Trianglearray

Engines, Libraries and Tools Xith3D Forums
#1

Hi,

Anybody Please help,

I have loaded a terrain model into my scene and then got the shape3D.Geometry() for my model and loaded the geometry into a triangleArray.
I can now access vertices info etc.
Does anybody know how I can pinpoint a triangle on my model terrain - based on a single x,z (no y) coordinate

ie - looking down the y axis if I am at point x,z - on my terrain model - what triangle is that point in.

grateful for any ideas or help

Thanks

rmdire

#2

I am doing something like this for a simulation of a robotic vehicle driving on a heightfield based terrain.

You may use a rayIntersectsTriangle algorithm.

For my simulation I use a complete geometry outside Xith since I just started to work with Xith and have no idea about the collision stuff.

The following algorithm does the computation based on a plane defined by 3 points in 3d space. So its used like plane.rayIntersectsTriangle(beamStart,beamEnd, returnHitPoint);
Original is:
// From the rayIntersectsTriangle Algorithm of Tomas M?ller & Prosolvia Clarus AB

// ========================================= public boolean rayIntersectsTriangle (minimally faster (5-10%) than v2 (does not create new intersection point)
public boolean rayIntersectsTriangle3(point3d _s1, point3d _s2, point3d _rtn) {
//System.out.println(“RAY TRIANGLE INTERSECTION !”);

  vector3d dir = new vector3d(_s2,_s1,false);

  vector3d edge1 = va;
  vector3d edge2 = vb;
  
  vector3d pvec = new vector3d(dir,edge2,false);
  
  double det = edge1.dotProduct(pvec);
  
  if ( det > -EPSILON && det < EPSILON ) { 
      //System.out.println("IS PARALLEL !");
      return false;
  }
  
  double inv_det = 1.0 / det;
  
  vector3d tvec = new vector3d(_s1,a,false);
  
  double u = tvec.dotProduct(pvec) * inv_det;
  
  //System.out.println("u="+u);

  if (u < 0.0 || u > 1.0) { 
      //System.out.println("NO INTERSECTION [1]!");
      return false;

  }

  vector3d qvec = new vector3d(tvec,edge1,false);
  
  double v = dir.dotProduct(qvec) * inv_det;
  //System.out.println("v="+v);
  if ( v < 0.0 || (u+v) > 1.0 ) { 
      // System.out.println("NO INTERSECTION [2] !");
      return false;
  }
  
  double t = edge2.dotProduct(qvec)*inv_det;
  
  _s1.translate(dir,t,_rtn);
  
  //System.out.println("FOUND INTERSECTION ["+t+"] @ "+_rtn.x+" "+_rtn.y+" "+_rtn.z+" !");
  return true;
}

Warning: the code may possibly incomplete due to cut and paste formatting problems.

This algorithm is based on my point,vector,plane classes which I coded without knowledge of javax.vecmath and I had no time yet to port them.

If thats what you are looking for contact me and I’ll send (explay) the code.

evilscientist at gmx net

#3

HI Cascade

I’ve tried to email you - but you don’t seem to be getting the mail.

anyway - Yes please - any help and example code would be much appreciated.

Should this be of any interest to you - I have some code which Calculates the weight into a triangle from a specified point within the triangle area and uses that to calculate the y .

Cheers

rmdire