Index: vtkQuadricDecimation.cxx
===================================================================
RCS file: /cvsroot/VTK/VTK/Graphics/vtkQuadricDecimation.cxx,v
retrieving revision 1.39.50.1
diff -u -r1.39.50.1 vtkQuadricDecimation.cxx
--- vtkQuadricDecimation.cxx	17 Jul 2008 20:29:18 -0000	1.39.50.1
+++ vtkQuadricDecimation.cxx	12 May 2009 10:54:30 -0000
@@ -337,6 +337,18 @@
       continue;
       }
     
+    // check for a non-collapsible edges -- AWT
+    if ( !this->IsEdgeCollapsible( endPtIds[0], endPtIds[1] ) )
+      {
+      vtkDebugMacro(<<"Non-collapsible edge detected " << edgeId << " " <<  cost);
+      // return the point to the queue but with the max cost so that 
+      // when it is recomputed it will be reconsidered
+      this->EdgeCosts->Insert(VTK_DOUBLE_MAX, edgeId);
+
+      edgeId = this->EdgeCosts->Pop(0, cost);
+      continue;
+      }
+
     this->NumberOfEdgeCollapses++;
         
     // Set the new coordinates of point0.
@@ -407,6 +419,96 @@
 }
 
 //----------------------------------------------------------------------------
+int vtkQuadricDecimation::IsEdgeCollapsible( vtkIdType pt0Id, vtkIdType pt1Id )
+{
+  bool isCollapsible = true;
+
+  vtkIdType i, j, k, m, vv;
+  vtkIdType npts, *pts;
+
+  vtkIdList* uniqueVertices  = vtkIdList::New( );
+  vtkIdList* neighbourCells  = vtkIdList::New( );
+  vtkIdList* thirdVertexList = vtkIdList::New( );
+
+  // Look for the cells which use this edge
+  this->Mesh->GetPointCells(pt0Id, this->CollapseCellIds);
+  for (i = 0; i < this->CollapseCellIds->GetNumberOfIds() && isCollapsible; i++) 
+    {
+    const vtkIdType cellId = this->CollapseCellIds->GetId(i);
+    this->Mesh->GetCellPoints(cellId, npts, pts);
+    for (j = 0; j < 3 && isCollapsible; j++) 
+      {
+      if (pts[j] == pt1Id) 
+        {
+        // Cell cellId uses this edge.  Now to look and see how many
+        // unique vertices are used by its neighbours
+
+        vtkIdType thisTriangleVerts[] = { pt0Id, pt1Id, VTK_INT_MAX };
+
+        // Work out which is the vertex of cell cellId not in the edge
+        this->Mesh->GetCellPoints( cellId, thirdVertexList );
+        for ( vv = 0; vv < 3; ++vv )
+          {
+          thisTriangleVerts[2] = thirdVertexList->GetId( vv );
+
+          if ( thisTriangleVerts[2] != thisTriangleVerts[0] && thisTriangleVerts[2] != thisTriangleVerts[0] )
+            break;
+          }
+
+        // Don't know why this should happen, but it shows there's
+        // something weird afoot
+        if ( vv == 3 )
+          continue;
+
+        // Going to build a list of vertices used
+        uniqueVertices->SetNumberOfIds( 0 );
+        uniqueVertices->Allocate( neighbourCells->GetNumberOfIds( ) );
+
+        for ( k = 0; k < 3 && isCollapsible; ++k )
+          {
+          this->Mesh->GetCellEdgeNeighbors( cellId, thisTriangleVerts[k], thisTriangleVerts[(k+1)%3], neighbourCells );
+
+          for ( m = 0; m < neighbourCells->GetNumberOfIds( ) && isCollapsible; ++m )
+            {
+            // Look for the vertex which isn't on the shared edge
+            vtkIdType farVertex = VTK_INT_MAX;
+
+            this->Mesh->GetCellPoints( cellId, thirdVertexList );
+            for ( vv = 0; vv < 3; ++vv )
+              {
+              farVertex = thirdVertexList->GetId( vv );
+
+              if ( farVertex != thisTriangleVerts[k] && farVertex != thisTriangleVerts[(k+1)%3] )
+                break;
+              }
+
+            if ( vv == 3 )
+              continue;
+
+            if ( -1 == uniqueVertices->IsId( farVertex ) )
+              {
+              uniqueVertices->InsertNextId( farVertex );
+              }
+            else
+              {
+              // We've seen this vertex before - this means we've found a 
+              // non-collapsible combination
+              isCollapsible = false;
+              }
+            }
+          }
+        }
+      }
+    }
+
+  thirdVertexList->Delete( );
+  uniqueVertices->Delete( );
+  neighbourCells->Delete( );
+
+  return ( isCollapsible ) ? 1 : 0;
+}
+
+//----------------------------------------------------------------------------
 void vtkQuadricDecimation::InitializeQuadrics(vtkIdType numPts)
 {
   vtkPolyData *input = this->Mesh;
Index: vtkQuadricDecimation.h
===================================================================
RCS file: /cvsroot/VTK/VTK/Graphics/vtkQuadricDecimation.h,v
retrieving revision 1.22
diff -u -r1.22 vtkQuadricDecimation.h
--- vtkQuadricDecimation.h	29 Nov 2004 18:41:29 -0000	1.22
+++ vtkQuadricDecimation.h	12 May 2009 09:58:02 -0000
@@ -169,6 +169,8 @@
   vtkIdType GetEdgeCellId(vtkIdType p1Id, vtkIdType p2Id);
 
   int IsGoodPlacement(vtkIdType pt0Id, vtkIdType pt1Id, const double *x);
+  int IsEdgeCollapsible( vtkIdType pt0Id, vtkIdType pt1Id );
+
   int TrianglePlaneCheck(const double t0[3], const double t1[3], 
                          const double t2[3],  const double *x);
   void ComputeNumberOfComponents(void);