Getting Outline of Overlapped Entities

Sometimes, in a drawing there are some entities overlapping on each other, for example, a few closed polylines. How do we find out a closed polyline that is the outline of these overlapped polygons?

This picture shows 3 polygons overlapping each other:

This picture shows the outline (in red) we want to obtain:

In this article, to simplify the code and the discussion, let me limit the entities are all closed Polylines, and each Polyline overlaps with at least 1 other Polyline (so that a continuous outline can be formed); also I only care to get an exterior outline and ignore possible islands inside the outline. Obviously, I should expect to obtain a Polyline, as the red outline shown in picture above, or a series of points that are the vertices of the Polyline.

How to proceed with .NET API code to do this? After some tries, I settled with these 2 steps:

1. Converting each Polyline to Region, then use Region.BooleanOperation() method to merge/unite all the closed Polylines into one single Region;

2. Use Brep API to generate a BrepEntity from the Region, then find the exterior loop of the BrepEntity. These exterior loop provides all the vertices of outline Polyline.

Here is the code that implements the thought of the process:

using System.Collections.Generic;
using System.Linq;
using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
using CadDb = Autodesk.AutoCAD.DatabaseServices;
 
using Autodesk.AutoCAD.BoundaryRepresentation;
 
namespace GetOutLine
{
    public class OutLiner
    {
        private Document _dwg;
 
        public OutLiner(Document dwg)
        {
            _dwg = dwg;
        }
 
        public void DrawOutline(IEnumerable<ObjectId> entIds)
        {
            using (var polyline = GetOutline(entIds))
            {
                using (var tran = _dwg.TransactionManager.StartTransaction())
                {
                    var space = (BlockTableRecord)tran.GetObject(
                        _dwg.Database.CurrentSpaceId, OpenMode.ForWrite);
                    space.AppendEntity(polyline as Entity);
                    tran.AddNewlyCreatedDBObject(polyline as Entity, true);
                    tran.Commit();
                }
            }
        }
 
        public Entity GetOutline(IEnumerable<ObjectId> entIds)
        {
            var regions = new List<Region>();
 
            using (var tran = _dwg.TransactionManager.StartTransaction())
            {
                foreach (var entId in entIds)
                {
                    var poly = tran.GetObject(entId, OpenMode.ForRead) as Polyline;
                    if (poly!=null)
                    {
                        var rgs = GetRegionFromPolyline(poly);
                        regions.AddRange(rgs);
                    }
                    
                }
 
                tran.Commit();
            }
 
            using (var region = MergeRegions(regions))
            {
                if (region != null)
                {
                    var brep = new Brep(region);
                    var points = new List<Point2d>();
                    var faceCount = brep.Faces.Count();
                    var face = brep.Faces.First();
                    foreach (var loop in face.Loops)
                    {
                        if (loop.LoopType == LoopType.LoopExterior)
                        {
                            foreach (var vertex in loop.Vertices)
                            {
                                points.Add(new Point2d(vertex.Point.X, vertex.Point.Y));
                            }
                            break;
                        }
                    }
 
                    return CreatePolyline(points);
                }
                else
                {
                    return null;
                }
            }
        }
 
        #region private methods
 
        private List<Region> GetRegionFromPolyline(CadDb.Polyline poly)
        {
            var regions = new List<Region>();
 
            var sourceCol = new DBObjectCollection();
            var dbObj = poly.Clone() as CadDb.Polyline;
            dbObj.Closed = true;
            sourceCol.Add(dbObj);
 
            var dbObjs = Region.CreateFromCurves(sourceCol);
            foreach (var obj in dbObjs)
            {
                if (obj is Region) regions.Add(obj as Region);
            }
 
            return regions;
        }
 
        private Region MergeRegions(List<Region> regions)
        {
            if (regions.Count == 0) return null;
            if (regions.Count == 1) return regions[0];
 
            var region = regions[0];
            for (int i=1; i<regions.Count; i++)
            {
                var rg = regions[i];
                region.BooleanOperation(BooleanOperationType.BoolUnite, rg);
                rg.Dispose();
            }
 
            return region;
        }
 
        private CadDb.Polyline CreatePolyline(List<Point2d> points)
        {
            var poly = new CadDb.Polyline(points.Count());
 
            for (int i=0; i<points.Count;i++)
            {
                poly.AddVertexAt(i, points[i], 0.0, 0.3, 0.3);
            }
 
            poly.SetDatabaseDefaults(_dwg.Database);
            poly.ColorIndex = 1;
            
            poly.Closed = true;
 
            return poly;
        }
 
        #endregion
    }
}

Here is the command that makes above code in action:

using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Runtime;
using CadApp = Autodesk.AutoCAD.ApplicationServices.Application;
 
[assembly: CommandClass(typeof(GetOutLine.Commands))]
 
namespace GetOutLine
{
    public class Commands
    {
        [CommandMethod("Outline")]
        public static void RunMyCommand()
        {
            var dwg = CadApp.DocumentManager.MdiActiveDocument;
            var ed = dwg.Editor;
 
            try
            {
                var ids = SelectPolylines(ed);
                if (ids != null)
                {
                    var liner = new OutLiner(dwg);
                    liner.DrawOutline(ids);
                }
                else
                {
                    ed.WriteMessage("\n*Cancel*");
                }
            }
            catch (System.Exception ex)
            {
                ed.WriteMessage("\nCommand failed:\n{0}", ex.Message);
                ed.WriteMessage("\n*Cancel*");
            }
        }
 
        private static ObjectId[] SelectPolylines(Editor ed)
        {
            var vals = new TypedValue[]
            {
                new TypedValue((int)DxfCode.Start, "LWPOLYLINE")
            };
 
            var res = ed.GetSelection(new SelectionFilter(vals));
            if (res.Status == PromptStatus.OK)
                return res.Value.GetObjectIds();
            else
                return null;
        }   
    }
}

Watch these video clip as the proof of how the code works.

Extra Thought

Obviously, creating a BrepEntity based on Region garantees a exterior loop/boundary will be generated, thus the outline curve. I could extend the Region generating process beyond Polyline entity or closed Curve (Circle,..). For example, if the entity is an Arc, I can draw a Line from the Arc's start point to its end point, then use these 2 entities to generate a Region; for DBText or MText, I can use its bounding box as a Polyline to generate the Region. But things could become very complicated if the overlapped entities could be any possible type of entities.

Oh, beside the usual 3 AutoCAD .NET API assemblies, the project needs to set reference to BREP API assembly (acdbmgdbrep.dll).

Extract AutoCAD Civil3D's Label Text

I have been doing much less writing on my AutoCAD programming during past 2 years, because of a). I have been quite busy at work, thus much less time I can spend on the topics I might have interest to dig in; 2). since I became master marathon runner (in my late 50!) a couple years ago, most my out-of-work time has been running, endless running (50 to 70 km/week, 4 to 6 formal races/year).

Anyway, back to AutoCAD programming. My company moved from AutoCAD Map to AutoCAD Civil3D since AutoCAD 2015 a while ago. Naturally, some custom programming task against AutoCAD Civil3D features became my job description, which exposes me to a vast new realm that I did not touch before.

Recently, I was ask to provide a way to extract Civil3D label's text. Labels in Civil3D are very powerful and complicated annotation objects. As a "newbie" Civil3D programmer, I spent quite sometime in vain to search through Civl3D's API: nothing came up to let me retrieve the displayed text string of a label.

Eventually, I found this article "Get CogoPoint Label Text" in ADN blogs "Infrastructure Modeling DevBlog", by Augosto Goncalves.

The code shown in that article worked quite well, except that if a label's text is stacked (e.g. the label text is displayed as multiple lines of text), there would be missing space between the text string segment. See this video clip.

The "missing space" issue can be easily fixed by adding a space when retrieved text string segment being combined with previously retrieved text string segment. See the the code below, which is quoted from that article with minor modification (in red):

private string GetText(ObjectId id)
{
    // store the DBTexts
    StringBuilder entityText = new StringBuilder();
 
    Database db = Application.DocumentManager.
        MdiActiveDocument.Database;
    using (Transaction trans = db.
        TransactionManager.StartTransaction())
    {
        // open the entity
        Entity point = trans.GetObject(id,
            OpenMode.ForRead) as Entity;
 
        // do a full explode (considering explode again
        // all BlockReferences and MText)
        List<DBObject> objs = FullExplode(point);
        foreach (Entity ent in objs)
        {
            // now get the text of each DBText
            if (ent.GetType() == typeof(DBText))
            {
                DBText text = ent as DBText;
                entityText.AppendLine(" " + text.TextString);
            }
        }
        trans.Commit();
    }
 
    return entityText.ToString().Trim();
}

See this video clip showing the result of modified code. I was quite happy of adopting this code in my work to allow my program retrieve Civil3D label's displayed text, well, until I ran into "curved" labels (e.g. labels used to annotate curved line work, in which the text characters are aligned along the curve) - all the spaces between text segment that make the text string read-able were gone. Actually, when running my modified code shown above, a space added between every character of the label text.

See this video clip showing the result of retrieved label text on "curved" label.

To investigate the cause, I manually did the recursive exploding of a label, as the code does, either a straight one, or curved one. The manual exploding revealed:

• With straight label, after recursive exploding I end up with one or more DBText entities. Thus, I can combine all the DBTexts' TextString value with space in between to eventually  assemble a text string as the label's displayed text.
• With curved string, after the recursively exploding, the label is also eventually exploded into DBText entities with each single character as a DBText, thus the resulted DBTexts effectively lose their literal meaning.

I have to say that whoever created Civil3D label, he/she must be incredibly talented and invented the way to use curved label to annotate curved entities. But how do I get the labels displayed text, so that it still conveys the same literal meaning as the label? 

It turned out a solution came out rather easily when I discuss the difficulty I was facing (with the curved label) with an experienced Civil3D user, showing how a label ends up with after repeated explode. He suggests I drag the curved label first before the attempt of retrieving label text, because dragging a curved label makes it "straight" label. So, the solution is programmtically dragging the label, retrieving the label text with recursive exploding, and then placing the dragged label back (resetting the label).

Here is the code I wrote, based Augosto's, which drags a curved label before exploding it in memory, and then place the label back (to curved style):

using System;
using System.Collections.Generic;
using System.Text;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.Geometry;
using CadDb = Autodesk.AutoCAD.DatabaseServices;
using CivilDb = Autodesk.Civil.DatabaseServices;
 
namespace WSP.GEO.UDS.Civil3dUtilities
{
    public static class LabelTextExtractor
    {
        public static string GetDisplayedLabelText(CadDb.ObjectId labelId)
        {
            if (labelId.ObjectClass.DxfName.ToUpper()!="AECC_GENERAL_SEGMENT_LABEL")
            {
                throw new ArgumentException(
                    "argument mismatch: not an \"AECC_GENERAL_SEGMENT_LABEL\"");
            }
 
            StringBuilder lblText = new StringBuilder();
 
            using (var tran=labelId.Database.TransactionManager.StartTransaction())
            {
                var label = tran.GetObject(labelId, OpenMode.ForRead) as CivilDb.Label;
                if (label!=null)
                {
                    bool changed = !label.Dragged && label.AllowsDragging;
                    try
                    {
                        if (changed)
                        {
                            label.UpgradeOpen();
                            double delta = label.StartPoint.DistanceTo(label.EndPoint);
                            label.LabelLocation = 
                                new Point3d(label.LabelLocation.X + 
                                    delta, label.LabelLocation.Y + 
                                    delta, label.LabelLocation.Z);
                        }
 
                        var dbObjs = FullExplode(label);
                        foreach (var obj in dbObjs)
                        {
                            if (obj.GetType() == typeof(DBText))
                            {
                                lblText.Append(" " + (obj as DBText).TextString);
                            }
 
                            obj.Dispose();
                        }
                    }
                    finally
                    {
                        if (changed) label.ResetLocation();
                    }
                }
 
                tran.Commit();
            }
 
            return lblText.ToString().Trim();
        }
 
        #region private methods
 
        private static List<CadDb.DBObject> FullExplode(CadDb.Entity ent)
        {
            // final result
            List<CadDb.DBObject> fullList = new List<CadDb.DBObject>();
 
            // explode the entity
            DBObjectCollection explodedObjects = new DBObjectCollection();
            ent.Explode(explodedObjects);
            foreach (CadDb.Entity explodedObj in explodedObjects)
            {
                // if the exploded entity is a blockref or mtext
                // then explode again
                if (explodedObj.GetType() == typeof(CadDb.BlockReference) ||
                    explodedObj.GetType() == typeof(CadDb.MText))
                {
                    fullList.AddRange(FullExplode(explodedObj));
                }
                else
                    fullList.Add(explodedObj);
            }
            return fullList;
        }
 
        #endregion
    }
}

As the code shows, I do not even care if the label is straight or curved. I simply test if the label is allowed to be dragged and if yes, if being dragged. If both are yes, programmatically drag it before doing the recursive explodes. Afterwards, I rest the label's location, if it has been programmatically dragged.

Here is the video clip showing the result of retrieving label's text.