"Kirk Andrews" <kir### [at] tektonartcom> wrote in message 
news:web.48489920de6841fda5d4a01d0@news.povray.org...
> Ok, here's my problem right now:
>
> My tree and branches are made by following along a spline, placing 
> vertices at a
> specified radius in a circle around the spline.  In order for the branches 
> to
> have a consistent radius, I need the plane on which each circle of 
> vertices
> rests to be perpendicular to the the direction of the spline.
>
> I thought that would be simple enough, but I'm having trouble.  I've been 
> trying
> to use vcross() to get an perpendicular axis to rotate a vector around, 
> but
> every time the spline changes direction, the axis will flip and therefore 
> so
> does the order of my vertices.  The result is an effect similar to 
> twisting a
> tube balloon.
>
> Am I making any sense? Any help?  Any simple solution that I'm just not 
> seeing?

Yes. Keep a running note of the vector that you used to start the previous 
circular sweep. Then use the VPerp_Adjust function in math.inc to adjust it 
so that it is perpendicular to the direction of the spline for the next 
step. This avoids suddenly getting a vector that points in completely the 
opposite direction and should therefore avoid the irritating twisting 
effect.

Regards,
Chris B.

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