Re: [Plib-users] Problems using sgIsectPlanePlane(), point is always at 0, 0, 0

[Fay, J. F Dr C. U. A. AAC/X. <joh...@eg...>]

Elmi,

	Hello and welcome to the PLIB community.

	I think the problem is that you are using relative vectors when
making your planes.  If you say "sgMakePlane ( plane1, a1, b1, c1 )" and
"sgMakePlane ( plane2, a2, b2, c2 )" you will get nonzero values for
your "point".  The three vectors in "sgMakePlane" are the vectors from
the origin to the three points that define the plane.

John F. Fay
Technical Fellow, Modeling and Simulation
Jacobs Technology / TEAS Group
P.O. Box 1935, Eglin AFB, FL  32542-5000
850-883-3496


-----Original Message-----
From: Satz Klauer [mailto:sat...@go...] 
Sent: Tuesday, September 20, 2011 2:16 AM
To: pli...@li...
Subject: [Plib-users] Problems using sgIsectPlanePlane(),point is always
at 0, 0, 0

Hi,

 I want to find the intersection line out of two planes but fail
 miserably. That's what I'm doing:

 - I start with two triangles a1,b1,c1 and a2,b2,c2 that describe my
 planes and consist of three coordinates x,y and z for every corner of
 it
 - out of their corner coordinates x,y and z I calculate 3D vectors
 sgVec3 va1,vb1,vc1 by substracting the coordinate values b1-a1, c1-b1,
 a1-c1 and normalizing the result; the same is done for the second
 triangle's coordinates
 - then I call

 sgMakePlane (plane1,va1,vb1,vc1);
 sgMakePlane (plane2,va2,vb2,vc2);
 ret=sgIsectPlanePlane (point,dir,plane1,plane2 );

 But here the result is unusable and nearly every time the same.
 "point" always contains _very_ small values so that after rounding to
 6 decimal places the coordinates of it are 0,0,0.

 What am I doing wrong here?

 Elmi

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