Topic: Mobius Strip in C#

[Dale Bartlett]

Further to this post: [Mobius strip in AutoCAD 2008 or 2009 by C#], does anyone have code that builds a correct single-sided strip? This seems impossible although the old post indicates otherwise. Regards, Dale

[gile]

Hi,

I do not think it is possible to create a 'true' Moebius strip with AutoCAD.
But it is possible to use the artefact shown here by luke: overlapping two lofted surfaces to 'close' the strip.

Here's a F# implementation:

Code - F#: [Select]

1. module MoebiusStrip
2.  
3. open System
4. open Autodesk.AutoCAD.DatabaseServices
5. open Autodesk.AutoCAD.Geometry
6. open Autodesk.AutoCAD.Runtime
7.  
8. [<CommandMethod("Test")>]
9. let test () =
10.     let computePoint angle halfWidth =
11.         new Point3d(
12.             (100. + halfWidth * cos (angle / 2.)) * cos angle,
13.             (100. + halfWidth * cos (angle / 2.)) * sin angle,
14.             halfWidth * sin (angle / 2.))
15.     let angle = Math.PI / 15.
16.     let db = HostApplicationServices.WorkingDatabase
17.     use tr = db.TransactionManager.StartTransaction()
18.     let btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) :?> BlockTableRecord
19.     let lines =
20.         [| for i in 0 .. 30 do
21.             let a = angle * float i
22.             yield new Line(computePoint a 20., computePoint a -20.) :> Entity |]
23.     let addSurf (ents: Entity[]) =
24.         let surf = new LoftedSurface()
25.         surf.CreateLoftedSurface(ents, null, null, new LoftOptions())
26.         btr.AppendEntity(surf) |> ignore
27.         tr.AddNewlyCreatedDBObject(surf, true)
28.     addSurf lines.[0 .. 29]
29.     addSurf lines.[1 .. 30]
30.     lines |> Array.iter(fun l -> l.Dispose())
31.     tr.Commit()

And a C# implementation

Code - C#: [Select]

1. using System;
2. using System.Linq;
3. using Autodesk.AutoCAD.DatabaseServices;
4. using Autodesk.AutoCAD.Geometry;
5. using Autodesk.AutoCAD.Runtime;
6.  
7. [assembly: CommandClass(typeof(MoebiusStrip.CommandMethods))]
8.  
9. namespace MoebiusStrip
10. {
11.     public class CommandMethods
12.     {
13.         [CommandMethod("Test")]
14.         public void Test()
15.         {
16.             double angle = Math.PI / 15.0;
17.             Entity[] lines = new Entity[31];
18.             for (int i = 0; i < 31; i++)
19.             {
20.                 double a = i * angle;
21.                 lines[i] = new Line(ComputePoint(a, 20), ComputePoint(a, -20));
22.             }
23.             Database db = HostApplicationServices.WorkingDatabase;
24.             using (Transaction tr = db.TransactionManager.StartTransaction())
25.             {
26.                 BlockTableRecord btr =
27.                     (BlockTableRecord)tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite);
28.                 AddSurf(lines.Take(30).ToArray(), btr);
29.                 AddSurf(lines.Skip(1).ToArray(), btr);
30.                 foreach (Line l in lines)
31.                 {
32.                     l.Dispose();
33.                 }
34.                 tr.Commit();
35.             }
36.         }
37.  
38.         private void AddSurf(Entity[] ents, BlockTableRecord btr)
39.         {
40.             LoftedSurface surf = new LoftedSurface();
41.             surf.CreateLoftedSurface(ents, null, null, new LoftOptions());
42.             btr.AppendEntity(surf);
43.             btr.Database.TransactionManager.AddNewlyCreatedDBObject(surf, true);
44.         }
45.  
46.         private Point3d ComputePoint(double angle, double halfWidth)
47.         {
48.             return new Point3d(
49.                 (100 + halfWidth * Math.Cos(angle / 2.0)) * Math.Cos(angle),
50.                 (100 + halfWidth * Math.Cos(angle / 2.0)) * Math.Sin(angle),
51.                 halfWidth * Math.Sin(angle / 2.0));
52.         }
53.     }
54. }

[kaefer]

I do not think it is possible to create a 'true' Moebius strip with AutoCAD.

I think that may need some reservations to be true, like "it's not possible to create a Moebius strip with the .Net API". I've modified your approach to that end, creating two half-strips, and applied the UNION command manually.

I keep my fingers crossed that the modelling error reported by the UNION command doesn't indicate anything more serious than the loss of associativity.

Code - F#: [Select]

1.     ...
2.     let addSurf lines =
3.         let lsurf = new LoftedSurface(Closed = false)
4.         lsurf.CreateLoftedSurface(
5.             lines, null, null, new LoftOptions() )
6.         let oid = btr.AppendEntity lsurf
7.         tr.AddNewlyCreatedDBObject(lsurf, true)
8.     addSurf lines.[0 .. 15]
9.     addSurf lines.[15 .. 30]
10.     ...

[gile]

You're right kaefer.

The upper codes can be completed using SendStringtoExecute and it seems to 'close' the strip in spite of the displayed message.

Code - C#: [Select]

1. using System;
2. using System.Linq;
3. using Autodesk.AutoCAD.ApplicationServices;
4. using Autodesk.AutoCAD.DatabaseServices;
5. using Autodesk.AutoCAD.EditorInput;
6. using Autodesk.AutoCAD.Geometry;
7. using Autodesk.AutoCAD.Runtime;
8.  
9. [assembly: CommandClass(typeof(MoebiusStripCs.CommandMethods))]
10.  
11. namespace MoebiusStripCs
12. {
13.     public class CommandMethods
14.     {
15.         [CommandMethod("Test")]
16.         public void Test()
17.         {
18.             Document doc = Application.DocumentManager.MdiActiveDocument;
19.             Database db = doc.Database;
20.             Editor ed = doc.Editor;
21.             double angle = Math.PI / 15.0;
22.             Line[] lines = new Line[31];
23.             for (int i = 0; i < 31; i++)
24.             {
25.                 double a = i * angle;
26.                 lines[i] = new Line(ComputePoint(a, 20), ComputePoint(a, -20));
27.             }
28.             using (Transaction tr = db.TransactionManager.StartTransaction())
29.             {
30.                 BlockTableRecord btr =
31.                     (BlockTableRecord)tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite);
32.                 ObjectId id1 = AddSurf(lines.Take(16).ToArray(), btr);
33.                 ObjectId id2 = AddSurf(lines.Skip(15).ToArray(), btr);
34.                 foreach (Line l in lines)
35.                 {
36.                     l.Dispose();
37.                 }
38.                 ed.SetImpliedSelection(new ObjectId[] { id1, id2 });
39.                 tr.Commit();
40.                 doc.SendStringToExecute("_union _previous \n", false, false, false);
41.             }
42.         }
43.  
44.         private ObjectId AddSurf(Entity[] ents, BlockTableRecord btr)
45.         {
46.             LoftedSurface surf = new LoftedSurface();
47.             surf.CreateLoftedSurface(ents, null, null, new LoftOptions());
48.             ObjectId id = btr.AppendEntity(surf);
49.             btr.Database.TransactionManager.AddNewlyCreatedDBObject(surf, true);
50.             return id;
51.         }
52.  
53.         private Point3d ComputePoint(double angle, double halfWidth)
54.         {
55.             return new Point3d(
56.                 (100 + halfWidth * Math.Cos(angle / 2.0)) * Math.Cos(angle),
57.                 (100 + halfWidth * Math.Cos(angle / 2.0)) * Math.Sin(angle),
58.                 halfWidth * Math.Sin(angle / 2.0));
59.         }
60.     }
61. }
62.  

[gile]

It seems to work fine with solid3d unioning

Code - F#: [Select]

1. module MoebiusStrip
2.  
3. open System
4. open Autodesk.AutoCAD.ApplicationServices
5. open Autodesk.AutoCAD.DatabaseServices
6. open Autodesk.AutoCAD.EditorInput
7. open Autodesk.AutoCAD.Geometry
8. open Autodesk.AutoCAD.Runtime
9.  
10. [<CommandMethod("Moebius")>]
11. let MoebiusStrip () =
12.     let doc = Application.DocumentManager.MdiActiveDocument
13.     let ed = doc.Editor
14.     let db = doc.Database
15.  
16.     let computePoint angle halfWidth =
17.         new Point3d(
18.             (100. + halfWidth * cos (angle / 2.)) * cos angle,
19.             (100. + halfWidth * cos (angle / 2.)) * sin angle,
20.             halfWidth * sin (angle / 2.))
21.  
22.     let angle = Math.PI / 15.
23.     use tr = db.TransactionManager.StartTransaction()
24.     let btr = tr.GetObject(db.CurrentSpaceId, OpenMode.ForWrite) :?> BlockTableRecord
25.  
26.     let addSolid lines =
27.        use lsurf = new LoftedSurface()
28.        lsurf.CreateLoftedSurface(lines, null, null, new LoftOptions())
29.        let solid =lsurf.Thicken(2., true)
30.        let id = btr.AppendEntity solid
31.        tr.AddNewlyCreatedDBObject(solid, true)
32.        solid
33.  
34.     let lines =
35.         [| for i in 0 .. 30 do
36.             let a = angle * float i
37.             yield new Line(computePoint a 20., computePoint a -20.) :> Entity |]
38.     let solid1 = addSolid lines.[0 .. 15]
39.     let solid2 = addSolid lines.[15 .. 30]
40.     lines |> Array.iter(fun l -> l.Dispose())
41.     solid1.BooleanOperation(BooleanOperationType.BoolUnite, solid2)
42.     tr.Commit()

[qjchen]

Thank you, gile and kaefer for your sharing.

I have ever use mesh to generate for such type entity

in
http://www.theswamp.org/index.php?topic=41630.msg467330#msg467330

but not solid.