a ribbon surcface along wich you make a curve on surface and than attach to that. Now the math can check for up consistently whetever your solution says, and as added bonus you jut recieved twist controll.

[failure]

Actually curves base their tangents on tangency ( first drivate ) not on curvature ( second derivate ).

First derivate is allways defined for the type of curves that maya uses.

It is true that curves don't have a sense of up, but "tangent constraint" has - it even let you specify it.

[joojaa]

Yes yes, sorry im still sleeping, tangents are defined fine with one derivate, but 1 vecor is not enough to describe a objects rotation in space, The problem is how to define the rotation around a curve.

But nurbs curves DO have a sense of up in maya (and in general implementation mostly too), its based on curvature (first order derivate and the plane the curve forms therein, im not so keen on the meths but you can fine examples of this in graphics gem series), however its not reliable in all instances. Because a curvature change can make them flip around to the exact opposite of the solution you wanted.