How Get Adjacent Faces From a Selected Edge In Solid

By Madhukar Moogala

Using BREP API it is fairly straightforward to retrieve the adjacent faces from a particular edge in a 3dsolid. We will use Boundary Loop to get all the loops consuming the edge, from Loop we can get associated Face.

static public void GetAdjacentFaces()
{
    Document doc = Application.DocumentManager.MdiActiveDocument;
    Database db = doc.Database;
    Editor ed = doc.Editor;
    PromptKeywordOptions pko = new PromptKeywordOptions(
        "\nnSpecify sub-entity selection type:");
    pko.AllowNone = false;
    pko.Keywords.Add("Edge");
    pko.Keywords.Default = "Edge";
    PromptResult pkr = ed.GetKeywords(pko);
    if (pkr.Status != PromptStatus.OK)
        return;

    //User selection is Edge
    SubentityType subentityType = SubentityType.Edge;

    //Enable force pick sub-selection
    PromptSelectionOptions pso = new PromptSelectionOptions();
    pso.MessageForAdding = "\nSelect solid " + pkr.StringResult + ": ";
    pso.SingleOnly = true;
    pso.SinglePickInSpace = true;
    pso.ForceSubSelections = true;
    PromptSelectionResult psr = ed.GetSelection(pso);
    if (psr.Status != PromptStatus.OK)
        return;

    SelectionSet ss = psr.Value;
    SelectedObject so = ss[0];
    if (!so.ObjectId.ObjectClass.IsDerivedFrom(
            RXClass.GetClass(typeof(Solid3d))))
    {
        ed.WriteMessage(
            "\nYou didn't select a solid, please try again...");
        return;
    }

    //To store adjacent Faces
    List faceIds = new List();

    using (Transaction Tx = db.TransactionManager.StartTransaction())
    {
        Solid3d solid = Tx.GetObject(so.ObjectId, OpenMode.ForWrite)
                        as Solid3d;
        SelectedSubObject[] sso = so.GetSubentities();

        //Checks that selected type matches keyword selection
        if (subentityType != sso[0].FullSubentityPath.SubentId.Type)
        {
            ed.WriteMessage("\nInvalid Subentity Type: " +
                            sso[0].FullSubentityPath.SubentId.Type +
                            ", please try again...");
            return;
        }
        SubentityId subentityId = sso[0].FullSubentityPath.SubentId;

        //Creates subentity path to use with GetSubentity
        FullSubentityPath subEntityPath = new FullSubentityPath(
            new ObjectId[] { solid.ObjectId },
            subentityId);

        //Returns a non-database resident entity
        //that represents the subentity
        using (Entity entity = solid.GetSubentity(subEntityPath))
        {
            ed.WriteMessage("\nSubentity Entity Type: "
                            + entity.ToString());
        }

        //Creates entity path to generate Brep object from it
        FullSubentityPath entityPath = new FullSubentityPath(
            new ObjectId[] { solid.ObjectId },
            new SubentityId(SubentityType.Null, IntPtr.Zero));

        using (Edge edge = new Edge(subEntityPath))
        {
            if (subentityType == SubentityType.Edge)
            {
                if (subentityId == edge.SubentityPath.SubentId)
                {
                    ed.WriteMessage("\nCurve: " + edge.Curve.ToString());

                    //Now color the adjacent faces.
                    foreach(BoundaryLoop loop in edge.Loops)
                    {
                        //For simplest loop,outward loop is our interest
                        if(loop.LoopType == LoopType.LoopExterior)
                        {
                            AcBr.Face face = loop.Face;
                            faceIds.Add(face.SubentityPath.SubentId);
                        }
                    }
                    if(faceIds.Count > 1)
                        foreach (SubentityId subentId in faceIds)
                        {
                            Color col = Color.FromColorIndex(
                                ColorMethod.ByColor, 1);
                            solid.SetSubentityColor(subentId, col);
                        }
                }
            }
        }
        Tx.Commit();
    }
}

Demo Video