Create Principal Curvature Curves

This guide demonstrates how to use the ON_EvPrincipalCurvatures function in C/C++.

Problem

You are looking for a way to create Principal Curvature lines starting with points on a surface. There is an ON an ON_EvPrincipalCurvatures function, it’s not clear how it should be used.

Solution

The Rhino C/C++ SDK does not have a function to create these Principal Curvature curves. But, using ON_EvPrincipalCurvatures, you can calculate them.

Before using ON_EvPrincipalCurvatures, you will need to calculate the second derivative of the surface that the test location. Then, it is just a matter or cooking up some curves based on the results.

Sample

The following is an example of how you might write such a command…

CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
{
  CRhinoGetObject go;
  go.SetCommandPrompt( L"Select surface" );
  go.SetGeometryFilter( CRhinoGetObject::surface_object );
  go.GetObjects( 1, 1 );
  if( go.CommandResult() != CRhinoCommand::success )
    return go.CommandResult();

  const CRhinoObject* obj = go.Object(0).Object();
  const ON_BrepFace* face = go.Object(0).Face();
  if( 0 == obj | 0 == face )
    return CRhinoCommand::failure;

  CRhinoGetPoint gp;
  gp.SetCommandPrompt( L"Select point on surface" );
  gp.Constrain( *face, obj->Attributes().m_wire_density );
  gp.GetPoint();
  if( gp.CommandResult() != CRhinoCommand::success )
    return gp.CommandResult();

  double s, t;
  const ON_Surface* srf = gp.PointOnSurface( &s, &t );
  if( 0 == srf )
    return CRhinoCommand::failure;

  ON_3dPoint P;
  ON_3dVector Ds, Dt, Dss, Dst, Dtt;
  if( !srf->Ev2Der(s, t, P, Ds, Dt, Dss, Dst, Dtt) )
    return CRhinoCommand::failure; // failed to evaluate derivatives

  ON_3dVector N;
  if( !srf->EvNormal(s, t, N) )
    return CRhinoCommand::failure; // failed to evaluate normal

  double gauss, mean, k[2];
  ON_3dVector K[2];
  if( !ON_EvPrincipalCurvatures(Ds, Dt, Dss, Dst, Dtt, N, &gauss, &mean, &k[0], &k[1], K[0], K[1]) )
    return CRhinoCommand::failure; // failed to evaluate principal curvatures

  int i;
  for( i = 0; i < 2; i++ )
  {
    if( fabs(k[i]) <= 1.0e-4 | fabs(k[i]) >= 1.0e4 )
    {
      // just draw a line as curvature is huge/tiny
      ON_Line line( P - K[i] * 5.0, P + K[i] * 5.0 );
      context.m_doc.AddCurveObject( line );
    }
    else
    {
      double r = 1.0 / k[i];
      ON_3dPoint center = P + r * N;
      ON_3dPoint start = center - r * K[i];
      ON_3dPoint end = center + r * K[i];
      ON_Arc arc( start, P, end );
      context.m_doc.AddCurveObject( arc );
    }
  }
  context.m_doc.Redraw();

  return CRhinoCommand::success;
}