Summary: Demonstrates how to split the edges of breps using the Rhino SDK.

I use the SplitEdge command to divide the edges of a surface at designated locations. Is there a way to do this from a plug-in?

The following function will split the edge of a brep at at a given location.

 /*

Description:
  Splits a brep edge into two edges.
Parameters:
  brep       - [in/out] The brep to modify.
  edge_index - [in] The index of the edge to split.
  edge_t     - [in] The parameter on the edge to split at.
Returns:
  True if successful, false otherwise.
*/
static bool SplitBrepEdge( ON_Brep& brep, int edge_index, double edge_t )
{
  bool rc = true;
  const ON_BrepEdge& edge = brep.m_E[edge_index];
 
  int trim_count = edge.TrimCount();
  ON_SimpleArray<double> trim_t( trim_count );
  trim_t.SetCount( trim_count );
 
  int i;
  for( i = 0; i < trim_count; i++ )
  {
    // Given a trim and parameter on the corresponding 3d edge,
    // get the corresponding parameter on the 2d trim curve.
    double t = 0.0;
    rc = brep.GetTrimParameter( edge.m_ti[i], edge_t, &t );
    if( rc )
      trim_t[i] = t;
    else
      break;
  }
 
  if( rc )
  {
    // Splits an edge into two edges.  The input edge
    // becomes the left portion and a new edge is created
    // for the right portion.
    rc = brep.SplitEdge( edge_index, edge_t, trim_t, -1 );
 
    // Delete any unreferenced objects from the brep arrays, 
    // reindexes as needed, and shrinks arrays to minimum required size.
    brep.Compact();
 
    // Set the brep's vertex tolerances.
    brep.SetVertexTolerances( true );
  }
 
  return rc;
}

You can use the above function as follows:

CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
{
  // Don't split kinky surfaces...
  CRhinoKeepKinkySurfaces keep_kinky_srfs;
 
  CRhinoGetObject go;
  go.SetCommandPrompt( L"Select edge to split" );
  go.SetGeometryFilter( CRhinoGetObject::curve_object );
  go.SetGeometryAttributeFilter( CRhinoGetObject::edge_curve );
  go.EnableReferenceObjectSelect( false );
  go.GetObjects( 1, 1 );
  if( go.CommandResult() != success )
    return go.CommandResult();
 
  const CRhinoObjRef objref = go.Object(0);
  const CRhinoObject* obj = objref.Object();
  const ON_BrepEdge* edge = objref.Edge();
  const ON_Brep* brep = objref.Brep();
  if( 0 == obj | 0 == edge | 0 == brep )
    return failure;
 
  CRhinoGetPoint gp;
  gp.SetCommandPrompt( L"Point to split edge" );
  gp.Constrain( *edge );
  gp.GetPoint();
  if( gp.CommandResult() != success )
    return gp.CommandResult();
 
  const ON_BrepTrim* trim = 0;
  double edge_t = 0;
  edge = gp.PointOnEdge( &edge_t, trim );
  if( edge )
  {
    ON_Brep newbrep( *brep );
    if( SplitBrepEdge(newbrep, edge->m_edge_index, edge_t) )
    {
      context.m_doc.ReplaceObject( CRhinoObjRef(obj), newbrep );
      context.m_doc.Redraw();
    }
  }
 
  return success;
}