Draft implementation of LOD API in OCCT:

- implemented interfaces for LOD at OpenGl, Graphic3d and MeshVS levels;
- added LOD managers at Graphic3d and OpenGl levels;
- simple mechanism of LOD switching is implemented in LOD manager;
- distance to COG metric is temporarily used;
- the metric could be redefined using custom LOD selectors;
- LODs are added to MeshVS_Mesh in Compute method and are listed as its data sources;
- added commands meshgenlod and meshlod to generate LODs for mesh and its topological representation
  and bind them to its OpenGl structures.

src/Graphic3d/Graphic3d_LOD.hxx

@@ -0,0 +1,96 @@
+// Created on: 2015-10-29
+// Created by: Varvara POSKONINA
+// Copyright (c) 2005-2014 OPEN CASCADE SAS
+//
+// This file is part of Open CASCADE Technology software library.
+//
+// This library is free software; you can redistribute it and/or modify it under
+// the terms of the GNU Lesser General Public License version 2.1 as published
+// by the Free Software Foundation, with special exception defined in the file
+// OCCT_LGPL_EXCEPTION.txt. Consult the file LICENSE_LGPL_21.txt included in OCCT
+// distribution for complete text of the license and disclaimer of any warranty.
+//
+// Alternatively, this file may be used under the terms of Open CASCADE
+// commercial license or contractual agreement.
+
+#ifndef _Graphic3d_LOD_Header
+#define _Graphic3d_LOD_Header
+
+#include <Graphic3d_Camera.hxx>
+#include <Graphic3d_SequenceOfGroup.hxx>
+#include <Graphic3d_Structure.hxx>
+#include <Standard.hxx>
+#include <Standard_Macro.hxx>
+#include <Standard_Transient.hxx>
+
+struct Graphic3d_RangeOfLOD
+{
+public:
+  Graphic3d_RangeOfLOD (const Standard_Real theFrom, const Standard_Real theTo)
+    : myTo (theTo),
+      myFrom (theFrom)
+  {
+    Standard_ASSERT_RAISE (theFrom < theTo,
+      "The upper boundary of the interval must be greater than lower one!");
+  }
+
+  Standard_Boolean IsIn (const Standard_Real theVal) const
+  {
+    return (myFrom < theVal) && (theVal < myTo);
+  }
+
+  Standard_Boolean IsLess (const Standard_Real theVal) const
+  {
+    return theVal < myFrom;
+  }
+
+  Standard_Boolean IsGreater (const Standard_Real theVal) const
+  {
+    return myTo < theVal;
+  }
+
+  bool operator < (const Graphic3d_RangeOfLOD& theOther) const
+  {
+    return myFrom < theOther.myFrom;
+  }
+
+private:
+  Standard_Real myFrom;
+  Standard_Real myTo;
+};
+
+class Graphic3d_LOD : public Standard_Transient
+{
+public:
+  Standard_EXPORT virtual ~Graphic3d_LOD();
+
+  Standard_EXPORT void SetRange (const Standard_Real theFrom, const Standard_Real theTo);
+
+  Standard_EXPORT const Graphic3d_RangeOfLOD& GetRange() const
+  {
+    return myRange;
+  }
+
+  Standard_EXPORT virtual Handle(Graphic3d_Group) NewGroup (const Handle(Graphic3d_Structure)& /*theParentStruct*/)
+  {
+    return NULL;
+  };
+
+  const Graphic3d_SequenceOfGroup& GetDrawGroups() const
+  {
+    return myGroups;
+  }
+
+  DEFINE_STANDARD_RTTIEXT (Graphic3d_LOD, Standard_Transient)
+
+protected:
+  Standard_EXPORT Graphic3d_LOD() : myRange (-DBL_MAX, DBL_MAX) {};
+
+protected:
+  Graphic3d_SequenceOfGroup myGroups;
+  Graphic3d_RangeOfLOD      myRange;
+};
+
+DEFINE_STANDARD_HANDLE (Graphic3d_LOD, Standard_Transient)
+
+#endif // _Graphic3d_LOD_Header