face normal direction

Dear Rob Bachrach
i am a student. i have a task to make IGES to STl , and have the same problem that some face's normal is reversed.
can you give me a example that how to Fix the normal .

Thanks

This is the code :

Standard_Real aDeflection;
// aDeflection = ... ;

TopoDS_Shape aShape

// removes all the triangulations of the faces ,
//and all the polygons on the triangulations of the edges:
BRepTools::Clean(aShape);

// adds a triangulation of the shape aShape with the deflection aDeflection:
BRepMesh::Mesh(aShape,aDeflection);

TopExp_Explorer aExpFace,aExpEdge;
for(aExpFace.Init(aShape,TopAbs_FACE);aExpFace.More();aExpFace.Next())
{
TopoDS_Face aFace = TopoDS::Face(aExpFace.Current());
TopLoc_Location aLocation;

// takes the triangulation of the face aFace:
Handle_Poly_Triangulation aTr = BRep_Tool::Triangulation(aFace,aLocation);

if(!aTr.IsNull()) // if this triangulation is not NULL
{
// takes the array of nodes for this triangulation:
const TColgp_Array1OfPnt& aNodes = aTr->Nodes();
// takes the array of triangles for this triangulation:
const Poly_Array1OfTriangle& triangles = aTr->Triangles();

// create array of node points in absolute coordinate system
TColgp_Array1OfPnt aPoints(1, aNodes.Length());
for( Standard_Integer i = 1; i < aNodes.Length()+1; i++)
aPoints(i) = aNodes(i).Transformed(aLocation);

// Takes the node points of each triangle of this triangulation.
// takes a number of triangles:
Standard_Integer nnn = aTr->NbTriangles();
Standard_Integer nt,n1,n2,n3;
for( nt = 1 ; nt < nnn+1 ; nt++)
{
if（aFace.Orientation() == TopAbs_REVERSED）
{
// takes the node indices of each triangle in n1,n2,n3:
triangles(nt).Get(n1,n2,n3);
// takes the node points:
gp_Pnt aPnt1 = aPoints(n1);
gp_Pnt aPnt2 = aPoints(n2);
gp_Pnt aPnt3 = aPoints(n3);
}
else
{
// takes the node indices of each triangle in n1,n2,n3:
triangles(nt).Get(n1,n3,n2);
// takes the node points:
gp_Pnt aPnt1 = aPoints(n1);
gp_Pnt aPnt2 = aPoints(n3);
gp_Pnt aPnt3 = aPoints(n2);
}

}

// Takes the polygon associated to an edge.
aExpEdge.Init(aFace,TopAbs_EDGE);
TopoDS_Edge aEdge;
// for example,working with the first edge:
if(aExpEdge.More())
aEdge = TopoDS::Edge(aExpEdge.Current());

if(!aEdge.IsNull()) // if this edge is not NULL
{
// takes the polygon associated to the edge aEdge:
Handle_Poly_PolygonOnTriangulation aPol =
BRep_Tool::PolygonOnTriangulation(aEdge,aTr,aEdge.Location());

if(!aPol.IsNull()) // if this polygon is not NULL
// takes the array of nodes for this polygon
// (indexes in the array of nodes for triangulation of theFace):
const TColStd_Array1OfInteger& aNodesOfPol = aPol->Nodes();
}
}
}

The question is there: when read the iges file, the aFace.Orientation() is always TopAbs_FORWARD,
it make the normal wrong.

but when read step file , it's face normal is right

if（aFace.Orientation() == TopAbs_REVERSED）
{
// takes the node indices of each triangle in n1,n2,n3:
triangles(nt).Get(n1,n2,n3);
// takes the node points:
gp_Pnt aPnt1 = aPoints(n1);
gp_Pnt aPnt2 = aPoints(n2);
gp_Pnt aPnt3 = aPoints(n3);
}
else
{
// takes the node indices of each triangle in n1,n2,n3:
triangles(nt).Get(n1,n3,n2);
// takes the node points:
gp_Pnt aPnt1 = aPoints(n1);
gp_Pnt aPnt2 = aPoints(n3);
gp_Pnt aPnt3 = aPoints(n2);
}

hello , i am a student. and i have a task that iges/step to stl.

when iges to stl , i find that some face's normal is reversed, and the problem i think is flow:
face.Orientation() is always TopAbs_FORWARD,

but , when step to stl , it's fine. the face normal is right.
can you give me some advise ,,thank you very much.

this is my code:
TopoDS_Shape shape = Reader.OneShape();
shape.Modified();
//shape.Checked();
double const deflection = 0.5;
double const angulardeflection = 0.5;
BRepTools::Clean(shape);
BRepMesh_IncrementalMesh discr(shape, deflection, false, angulardeflection); //shape or face

TopoDS_Face face;

for (TopExp_Explorer ex(shape, TopAbs_FACE); ex.More(); ex.Next())
{
TopoDS_Face face = TopoDS::Face(ex.Current());
TopLoc_Location loc;

BRepAdaptor_Surface sf(face, Standard_False);
BRepLProp_SLProps prop(sf, 1, 1e-5);

Handle(Poly_Triangulation) triangulation = BRep_Tool::Triangulation (face, loc);

//Handle(Poly_Triangulation) triangulation1 = BRepTools::Triangulation(face, 0.01);

if (!triangulation.IsNull())
{
gp_Pnt2d uv;
gp_Pnt pnt;
gp_Vec nn;

int ntriangles = triangulation -> NbTriangles();
int npoints = triangulation->NbNodes();

const TColgp_Array1OfPnt& aNodes = triangulation->Nodes();
int num = aNodes.Length();

TColgp_Array1OfPnt aPoints(1, aNodes.Length());
for( Standard_Integer i = 1; i < aNodes.Length()+1; i++)
{
aPoints(i) = aNodes(i).Transformed(loc);

mesh.add_vertex( TNOMS3D::OPoint( aPoints(i).X(), aPoints(i).Y(), aPoints(i).Z() ) );
}

const Poly_Array1OfTriangle& triangles =triangulation ->Triangles();

Standard_Integer v1,v2,v3;
for (int j = 1; j <= ntriangles; j++)
{
if (face.Orientation() == TopAbs_REVERSED)
{
triangles(j).Get(v1,v3,v2);
mesh.add_face( OpenMesh::VertexHandle(index+v1-1), OpenMesh::VertexHandle(index+v2-1), OpenMesh::VertexHandle(index+v3-1));
}
else
{
triangles(j).Get(v1,v2,v3);
mesh.add_face( OpenMesh::VertexHandle(index+v1-1), OpenMesh::VertexHandle(index+v2-1), OpenMesh::VertexHandle(index+v3-1));
}

//mesh.add_face( OpenMesh::VertexHandle(v1-1), OpenMesh::VertexHandle(v2-1), OpenMesh::VertexHandle(v3-1));

Poly_Triangle triangle = (triangulation -> Triangles())(j);
//gp_Vec nn[3];
for(int k=1; k<=3; k++)
{
uv = (triangulation -> UVNodes())(triangle(k));
prop.SetParameters (uv.X(), uv.Y());

if (prop.IsNormalDefined())
nn = prop.Normal();
else
{
// n = gp_Vec (0,0,0);
gp_Vec a(aPoints(v1),aPoints(v2));
gp_Vec b(aPoints(v1),aPoints(v3));
nn = b^a;
}
if (face.Orientation() == TopAbs_REVERSED) nn *= -1;

if( k==1 )
{
mesh.set_normal( OpenMesh::VertexHandle(index+v1-1),TNOMS3D::ONormal(nn.X(),nn.Y(),nn.Z()) );
}
if( k==2 )
{
mesh.set_normal( OpenMesh::VertexHandle(index+v2-1),TNOMS3D::ONormal(nn.X(),nn.Y(),nn.Z()) );
}
if( k==3 )
{
mesh.set_normal( OpenMesh::VertexHandle(index+v3-1),TNOMS3D::ONormal(nn.X(),nn.Y(),nn.Z()) );
}
}
}

index = index+npoints;

}

}