Subject:
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calc_z4.c (Was: Math Problem Again)
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Newsgroups:
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lugnet.cad.dev
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Date:
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Thu, 22 Oct 2009 22:42:19 GMT
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Viewed:
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16756 times
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In lugnet.cad.dev, Paul Easter wrote:
> In lugnet.cad.dev, Ronan Webb wrote:
> > In lugnet.cad.dev, Tore Eriksson wrote:
> > > You are given three 3d cooridates.
> >
> > (snip) <
> > > If possible, I would like an Excel spreadsheet, but just the formula will do.
> > >
> > > TIA,
> > > /Tore
> >
> >
> >
> > http://astronomy.swin.edu.au/~pbourke/geometry/planeeq/
> >
> > (snip) <
> > Having worked these out (i.e. this has given you the A,B,C,D in
> > A.x + B.y + C.z + D = 0) just plug in your fourth point's x and y co-ords
> > (say x4 and y4) into this equation i.e. let x=x4 and let y=y4
> > This gives you the equation for z4 that you are looking for viz.
> > A.x4 + B.y4 +C.z4 +D =0 (where now the only thing you don't know is z4)
> > rearranging this is just
> > z4=(-D -A.x4 - B.x4)/C
> >
> >
> > Then again, it's late :)
>
> Hey! cool! where was this info when I wrote my calculator program? It would have made it alot easier. The calculator grew into a slightly different purpose though. it was for helping determine where to trim lines and quads at intersection points.
>
> The last formula had a typo, here it is corrected.
> z4=(-D -A.x4 - B.y4)/C
>
> Tore, I will email you the spreadsheet I put this information into.
>
> Paul
Sorry guys, just wanted to store this function in a safer place than my hard
drive. :)
I just don't want to remake it again...!
Feel free to copy/paste/edit/port if you need it!
/Tore
// calc_z4 by Tore Eriksson
// credits to Ronan Webb for the superb formula
double calc_z4(double x1,double y1,double z1,
double x2,double y2,double z2,
double x3,double y3,double z3,
double x4,double y4)
{
double a,b,c,minus_d,z4;
a = y1*(z2-z3) + y2*(z3-z1) + y3*(z1-z2);
b = z1*(x2-x3) + z2*(x3-x1) + z3*(x1-x2);
c = x1*(y2-y3) + x2*(y3-y1) + x3*(y1-y2);
minus_d = x1*(y2*z3-y3*z2) + x2*(y3*z1-y1*z3) + x3*(y1*z2-y2*z1);
z4 = (minus_d - a*x4 - b*y4)/c;
return z4;
}
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Message has 1 Reply:
Message is in Reply To:
 | | Re: Math Problem Again
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| (...) Hey! cool! where was this info when I wrote my calculator program? It would have made it alot easier. The calculator grew into a slightly different purpose though. it was for helping determine where to trim lines and quads at intersection (...) (20 years ago, 14-Nov-03, to lugnet.cad.dev)
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