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?

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"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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> 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.

Ah!  Thank you.  Works great.  I didn't think to look in math.inc for more
vector functions.

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"Kirk Andrews" <kir### [at] tektonartcom> wrote in message 
news:web.484985d0de6841fdb555cae70@news.povray.org...
>> 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.
>
> Ah!  Thank you.  Works great.  I didn't think to look in math.inc for more
> vector functions.

My pleasure. I remember it took me quite some time a few years ago to 
discover it when I wanted to add some clothing around a limb and I don't 
think I've ever used it for anything else since :o)

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