Testing whether a point is inside or above an AutoCAD solid using .NET

Today’s post resulted from an internal discussion: Miroslav Schonauer – with the help of Jan Liska – put together some code for a recent consulting engagement that they felt was important to share with the community. They wanted to test point containment for a particular 3D solid, but also to test whether the selected point – if outside the solid – was above it.

They achieved this using AutoCAD’s Brep API. Here is the C# they put together (with some minor, cosmetic edits from my side):

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.BoundaryRepresentation;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;
using System;
 
namespace BrepTest
{
  public class Commands
  {
    [CommandMethod("PISO")]
    public void PointInSolid()
    {
      var doc = Application.DocumentManager.MdiActiveDocument;
      var db = doc.Database;
      var ed = doc.Editor;
 
      try
      {
        // Select Solid3d
 
        var peo = new PromptEntityOptions("\nSelect a 3D solid");
        peo.SetRejectMessage("\nMust be a 3D solid.");
        peo.AddAllowedClass(typeof(Solid3d), true);
 
        var per = ed.GetEntity(peo);
        if (per.Status != PromptStatus.OK)
          return;
 
        using (var tr = db.TransactionManager.StartTransaction())
        {
          // Open the selected Solid3d
 
          var sol = tr.GetObject(per.ObjectId, OpenMode.ForRead) as Solid3d;
          if (sol != null) // Should always be true
          { 
            // Create the sub-entity path to it
 
            var ids = new ObjectId[] { per.ObjectId };
            var path =
              new FullSubentityPath(
                ids, new SubentityId(SubentityType.Null, IntPtr.Zero)
              );
 
            // Create a Brep from the path
 
            using (var brep = new Brep(path))
            {
              while (true)
              {
                // Select point
 
                var ppr = ed.GetPoint("\nSelect a point");
                if (ppr.Status != PromptStatus.OK)
                  break;
                var pt = ppr.Value;
 
                // 1) Check point containment w.r.t selected solid
 
                var pc = new PointContainment();
                using (var brepEnt = brep.GetPointContainment(pt, out pc))
                {
                  ed.WriteMessage("\nPt {0}: {1}", pt, pc);
                }
 
                // 2) If Outside, check whether the solid is "below":
 
                if (pc == PointContainment.Outside)
                {
                  // Fire a -Z Ray from the point to find if it hits the Brep
 
                  var ray = new Ray3d(pt, -Vector3d.ZAxis);
                  var hits = brep.GetLineContainment(ray, 1);
 
                  if (hits == null || hits.Length == 0)
                  {
                    ed.WriteMessage("\nSolid is NOT below point.");
                  }
                  else
                  {
                    ed.WriteMessage(
                      "\nSolid IS below point, distance = {0}",
                      pt.DistanceTo(hits[0].Point)
                    );
                  }
                }
              }
            }
          }
          tr.Commit();
        }
      }
      catch (System.Exception ex)
      {
        ed.WriteMessage("Error: {0}\r\n at {1}", ex.Message, ex.StackTrace);
      }
    }
  }
}

To give it a try, just run the PISO command (for PointInSolid), then select a Solid3d and start picking points. You’ll see on the application report via the command-line whether the selected point is contained within the solids and, if not, whether the point is above it (i.e. that the solid is below the point).

A big thanks to Miro & Jan for sharing this useful code! :-)