"Tor Olav Kristensen" <tor### [at] TOBEREMOVEDgmailcom> wrote:
> Jim Charter <jrc### [at] msncom> wrote:
> > Can anyone show me proper usage of the Shear_Trans macro.
> >
> > This is a modification I made to the "Basic Scene" supplied on the
> > insert menu.  Can't get it to parse.
> >
> > #local A=<1,2,3>;
> > #local B=<1,2,3>;
> > #local C=<1,2,3>;
> ....
> >    Shear_Trans(A,B,C)
> ....
>
> Hi Jim,
>
> You have to make sure that no two of those three vectors are parallel.
>
> I.e.: These three conditions have to be met:
>
> vlength(vcross(A, B)) > 0
> vlength(vcross(B, C)) > 0
> vlength(vcross(C, A)) > 0
>
> (Note that there are also other ways to check for parallel vectors.)

I think the most elegant way is to use the scalar triple product:

vdot(A, vcross(B, C)) != 0

See:
http://en.wikipedia.org/wiki/Triple_product

--
Tor Olav
http://subcube.com

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