Summary: Demonstrates how to project points onto Brep objects.

Is there a way that I can project a 2-d point (x,y) onto a brep object, using the Rhino SDK, in order to acquire the z-coordinate?

Use the RhinoProjectPointsToBreps SDK function. In order to use this function you will need to provide the following:

1. An array of one or more Brep objects.
2. An array of one or more points to project.
3. A projection direction (vector).

The following sample code demonstrates how you can use this function to accomplish what you want.

 
CRhinoCommand::result CCommandFoobarCpp::RunCommand( const CRhinoCommandContext& context )
{
  CRhinoGetObject go;
  go.SetCommandPrompt(L"Select surface or polysurface");
  go.SetGeometryFilter(
    CRhinoGetObject::surface_object | 
    CRhinoGetObject::polysrf_object
    );
  go.GetObjects(1, 1);
  if (go.CommandResult() != CRhinoCommand::success)
    return go.CommandResult();
 
  const ON_Brep* brep = go.Object(0).Brep();
  if (0 == brep)
    return CRhinoCommand::failure;
 
  ON_3dPoint point(0.0, 0.0, 0.0); // some point on the world x-y plane
 
  // Prepare input to RhinoProjectPointsToBreps
 
  ON_SimpleArray<const ON_Brep*> Breps;
  Breps.Append(brep);
 
  ON_3dPointArray Points;
  Points.Append(point);
 
  ON_3dVector ProjDir(0.0, 0.0, 1.0); // world z-axis
 
  ON_3dPointArray OutPoints;
  ON_SimpleArray<int> Indices;
 
  bool rc = RhinoProjectPointsToBreps(
    Breps, 
    Points, 
    ProjDir, 
    OutPoints, 
    Indices,
    context.m_doc.AbsoluteTolerance()
    );
 
  if (rc == true)
  {
    for (int i = 0; i < OutPoints.Count(); i++)
    {
      ON_3dPoint pt = OutPoints[i];
      context.m_doc.AddPointObject(pt);
    }
    context.m_doc.Redraw();
  }
 
  return CRhinoCommand::success;
}

 
public override IRhinoCommand.result RunCommand(IRhinoCommandContext context)
{
  MRhinoGetObject go = new MRhinoGetObject();
  go.SetCommandPrompt("Select surface or polysurface");
  go.SetGeometryFilter(
    IRhinoGetObject.GEOMETRY_TYPE_FILTER.surface_object | 
    IRhinoGetObject.GEOMETRY_TYPE_FILTER.polysrf_object
    );
  go.GetObjects(1, 1);
  if (go.CommandResult() != IRhinoCommand.result.success)
    return go.CommandResult();
 
  IOnBrep brep = go.Object(0).Brep();
  if (brep == null)
    return IRhinoCommand.result.failure;
 
  On3dPoint point = new On3dPoint(0.0, 0.0, 0.0); // some point on the world x-y plane
 
  // Prepare input to RhinoProjectPointsToBreps
 
  IOnBrep[] Breps = new IOnBrep[1];
  Breps[0] = brep;
 
  On3dPointArray Points = new On3dPointArray();
  Points.Append(point);
 
  On3dVector ProjDir = new On3dVector(0.0, 0.0, 1.0); // world z-axis
 
  On3dPointArray OutPoints = new On3dPointArray();
  Arrayint Indices = new Arrayint();
 
  // Do the projection
  bool rc = RhUtil.RhinoProjectPointsToBreps(
    Breps, 
    Points, 
    ProjDir, 
    out OutPoints, 
    out Indices,
    context.m_doc.AbsoluteTolerance()
    );
 
  if (rc == true)
  {
    for (int i = 0; i < OutPoints.Count(); i++)
    {
      On3dPoint pt = OutPoints[i];
      context.m_doc.AddPointObject(pt);
    }
    context.m_doc.Redraw();
  }
 
  return IRhinoCommand.result.success;
}