The index of the triangle's first vertex. 
   Namespace:   Autodesk.Revit.DB.DirectContext3D  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 18.0.0.0 (18.2.0.0) 
  Since:  2017 
  Namespace:   Autodesk.Revit.DB.DirectContext3D  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 18.0.0.0 (18.2.0.0) 
  Since:  2017