Summary: Demonstrates how to move mesh vertices using the Rhino SDK.

How can I modify a particular point, or vertex, of CRhinoMeshObject object?

A CRhinoMeshObject's geometric data member is an ON_Mesh object. For information on the ON_Mesh class, the the SDK's opennurbs_mesh.h header file.

Mesh vertices are stored on an ON_Mesh in an m_V data member, which is simply an array of points. So, if you want to modify the vertices of a mesh, you need to modify the data in this array.

I should also add that in order to modify anything in Rhino, you might:

1. Get the object.
2. Make a copy of the object.
3. Modify this copied object.
4. Call one of the CRhinoDoc::ReplaceObject overrides to update the object.

The following sample demonstrates how you might do this from a command.

CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
{
  CRhinoGetObject gv;
  gv.SetCommandPrompt( L"Select mesh vertex to move" );
  gv.SetGeometryFilter( CRhinoGetObject::meshvertex_object );
  gv.GetObjects( 1, 1 );
  if( gv.CommandResult() != success )
    return gv.CommandResult();
 
  const CRhinoObject* obj = gv.Object(0).Object();
  const ON_MeshVertexRef* vertex = gv.Object(0).MeshVertex();
  if( 0 == obj | 0 == vertex )
    return failure;
 
  const ON_Mesh* mesh = vertex->m_mesh;
  if( 0 == mesh )
   return failure;
 
  ON_3dPoint pt = mesh->m_V[vertex->m_mesh_vi];
 
  CRhinoGetPoint gp;
  gp.SetCommandPrompt( L"New location" );
  gp.SetBasePoint( pt );
  gp.DrawLineFromPoint( pt, TRUE );
  gp.GetPoint();
  if( gp.CommandResult() != success )
    return gp.CommandResult();
 
  ON_Mesh dupe_mesh( *mesh );
  dupe_mesh.SetVertex( vertex->m_mesh_vi, gp.Point() );
 
  // Since we've modified ON_Mesh.m_V array, 
  // we need to invalidate a few things so they
  // can be recalculated based on the new data.
  dupe_mesh.InvalidateVertexBoundingBox();
  dupe_mesh.InvalidateVertexNormalBoundingBox();
  dupe_mesh.InvalidateCurvatureStats();
  dupe_mesh.m_FN.SetCount(0);
  dupe_mesh.m_N.SetCount(0);
  dupe_mesh.ComputeFaceNormals();
  dupe_mesh.ComputeVertexNormals();
  dupe_mesh.SetClosed(-1);
 
  if( dupe_mesh.IsValid() )
  {
    context.m_doc.ReplaceObject( CRhinoObjRef(obj), dupe_mesh );
    context.m_doc.Redraw();
  }
 
  return success;
}