RE: [Plib-devel] Is a point inside a triangle in 3-d

[Fay J. F C. AAC/W. <joh...@eg...>]

OK, I have a well-tested point-inside-a-triangle function in two dimensions.

I have a poorly-tested point-inside-a-triangle function in three dimensions.
The problem is not well-posed (the point may not lie on the plane of the
triangle) and it will give different answers from the present function in
SG.  The differences are especially apparent if the point does not lie in
the plane of the triangle, in which case the present code can easily give an
improper answer.

I have a somewhat-tested intersection-of-two-lines function in three
dimensions.  It is consistent with the documentation although it gives
different results from the present algorithm if the two lines do not
actually intersect.

I did a bit of work on a function to return the line of intersection between
two planes but realized that the general vector algebra solution would be
less efficient than the present code so I gave it up.

John F. Fay
joh...@eg...



-----Original Message-----
From: pli...@li...
[mailto:pli...@li...]On Behalf Of Norman Vine
Sent: Friday, March 26, 2004 9:37 AM
To: pli...@li...
Subject: RE: [Plib-devel] Is a point inside a triangle in 3-d


John Fay writes:

>       I've been mulling the 3-d version of the point/triangle problem and
I have a question or two.
>        (1) The current code assumes that the point and the triangle are in
the same plane.  Is this a good thing?
>        (2) Is there a need for a faster point-in-triangle function?


One always needs a faster intersection routine :-)

To see how this one is used see < URL all one line >

http://cvs.flightgear.org/cgi-bin/viewcvs/viewcvs.cgi/FlightGear/src/Scenery
/hitlist.cxx?rev=1.9&cvsroot=FlightGear-0.9&content-type
=text/vnd.viewcvs-markup

The code is not the easiest to follow but a generic line intersection
with a SSG tree is in there

< the line is described by its origin and direction >

void FGHitList::Intersect( ssgBranch *scene, sgdVec3 orig, sgdVec3 dir ) {
    sgdMat4 m;
    clear();
    sgdMakeIdentMat4 ( m ) ;
    IntersectBranch( scene, m, orig, dir );
}

Cheers

Norman




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