Trying to find a, unit long) vector parrallel to the intersection line of two
planes, and in an attempt to understand vcross, I did the following:

take 3 points: A, B, C.
N1= vnormalize(A-B)
N2= vnormalize(A-C)
Nx is the normal of a plane

L= vnormalize(vcross(N1,N2))

Is L the vector I'm looking for?
If not, how do I find the vector, and what is the meaning of vcross?

ingo
--
Met dank aan de muze met het glazen oog.

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On Wed, 11 Aug 1999 22:30:39 +0200, ingo wrote:
>Trying to find a, unit long) vector parrallel to the intersection line of two
>planes, and in an attempt to understand vcross, I did the following:
>
>take 3 points: A, B, C.
>N1= vnormalize(A-B)
>N2= vnormalize(A-C)
>Nx is the normal of a plane
>
>L= vnormalize(vcross(N1,N2))
>
>Is L the vector I'm looking for?
>If not, how do I find the vector, and what is the meaning of vcross?

Where are A, B, and C?

If N1 and N2 are the normals of the planes, then L is indeed the vector you're
looking for.  To find the normal of a plane given three noncollinear points 
A, B, C on the plane, you'd use vnormalize(vcross(B-A,C-A)).  Repeat for three 
points in the other plane.  You can leave off the vnormalize in this case 
because you'll be normalizing your final result.

vcross is the vector cross-product, which is a vector perpendicular to both of
the two given vectors and with a length equal to twice the area of the triangle
between the two vectors.

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Ron Parker heeft geschreven in bericht <37b1df4f@news.povray.org>...
>Where are A, B, and C?
>If N1 and N2 are the normals of the planes, then L is indeed the vector you're
>looking for.

In my case three points on a bicubic_patch, and N1, N2 are the normals of the
planes.

>To find the normal of a plane given three noncollinear points
>A, B, C on the plane, you'd use vnormalize(vcross(B-A,C-A)).  Repeat for three
>points in the other plane.  You can leave off the vnormalize in this case
>because you'll be normalizing your final result.


And this bit goes into my "knowledgebase".

>vcross is the vector cross-product, which is a vector perpendicular to both of
>the two given vectors and with a length equal to twice the area of the triangle
>between the two vectors.

The docs say something similar, but reading it and understanding it are two
things.

Thanks Ron,

ingo
--
Met dank aan de muze met het glazen oog.

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I should have a visible demonstration of this up on the cyclopedia soon... I find a
picture is worth a thousand words. I'll let you know when it's done



ingo wrote:

> Ron Parker heeft geschreven in bericht <37b1df4f@news.povray.org>...
> >Where are A, B, and C?
> >If N1 and N2 are the normals of the planes, then L is indeed the vector you're
> >looking for.
>
> In my case three points on a bicubic_patch, and N1, N2 are the normals of the
> planes.
>
> >To find the normal of a plane given three noncollinear points
> >A, B, C on the plane, you'd use vnormalize(vcross(B-A,C-A)).  Repeat for three
> >points in the other plane.  You can leave off the vnormalize in this case
> >because you'll be normalizing your final result.
>
> And this bit goes into my "knowledgebase".
>
> >vcross is the vector cross-product, which is a vector perpendicular to both of
> >the two given vectors and with a length equal to twice the area of the triangle
> >between the two vectors.
>
> The docs say something similar, but reading it and understanding it are two
> things.
>
> Thanks Ron,
>
> ingo
> --
> Met dank aan de muze met het glazen oog.

--
Joshua English
eng### [at] spiritonecom
IQC: 1946299
"It's a thankless job, but I've got a lot of Karma to burn off."

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