Aligning entity using 3 points

By Balaji Ramamoorthy

Here is a sample code to align a selected entity based on three source and their corresponding destination points. The order in which the point are selected is shown in the screenshot and a sample drawing to try the the code snippet.

Document activeDoc = Application.DocumentManager.MdiActiveDocument;
Database db = activeDoc.Database;
Editor ed = activeDoc.Editor;
 
// Prompt for entity selection
PromptEntityResult per 
= ed.GetEntity(new PromptEntityOptions("Select an entity : "));
if (per.Status != PromptStatus.OK)
    return;
 
ObjectId oid = per.ObjectId;
 
Matrix3d ucs2wcs = ed.CurrentUserCoordinateSystem;
 
// 3 source points
Point3d srcpt1 = Point3d.Origin;
Point3d srcpt2 = Point3d.Origin;
Point3d srcpt3 = Point3d.Origin;
 
// 3 destination points
Point3d destpt1 = Point3d.Origin;
Point3d destpt2 = Point3d.Origin;
Point3d destpt3 = Point3d.Origin;
 
// Source point 1
PromptPointResult ppr1 
    = ed.GetPoint(new PromptPointOptions("Select src point 1"));
if (ppr1.Status != PromptStatus.OK)
    return;
srcpt1 = ppr1.Value.TransformBy(ucs2wcs);
 
// Destination point 1
ppr1 = ed.GetPoint(new PromptPointOptions("Select dest point 1"));
if (ppr1.Status != PromptStatus.OK)
    return;
destpt1 = ppr1.Value.TransformBy(ucs2wcs);
 
// Display transient line to show the selection of points
IntegerCollection coll = new IntegerCollection();
Line tmpline1 = new Line(srcpt1, destpt1);
TransientManager.CurrentTransientManager.AddTransient
   (tmpline1, TransientDrawingMode.DirectShortTerm, 128, coll);
 
// Source point 2
PromptPointResult ppr2 
    = ed.GetPoint(new PromptPointOptions("Select src point 2"));
if (ppr2.Status != PromptStatus.OK)
    return;
srcpt2 = ppr2.Value.TransformBy(ucs2wcs);
 
// Destination point 2
ppr2 = ed.GetPoint(new PromptPointOptions("Select dest point 2"));
if (ppr2.Status != PromptStatus.OK)
    return;
destpt2 = ppr2.Value.TransformBy(ucs2wcs);
 
// Display transient line to show the selection of points
Line tmpline2 = new Line(srcpt2, destpt2);
TransientManager.CurrentTransientManager.AddTransient
    (tmpline2, TransientDrawingMode.DirectShortTerm, 128, coll);
 
// Source point 3
PromptPointResult ppr3 
    = ed.GetPoint(new PromptPointOptions("Select src point 3"));
if (ppr3.Status != PromptStatus.OK)
    return;
srcpt3 = ppr3.Value.TransformBy(ucs2wcs);
 
// Destination point 3
ppr3 = ed.GetPoint(new PromptPointOptions("Select dest point 3"));
if (ppr3.Status != PromptStatus.OK)
    return;
destpt3 = ppr3.Value.TransformBy(ucs2wcs);
 
// Display transient line to show the selection of points
Line tmpline3 = new Line(srcpt3, destpt3);
TransientManager.CurrentTransientManager.AddTransient
    (tmpline3, TransientDrawingMode.DirectShortTerm, 128, coll);
 
try
{
    Point3d fromOrigin = srcpt1;
    Vector3d fromXaxis = srcpt2 - srcpt1;
    Vector3d fromYaxis = srcpt3 - srcpt1;
    Vector3d fromZaxis = fromXaxis.CrossProduct(fromYaxis);
 
    Point3d toOrigin = destpt1;
    Vector3d toXaxis = 
            (destpt2 - destpt1).GetNormal() * fromXaxis.Length;
    Vector3d toYaxis 
           = (destpt3 - destpt1).GetNormal() * fromYaxis.Length;
    Vector3d toZaxis = toXaxis.CrossProduct(toYaxis);
 
    // Get the transformation matrix for aligning coordinate systems
    Matrix3d mat = new Matrix3d();
    mat = Matrix3d.AlignCoordinateSystem(
                                            fromOrigin, 
                                            fromXaxis, 
                                            fromYaxis, 
                                            fromZaxis, 
                                            toOrigin, 
                                            toXaxis, 
                                            toYaxis, 
                                            toZaxis
                                        );
 
    // Transform the selected entity
    using (Transaction tr = db.TransactionManager.StartTransaction())
    {
        Entity ent = tr.GetObject(oid, OpenMode.ForWrite) as Entity;
        ent.TransformBy(mat);
        tr.Commit();
    }
}
catch (Autodesk.AutoCAD.Runtime.Exception ex )
{
    ed.WriteMessage(ex.Message);
}
 
// Remove the transient lines
TransientManager.CurrentTransientManager.EraseTransient
                                              (tmpline1, coll);
tmpline1.Dispose();
 
TransientManager.CurrentTransientManager.EraseTransient
                                              (tmpline2, coll);
tmpline2.Dispose();
 
TransientManager.CurrentTransientManager.EraseTransient
                                              (tmpline3, coll);
tmpline3.Dispose();

The screenshots show the order of selected points and the aligned entity

Here is a sample drawing to try the sample code :

Download Test