protected:
  unsigned int queryID; // Query object ID

public:
  DEFINE_STANDARD_RTTIEXT(Graphic3d_OcclusionQuery, Standard_Transient)

//! Gets number of samples that have passed the rendering pipeline.
unsigned int GetNumSamplesPassed() const;

//! Helper method that returns if any samples have passed the rendering
//! pipeline.
bool AnySamplesPassed() const;

unsigned int GetNumSamplesPassed() const;

class Graphic3d_OcclusionQuery : public Standard_Transient {

class Graphic3d_ViewOcclusionMask : public Standard_Transient

private:
  const Handle(OpenGl_Context) & aCtx;

Standard_EXPORT OpenGl_OcclusionQuery::~OpenGl_OcclusionQuery() {
  aCtx->core15->glDeleteQueries(1, &queryID);
  queryID = 0;
}

Standard_EXPORT void OpenGl_OcclusionQuery::EndQuery() {
  aCtx->core15->glEndQuery(GL_SAMPLES_PASSED);
  aCtx->core15->glGetQueryObjectiv(queryID, GL_QUERY_RESULT, &samplesPassed);
}

aCtx->core15->glEndQuery(GL_SAMPLES_PASSED);

//! Returns True if the structure occulded in specified view, otherwise
//! returns False.
Standard_Boolean IsOccluded(const Standard_Integer theViewId) const 
{
  return (!OcclusionMask->IsVisible(theViewId));
}

//! Marks structure as Occluded by other strcuture in specified view,!
void SetOccluionSate(const Standard_Integer theViewId, const bool theIsVisible) const 
{
  OcclusionMask->SetVisible(theViewId, !theIsVisible);
}

// =======================================================================
// function : OccludedStructures
// purpose  :
// =======================================================================
void Graphic3d_CView::OccludedStructures(Graphic3d_MapOfStructure& theStructures) const

//! Returns number of occluded strcutures in the view
Standard_EXPORT void OccludedStructures(Graphic3d_MapOfStructure& theStructures) const;

Standard_EXPORT void UpdateOcclusion(const Handle(OpenGl_Workspace) & theWorkspace);

enum { Graphic3d_FrameStatsTimer_NB = Graphic3d_FrameStatsTimer_OcclusionCulling + 1 };

// Turn off writing to depth and color buffers 
aCtx->core11fwd->glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);

// Start record occlusion test computational cost  
const Handle(OpenGl_FrameStats) &aStats = theWorkspace->GetGlContext()->FrameStats();

const Handle(Graphic3d_Layer) &aLayer = aLayerIter.ChangeValue();

private:
  GLuint   myID;                     //!< OpenGL query ID.
  GLenum   myType;                   //!< OpenGL query Type.
  bool     inUse;                    //!< store bool answer is the query still inprogress or finished.
  bool     started;                  //!< bool answer is the query stated before or this the first time.

void OpenGl_OcclusionQuery::Release(OpenGl_Context *theCtx) {
  if (myID == 0)
    return;
  if (theCtx != NULL) {
    theCtx->core15->glDeleteQueries(1, &myID);
    myID = 0;
    started = false;
  }
}

if (myQuery->GetID()==0)
    myQuery->Create(aCtx, GL_ANY_SAMPLES_PASSED);

class OpenGl_OcclusionQuery : public OpenGl_Resource {

Standard_EXPORT virtual int IsResultsReady(const Handle(OpenGl_Context) & theCtx) const;

// function : RenderOccluder 
// purpose  : 
// =======================================================================
void OpenGl_Structure::RenderOccluder(const Handle(OpenGl_Workspace)& theWorkspace) const 
{
  const Handle(OpenGl_Context) &aCtx = theWorkspace->GetGlContext();
  aCtx->ApplyModelViewMatrix();
  renderBoundingBox(theWorkspace);
}

Handle(OpenGl_OcclusionQuery)      myQuery;             //! test the occlusion status of the structure;

  // re-validate occlusion results  
  myRenderParams.OcculsionQueryState = Graphic3d_RenderingParams::OcculsionQuery_On;
}
//=======================================================================
//function : renderStructs

aShadowPrs->CStructure()->OcclusionMask =new Graphic3d_ViewOcclusionMask();