# Building indexed vertice/face structure.

I have a piece of code with which I am extracting triangles from a shape like so ...

``````std::vector<double> points;
// complete winding of points in triangle order.
for (TopExp_Explorer exp (aShape, TopAbs_FACE); exp.More(); exp.Next())
{
TriangleAccessor aTool (TopoDS::Face (exp.Current()));
for (int iTri = 1; iTri <= aTool.NbTriangles(); iTri++)
{
gp_Vec aNorm;
gp_Pnt aPnt1, aPnt2, aPnt3;
aTool.GetTriangle (iTri, aNorm, aPnt1, aPnt2, aPnt3);
points.push_back( aPnt1.X() ); points.push_back( aPnt1.Y() ); points.push_back( aPnt1.Z() );
points.push_back( aPnt2.X() ); points.push_back( aPnt2.Y() ); points.push_back( aPnt2.Z() );
points.push_back( aPnt3.X() ); points.push_back( aPnt3.Y() ); points.push_back( aPnt3.Z() );
}
}``````

After this I am then building an indexed face list by checking those vertices to see which faces share them.

I see after a bit of coding that this is a naive approach because rounding issues will cause degenerate cases and the index information about each polygon is lost.

Now I am wondering if it is also possible using a topological explorer to extract edges that are shared from the shape and -then- relate those back to each triangle that I extract?

-OR- does a better mechanism exist for me to extract a properly indexed list of polygons and vertices?

What I am after is a mesh from this extraction that will test closed and manifold.

This is the code for the TriangleAccessor based on this ( http://jamesgregson.blogspot.com.au/2012/05/example-code-for-building.html )

``````// Auxiliary tools
namespace
{
// Tool to get triangles from triangulation taking into account face
// orientation and location
class TriangleAccessor
{
public:
TriangleAccessor (const TopoDS_Face& aFace)
{
TopLoc_Location aLoc;
myPoly = BRep_Tool::Triangulation (aFace, aLoc);
myTrsf = aLoc.Transformation();
myNbTriangles = (myPoly.IsNull() ? 0 : myPoly->Triangles().Length());
myInvert = (aFace.Orientation() == TopAbs_REVERSED);
if (myTrsf.IsNegative())
myInvert = ! myInvert;
}

int NbTriangles () const { return myNbTriangles; }

// get i-th triangle and outward normal
void GetTriangle (int iTri, gp_Vec &theNormal, gp_Pnt &thePnt1, gp_Pnt &thePnt2, gp_Pnt &thePnt3)
{
// get positions of nodes
int iNode1, iNode2, iNode3;
myPoly->Triangles()(iTri).Get (iNode1, iNode2, iNode3);
thePnt1 = myPoly->Nodes()(iNode1);
thePnt2 = myPoly->Nodes()(myInvert ? iNode3 : iNode2);
thePnt3 = myPoly->Nodes()(myInvert ? iNode2 : iNode3);

int a = iNode1;
int b = (myInvert ? iNode3 : iNode2);
int c = (myInvert ? iNode2 : iNode3);

// apply transormation if not identity
if (myTrsf.Form() != gp_Identity)
{
thePnt1.Transform (myTrsf);
thePnt2.Transform (myTrsf);
thePnt3.Transform (myTrsf);
}

// calculate normal
theNormal = (thePnt2.XYZ() - thePnt1.XYZ()) ^ (thePnt3.XYZ() - thePnt1.XYZ());
Standard_Real aNorm = theNormal.Magnitude();
if (aNorm > gp::Resolution())
theNormal /= aNorm;

}

private:
Handle(Poly_Triangulation) myPoly;
gp_Trsf myTrsf;
int myNbTriangles;
bool myInvert;
};

}``````

Would those nodes that are accessed during the triangle extraction also match nodes during an edge extraction. That is could I use the edge data to understand which edge on each triangle is shared with the edge on another triangle.

Thanks !