diff --git a/include/Algo/Decimation/approximator.h b/include/Algo/Decimation/approximator.h
index 320d2b8b71d55bb553cbe7fcaf67b7091e5c2d57..5a533562f944f7f48640b1ed37d3ed97b3c5f2c4 100644
--- a/include/Algo/Decimation/approximator.h
+++ b/include/Algo/Decimation/approximator.h
@@ -39,6 +39,7 @@ namespace Decimation
 enum ApproximatorType
 {
 	A_QEM,
+	A_QEMhalf,
 	A_MidEdge,
 	A_HalfCollapse,
 	A_CornerCutting,
diff --git a/include/Algo/Decimation/decimation.hpp b/include/Algo/Decimation/decimation.hpp
index ece16239ac5c50a72f2699dce8c7281d510a91ec..21f8fdb03ca535f5d2929224639b2be69550fe72 100644
--- a/include/Algo/Decimation/decimation.hpp
+++ b/include/Algo/Decimation/decimation.hpp
@@ -45,6 +45,9 @@ void decimate(
 		case A_QEM :
 			approximators.push_back(new Approximator_QEM<PFP>(map, position)) ;
 			break ;
+		case A_QEMhalf :
+			approximators.push_back(new Approximator_QEMhalf<PFP>(map, position)) ;
+			break ;
 		case A_MidEdge :
 			approximators.push_back(new Approximator_MidEdge<PFP>(map, position)) ;
 			break ;
@@ -62,7 +65,7 @@ void decimate(
 			break ;
 		case A_LightfieldFull :
 		{
-			approximators.push_back(new Approximator_QEM<PFP>(map, position)) ;
+			approximators.push_back(new Approximator_QEMhalf<PFP>(map, position)) ;
 			/*
 						PFP::TVEC3 frame[3] ;
 						frame[0] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "frame_T") ; // Tangent
@@ -94,28 +97,6 @@ void decimate(
 		case A_LightfieldHalf :
 		{
 			approximators.push_back(new Approximator_HalfCollapse<PFP>(map, position)) ;
-			/*
-						PFP::TVEC3 frame[3] ;
-						frame[0] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "frame_T") ; // Tangent
-						frame[1] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "frame_B") ; // Bitangent
-						frame[2] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "frame_N") ; // Normal
-						for (unsigned int i = 0 ; i < 3 ; ++i)
-							if (!frame[i].isValid()) {
-								std::cerr << "In function decimate : frame[" << i << "] is not valid" << std::endl ;
-							}
-
-						AttributeHandler<typename PFP::VEC3> colorPTM[6] ;
-						colorPTM[0] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_a") ;
-						colorPTM[1] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_b") ;
-						colorPTM[2] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_c") ;
-						colorPTM[3] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_d") ;
-						colorPTM[4] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_e") ;
-						colorPTM[5] = map.template getAttribute<typename PFP::VEC3>(VERTEX_ORBIT, "colorPTM_f") ;
-						for (unsigned int i = 0 ; i < 6 ; ++i)
-							if (!colorPTM[i].isValid()) {
-								std::cerr << "In function decimate : colorPTM[" << i << "] is not valid" << std::endl ;
-							}
-			*/
 			AttributeHandler<Geom::Matrix<3,3,typename PFP::REAL> > frame = map.template getAttribute<Geom::Matrix<3,3,typename PFP::REAL> >(VERTEX_ORBIT, "frame") ;
 			AttributeHandler<Geom::Matrix<3,6,typename PFP::REAL> > RGBfunctions = map.template getAttribute<Geom::Matrix<3,6,typename PFP::REAL> >(VERTEX_ORBIT, "colorPTM") ;
 			approximators.push_back(new Approximator_FrameHalf<PFP>(map, frame)) ;
diff --git a/include/Algo/Decimation/geometryApproximator.h b/include/Algo/Decimation/geometryApproximator.h
index 3aaabd3958464a8aeb82d82243821d7796fe7283..4b7218da93d08fc54350b1a6196af4d7bc883045 100644
--- a/include/Algo/Decimation/geometryApproximator.h
+++ b/include/Algo/Decimation/geometryApproximator.h
@@ -58,6 +58,28 @@ public:
 	void approximate(Dart d) ;
 } ;
 
+template <typename PFP>
+class Approximator_QEMhalf : public Approximator<PFP, typename PFP::VEC3>
+{
+public:
+	typedef typename PFP::MAP MAP ;
+	typedef typename PFP::VEC3 VEC3 ;
+	typedef typename PFP::REAL REAL ;
+
+protected:
+	AttributeHandler<Quadric<REAL> > m_quadric ;
+
+public:
+	Approximator_QEMhalf(MAP& m, AttributeHandler<VEC3>& pos, Predictor<PFP, VEC3>* pred = NULL) :
+		Approximator<PFP, VEC3>(m, pos, EDGE_ORBIT, pred)
+	{}
+	~Approximator_QEMhalf()
+	{}
+	ApproximatorType getType() const { return A_QEMhalf ; }
+	bool init() ;
+	void approximate(Dart d) ;
+} ;
+
 template <typename PFP>
 class Approximator_MidEdge : public Approximator<PFP, typename PFP::VEC3>
 {
diff --git a/include/Algo/Decimation/geometryApproximator.hpp b/include/Algo/Decimation/geometryApproximator.hpp
index 442cbe40c47a0554bcf913ce59fde11edbc18d31..d929e142a039c38ee0bd8d0fe15167c73e0938e6 100644
--- a/include/Algo/Decimation/geometryApproximator.hpp
+++ b/include/Algo/Decimation/geometryApproximator.hpp
@@ -105,6 +105,69 @@ void Approximator_QEM<PFP>::approximate(Dart d)
 		this->m_approx[d] = res ;
 }
 
+/************************************************************************************
+ *                            QUADRIC ERROR METRIC (for half-edge criteria)         *
+ ************************************************************************************/
+
+template <typename PFP>
+bool Approximator_QEMhalf<PFP>::init()
+{
+	m_quadric = this->m_map.template getAttribute<Quadric<REAL> >(VERTEX_ORBIT, "QEMquadric") ;
+
+	if(this->m_predictor)
+	{
+		return false ;
+	}
+	return true ;
+}
+
+template <typename PFP>
+void Approximator_QEMhalf<PFP>::approximate(Dart d)
+{
+	MAP& m = this->m_map ;
+
+	// get some darts
+	Dart dd = m.phi2(d) ;
+
+	Quadric<REAL> q1, q2 ;
+	if(!m_quadric.isValid()) // if the selector is not QEM, compute local error quadrics
+	{
+		// compute the error quadric associated to v1
+		Dart it = d ;
+		do
+		{
+			Quadric<REAL> q(this->m_attrV[it], this->m_attrV[m.phi1(it)], this->m_attrV[m.phi_1(it)]) ;
+			q1 += q ;
+			it = m.alpha1(it) ;
+		} while(it != d) ;
+
+		// compute the error quadric associated to v2
+		it = dd ;
+		do
+		{
+			Quadric<REAL> q(this->m_attrV[it], this->m_attrV[m.phi1(it)], this->m_attrV[m.phi_1(it)]) ;
+			q2 += q ;
+			it = m.alpha1(it) ;
+		} while(it != dd) ;
+	}
+	else // if the selector is QEM, use the error quadrics computed by the selector
+	{
+		q1 = m_quadric[d] ;
+		q2 = m_quadric[dd] ;
+	}
+
+	Quadric<REAL> quad ;
+	quad += q1 ;	// compute the sum of the
+	quad += q2 ;	// two vertices quadrics
+
+	VEC3 res ;
+	bool opt = quad.findOptimizedPos(res) ;	// try to compute an optimized position for the contraction of this edge
+	if(!opt)
+		this->m_approx[d] = this->m_attrV[d] ;
+	else
+		this->m_approx[d] = res ;
+}
+
 /************************************************************************************
  *							         MID EDGE                                       *
  ************************************************************************************/
diff --git a/include/Algo/Decimation/selector.hpp b/include/Algo/Decimation/selector.hpp
index f3a747ac7ab6f9b480e29d0a6e3964f166a70e23..a4f0bc708ed61673a5b5b39af67eaecd1491579a 100644
--- a/include/Algo/Decimation/selector.hpp
+++ b/include/Algo/Decimation/selector.hpp
@@ -404,9 +404,9 @@ void EdgeSelector_QEM<PFP>::updateAfterCollapse(Dart d2, Dart dd2)
 			do											// - edges for which only the collapsibility must be re-tested
 			{
 				updateEdgeInfo(vit2, false) ;
-				updateEdgeInfo(m.phi1(vit2), false) ; // OPTIM POSSIBLE : ne pas faire le 1er phi1(vit2) (car sera fait dans prochaine itération) ?
+				updateEdgeInfo(m.phi1(vit2), false) ;
 				vit2 = m.alpha_1(vit2) ;
-			} while(vit2 != stop) ;	// OPTIM POSSIBLE : ne pas faire vit2 == stop (car déjà fait dans initEdgeInfo(vit)) ?
+			} while(vit2 != stop) ;
 		}
 		else
 			updateEdgeInfo(vit, true) ;
@@ -473,7 +473,8 @@ void EdgeSelector_QEM<PFP>::computeEdgeInfo(Dart d, EdgeInfo& einfo)
 
 	m_positionApproximator->approximate(d) ;
 
-	REAL err = quad(m_positionApproximator->getApprox(d)) ;
+	REAL err = std::max(REAL(0),REAL(quad(m_positionApproximator->getApprox(d)))) ;
+
 	einfo.it = edges.insert(std::make_pair(err, d)) ;
 	einfo.valid = true ;
 }
@@ -609,7 +610,7 @@ void EdgeSelector_QEMml<PFP>::recomputeQuadric(const Dart d, const bool recomput
            	quadric[d] += Quadric<REAL>(this->m_position[d],this->m_position[this->m_map.phi2(dFront)],this->m_position[dBack]) ;
        	}
        	if (recomputeNeighbors)
-       		recomputeQuadric(this->m_map.phi2(dFront), false) ;
+       		recomputeQuadric(dBack, false) ;
 
     } while(dFront != dInit) ;
 }
@@ -638,7 +639,7 @@ void EdgeSelector_QEMml<PFP>::updateAfterCollapse(Dart d2, Dart dd2)
 		{
 			updateEdgeInfo(vit2, true) ;
 			updateEdgeInfo(m.phi1(vit2), false) ;
-			vit2 = m.alpha_1(vit2) ;	// OPTIM POSSIBLE : ne pas faire vit2 == stop car déjà fait dans init/updateEdgeInfo(vit)
+			vit2 = m.alpha_1(vit2) ;
 		} while(vit2 != stop) ;
 
 		vit = m.alpha1(vit) ;
@@ -728,6 +729,7 @@ bool HalfEdgeSelector_Lightfield<PFP>::init()
 		if(ok == 0 && (*it)->getApproximatedAttributeName() == "position")
 		{
 			m_positionApproximator = reinterpret_cast<Approximator<PFP, VEC3>* >(*it) ; // 1) position
+			assert(m_positionApproximator->getType() != A_QEM) ; // A_QEM is not compatible for half-edge crit
 			++ok ;
 		}
 		else if( ok == 1 && (*it)->getApproximatedAttributeName() == "frame")
@@ -855,7 +857,7 @@ void HalfEdgeSelector_Lightfield<PFP>::recomputeQuadric(const Dart d, const bool
            	quadric[d] += Quadric<REAL>(this->m_position[d],this->m_position[this->m_map.phi2(dFront)],this->m_position[dBack]) ;
        	}
        	if (recomputeNeighbors)
-       		recomputeQuadric(this->m_map.phi2(dFront), false) ;
+       		recomputeQuadric(dBack, false) ;
 
     } while(dFront != dInit) ;
 }
@@ -872,7 +874,9 @@ void HalfEdgeSelector_Lightfield<PFP>::updateAfterCollapse(Dart d2, Dart dd2)
 	do
 	{
 		updateHalfEdgeInfo(vit, true) ;
+		updateHalfEdgeInfo(m.phi2(vit), true) ;
 		updateHalfEdgeInfo(m.phi1(vit), true) ;
+		updateHalfEdgeInfo(m.phi2(m.phi1(vit)), true) ;
 
 		Dart stop = m.phi2(vit) ;
 		Dart vit2 = m.alpha_1(m.phi1(vit)) ;