Dynamically Drawing Geometry when Picking Points

This guide demonstrates how to derive a new class to dynamically draw geometry during a point picking operation using C/C++.


When using Rhino, you have probably noticed that many of the object creation commands, such as Line and Circle, and transformation commands, such as Move and Copy, dynamically draw objects as they are being created or transformed. This operation is performed by deriving a new class from Rhino’s point pick class, CRhinoGetPoint, and overriding two member functions: OnMouseMove() and DynamicDraw().

OnMouseMove() is called every time the mouse moves. This is a great place to perform calculations, such as transformations.

DynamicDraw() is called as the mouse moves, as well. Every time the mouse moves, DynamicDraw() will be called once per viewport.

NOTE: Rhino calls DynamicDraw() happen after the call to OnMouseMove().


The following sample demonstrates how to derive a new class from CRhinoGetPoint and override OnMouseMove() and DynamicDraw() to dynamically draw geometry. In this sample, we are going to dynamically draw a circle while the user is specifying its radius.

class CGetCircleRadiusPoint : public CRhinoGetPoint
  void SetCenterPoint( const ON_3dPoint center_point );
  bool CalculateCircle( CRhinoViewport& vp, const ON_3dPoint& pt );

  void OnMouseMove( CRhinoViewport& vp,
                    UINT flags,
                    const ON_3dPoint& pt,
                    const CPoint* pt2d );

  void DynamicDraw( HDC hDC, CRhinoViewport& vp, const ON_3dPoint& pt );
  const ON_Circle& Circle() const;
  ON_Circle m_circle;
  ON_3dPoint m_center_point;
  bool m_draw_circle;

  m_draw_circle = false;

void CGetCircleRadiusPoint::SetCenterPoint( const ON_3dPoint center_point )
  m_center_point = center_point;

bool CGetCircleRadiusPoint::CalculateCircle( CRhinoViewport& vp, const ON_3dPoint& pt )
  double radius = m_center_point.DistanceTo( pt );
  if( radius < ON_SQRT_EPSILON )
    return false;

  ON_Plane plane = vp.ConstructionPlane().m_plane;
  plane.SetOrigin( m_center_point );
  m_circle.Create( plane, radius );
  return m_circle.IsValid() ? true : false;

void CGetCircleRadiusPoint::OnMouseMove( CRhinoViewport& vp,
                                         UINT flags,
                                         const ON_3dPoint& pt,
                                         const CPoint* pt2d )
  m_draw_circle = CalculateCircle( vp, pt );
  CRhinoGetPoint::OnMouseMove( vp, flags, pt, pt2d );

void CGetCircleRadiusPoint::DynamicDraw( HDC hDC,
                                         CRhinoViewport& vp,
                                         const ON_3dPoint& pt )
  if( m_draw_circle )
    ON_Color color = RhinoApp().AppSettings().TrackingColor();
    ON_Color saved_color = vp.SetDrawColor( color );
    vp.DrawCircle( m_circle );
    vp.SetDrawColor( saved_color );
  CRhinoGetPoint::DynamicDraw( hDC, vp, pt );
const ON_Circle& CGetCircleRadiusPoint::Circle() const
  return m_circle;

Finally, here is our CRhinoGetPoint derived class in action:

CRhinoCommand::result CCommandTest::RunCommand(const CRhinoCommandContext& context)
  CRhinoGetPoint gp;
  gp.SetCommandPrompt( L"Center point" );
  gp.ConstrainToConstructionPlane( FALSE );
  if( gp.CommandResult() != CRhinoCommand::success )
    return gp.CommandResult();

  ON_3dPoint center_point = gp.Point();

  CGetCircleRadiusPoint gc;
  gc.SetCommandPrompt( L"Radius" );
  gc.ConstrainToConstructionPlane( FALSE );
  gc.SetBasePoint( center_point );
  gc.DrawLineFromPoint( center_point, TRUE );
  gc.SetCenterPoint( center_point );
  if( gc.CommandResult() != CRhinoCommand::success )
    return gc.CommandResult();

  if( gc.CalculateCircle( gc.View()->Viewport(), gc.Point() ))
    ON_Circle circle = gc.Circle();
    context.m_doc.AddCurveObject( circle );
  return CRhinoCommand::success;