Dynamically Drawing Text Strings

This guide demonstrates how to dynamically draw text strings using C/C++.


On occasion, it is useful to dynamically display some text while in the middle of a point picking operation. Rhino’s VariableFilletSrf command is a good example of a command that does this.

To add this capability to an plugin command, you need to:

  1. Derive a new class from CRhinoGetPoint.
  2. Override the CRhinoGetPoint::DynamicDraw virtual function.
  3. From within the DynamicDraw override, call CRhinoViewport::DrawString.


The following example code demonstrates how to derive a new class from CRhinoGetPoint, override the CRhinoGetPoint::DynamicDraw member, and draw text dynamically.

class CDrawStringGetPoint : public CRhinoGetPoint
  CDrawStringGetPoint() {}
  void DynamicDraw( HDC hdc, CRhinoViewport& vp, const ON_3dPoint& pt );

void CDrawStringGetPoint::DynamicDraw( HDC hdc, CRhinoViewport& vp, const ON_3dPoint& pt )
  // Format active point as a string
  ON_wString str;
  RhinoFormatPoint( pt, str );

  // Build world-to-screen coordinate transformation
  ON_Xform w2s;
  vp.VP().GetXform( ON::world_cs, ON::screen_cs, w2s );

  // Transform point from world to screen coordinates
  ON_3dPoint screenpoint = w2s * pt;

  // Offset point so text does not overlap cursor
  screenpoint.x += 5.0;
  screenpoint.y += -5.0;

  // Draw string using the system font
  vp.DrawString( str, str.Length(), screenpoint, false, 0, 12, L"System" );

  // Allow base class to draw
  CRhinoGetPoint::DynamicDraw( hdc, vp, pt );

You can use the above class as you would a CRhinoGetPoint object. Just create a new CDrawStringGetPoint object, initialize the class by calling base class members, and call it’s GetPoint member. For example:

CRhinoCommand::result CCommandTest::RunCommand( const CRhinoCommandContext& context )
  CDrawStringGetPoint gp;
  gp.SetCommandPrompt( L"Pick test point" );
  if( gp.CommandResult() != success )
    return gp.CommandResult();

  // TODO...

  return success;