inside/outside decision problem

[Madhavan Lakshmiraghavan]

Jens,

I just dealt with a similar problem. Basically, you want the following
equation:

Do this.
  -  pick a point on polygon A unconnected to B. Let its coordinates be  (Xa,
Ya, Za)
  - randonly pick a point on polygon B. Let its coordinates be (Xb, Yb, Zb)
  - Find the components of the unit normal for polygon B. Let it be (n1, n2,
n3)

Then,
D = n1*(Xa-Xb) + n2*(Ya-Yb) + n3*(Za-Zb)

where D is the perpendiculat distance from the above-picked point on polygon A
to the plane of polygon B.

If D is positive, then that point of polygon A lies 'above' polygon B assuming
you define 'above' as the direction of the positive normal of B.  Redefine your
concept of 'inside' or 'outside' in  terms of 'above' or 'below'.

Hope this helps.

Raghavan

--- Jens Glagau wrote:
Dear vtkusers,

i want to decide wether a polygon A is inside or outside of a given
surface. Polygon A is connected with the surface with at least one edge
(at polygon B, which is of course part of the surface). All points of
polygon A are either outside of the surface or on the surface.

The points of polygons are known, therefore especially the normal of
polygon B and i know the point-ids which are in both polygons.

Of course, this is a mathematical question and not one related directly to
vtk, but i hope somebody sends me an answer. If my text is insufficent or
unprecise, please tell me.


Thanks in advance

 Jens
--- end of quote ---
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