Retrieving a Solid3d face normal through brep API

By Philippe Leefsma

The following C# post illustrates how to retrieve a solid face normal through some of the functionalities of the BRep API:

// Returns normal vector to a solid3d face [Philippe Leefsma, 7/11/2012]

[CommandMethod("FaceNormal")]

public void FaceNormal()

{

    Document doc = Application.DocumentManager.MdiActiveDocument;

    Database db = doc.Database;

    Editor ed = doc.Editor;

 

    PromptEntityOptions peo = new PromptEntityOptions(

        "\nSelect a 3D solid: ");

 

    peo.SetRejectMessage("\nInvalid selection…");

    peo.AddAllowedClass(typeof(Solid3d), true);

 

    PromptEntityResult per = ed.GetEntity(peo);

 

    if (per.Status != PromptStatus.OK)

        return;

 

    PromptPointOptions ppo = new PromptPointOptions(

        "\nSelect a point on a face: ");

 

    ppo.AllowArbitraryInput = false;

 

    PromptPointResult ppr = ed.GetPoint(ppo);

 

    if (ppr.Status != PromptStatus.OK)

        return;

 

    Point3d point = ppr.Value;

 

    using (Transaction Tx = db.TransactionManager.StartTransaction())

    {

        Solid3d solid = Tx.GetObject(per.ObjectId, OpenMode.ForRead)

            as Solid3d;

 

        ObjectId[] ids = new ObjectId[] { per.ObjectId };

 

        FullSubentityPath path = new FullSubentityPath(

            ids, new SubentityId(SubentityType.Null, IntPtr.Zero));

 

        using (Brep brep = new Brep(path))

        {

            foreach (Autodesk.AutoCAD.BoundaryRepresentation.Face face in

                brep.Faces)

            {

                PointContainment ptContainment = new PointContainment();

 

                using (BrepEntity brepEnt = face.GetPointContainment(

                    point,

                    out ptContainment))

                {

                    if (ptContainment == PointContainment.Inside ||

                        ptContainment == PointContainment.OnBoundary)

                    {

                        ed.WriteMessage("\nFace Normal at " +

                            PointToStr(point) + " = " +

                            VectToStr(GetFaceNormal(face, point)));

                    }

                }

            }

        }

 

        Tx.Commit();

    }

}

 

// Small utility methods

private string VectToStr(Vector3d v)

{

    return "[" +

     &#16
0;  v.X.ToString("F2") + ", " +

        v.Y.ToString("F2") + ", " +

        v.Z.ToString("F2")

        + "]";

}

 

Vector3d GetFaceNormal(

    Autodesk.AutoCAD.BoundaryRepresentation.Face face,

    Point3d point)

{

    PointOnSurface ptOnSurf = face.Surface.GetClosestPointTo(point);

 

    Point2d param = ptOnSurf.Parameter;

 

    Vector3d normal = ptOnSurf.GetNormal(param);

 

    // It seems we cannot trust the

    // face.Surface.IsNormalReversed property,

    // so I am applying a small offset to the point on surface

    // and check if the offset point is inside the solid

    // in case it is inside (or on boundary),

    // the normal must be reversed

 

    Vector3d normalTest = normal.MultiplyBy(1E-6 / normal.Length);

  

    Point3d ptTest = point.Add(normalTest);

 

    PointContainment ptContainment = new PointContainment();

 

    bool reverse = false;

 

    using(BrepEntity brepEnt =

        face.Brep.GetPointContainment(

            ptTest,

            out ptContainment))

    {

        if(ptContainment != PointContainment.Outside)

        {

            reverse = true;

        }

    }

 

    return normal.MultiplyBy(reverse ? -1.0 : 1.0);

}

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