/*=========================================================================

  Program:   Visualization Toolkit
  Module:    $RCSfile: vtkQuad.cxx,v $

  Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
  All rights reserved.
  See Copyright.txt or http://www.kitware.com/Copyright.htm for details.

     This software is distributed WITHOUT ANY WARRANTY; without even
     the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
     PURPOSE.  See the above copyright notice for more information.

=========================================================================*/
#include "vtkQuad.h"

#include "vtkObjectFactory.h"
#include "vtkCellArray.h"
#include "vtkCellData.h"
#include "vtkLine.h"
#include "vtkTriangle.h"
#include "vtkMath.h"
#include "vtkPlane.h"
#include "vtkPointData.h"
#include "vtkPointLocator.h"
#include "vtkPoints.h"

vtkCxxRevisionMacro(vtkQuad, "$Revision: 1.3.12.1 $");
vtkStandardNewMacro(vtkQuad);

static const double VTK_DIVERGED = 1.e6;

//----------------------------------------------------------------------------
// Construct the quad with four points.
vtkQuad::vtkQuad()
{
  this->Points->SetNumberOfPoints(4);
  this->PointIds->SetNumberOfIds(4);
  for (int i = 0; i < 4; i++)
    {
    this->Points->SetPoint(i, 0.0, 0.0, 0.0);
    this->PointIds->SetId(i,0);
    }
  this->Line = vtkLine::New();
  this->Triangle = vtkTriangle::New();
}

//----------------------------------------------------------------------------
vtkQuad::~vtkQuad()
{
  this->Line->Delete();
  this->Triangle->Delete();
}

//----------------------------------------------------------------------------
static const int VTK_QUAD_MAX_ITERATION=20;
static const double VTK_QUAD_CONVERGED=1.e-04;

inline static void ComputeNormal(vtkQuad *self, double pt1[3], double pt2[3],
                                 double pt3[3], double n[3])
{
  vtkTriangle::ComputeNormal (pt1, pt2, pt3, n);
  
  // If first three points are co-linear, then use fourth point
  //
  double pt4[3];
  if ( n[0] == 0.0 && n[1] == 0.0 && n[2] == 0.0 )
    {
    self->Points->GetPoint(3,pt4);
    vtkTriangle::ComputeNormal (pt2, pt3, pt4, n);
    }
}

//----------------------------------------------------------------------------
int vtkQuad::EvaluatePosition(double x[3], double* closestPoint,
                             int& subId, double pcoords[3], 
                             double& dist2, double *weights)
{
  int i, j;
  double pt1[3], pt2[3], pt3[3], pt[3], n[3];
  double det;
  double maxComponent;
  int idx=0, indices[2];
  int iteration, converged;
  double  params[2];
  double  fcol[2], rcol[2], scol[2], cp[3];
  double derivs[8];

  subId = 0;
  pcoords[0] = pcoords[1] = params[0] = params[1] = 0.5;

  // Get normal for quadrilateral
  //
  this->Points->GetPoint(0, pt1);
  this->Points->GetPoint(1, pt2);
  this->Points->GetPoint(2, pt3);
  ComputeNormal (this, pt1, pt2, pt3, n);

  // Project point to plane
  //
  vtkPlane::ProjectPoint(x,pt1,n,cp);

  // Construct matrices.  Since we have over determined system, need to find
  // which 2 out of 3 equations to use to develop equations. (Any 2 should 
  // work since we've projected point to plane.)
  //
  for (maxComponent=0.0, i=0; i<3; i++)
    {
    if (fabs(n[i]) > maxComponent)
      {
      maxComponent = fabs(n[i]);
      idx = i;
      }
    }
  for (j=0, i=0; i<3; i++)  
    {
    if ( i != idx )
      {
      indices[j++] = i;
      }
    }

  // Use Newton's method to solve for parametric coordinates
  //  
  for (iteration=converged=0; !converged
         && (iteration < VTK_QUAD_MAX_ITERATION);
       iteration++) 
    {
    //  calculate element interpolation functions and derivatives
    //
    this->InterpolationFunctions(pcoords, weights);
    this->InterpolationDerivs(pcoords, derivs);

    //  calculate newton functions
    //
    for (i=0; i<2; i++) 
      {
      fcol[i] = rcol[i] = scol[i] = 0.0;
      }
    for (i=0; i<4; i++)
      {
      this->Points->GetPoint(i, pt);
      for (j=0; j<2; j++)
        {
        fcol[j] += pt[indices[j]] * weights[i];
        rcol[j] += pt[indices[j]] * derivs[i];
        scol[j] += pt[indices[j]] * derivs[i+4];
        }
      }

    for (j=0; j<2; j++)
      {
      fcol[j] -= cp[indices[j]];
      }

    //  compute determinants and generate improvements
    //
    if ( (det=vtkMath::Determinant2x2(rcol,scol)) == 0.0 )
      {
      return -1;
      }

    pcoords[0] = params[0] - vtkMath::Determinant2x2 (fcol,scol) / det;
    pcoords[1] = params[1] - vtkMath::Determinant2x2 (rcol,fcol) / det;

    //  check for convergence
    //
    if ( ((fabs(pcoords[0]-params[0])) < VTK_QUAD_CONVERGED) &&
         ((fabs(pcoords[1]-params[1])) < VTK_QUAD_CONVERGED) )
      {
      converged = 1;
      }
    // Test for bad divergence (S.Hirschberg 11.12.2001)
    else if ((fabs(pcoords[0]) > VTK_DIVERGED) || 
             (fabs(pcoords[1]) > VTK_DIVERGED))
      {
      return -1;
      }

    //  if not converged, repeat
    //
    else 
      {
      params[0] = pcoords[0];
      params[1] = pcoords[1];
      }
    }

  //  if not converged, set the parametric coordinates to arbitrary values
  //  outside of element
  //
  if ( !converged )
    {
    return -1;
    }

  this->InterpolationFunctions(pcoords, weights);

  if ( pcoords[0] >= -0.001 && pcoords[0] <= 1.001 &&
       pcoords[1] >= -0.001 && pcoords[1] <= 1.001 )
    {
    if (closestPoint)
      {
      dist2 = 
        vtkMath::Distance2BetweenPoints(cp,x); //projection distance
      closestPoint[0] = cp[0];
      closestPoint[1] = cp[1];
      closestPoint[2] = cp[2];
      }
    return 1;
    }
  else
    {
    double t;
    double pt4[3];
    
    if (closestPoint)
      {
      this->Points->GetPoint(3, pt4);
      
      if ( pcoords[0] < 0.0 && pcoords[1] < 0.0 )
        {
        dist2 = vtkMath::Distance2BetweenPoints(x,pt1);
        for (i=0; i<3; i++)
          {
        closestPoint[i] = pt1[i];
          }
        }
      else if ( pcoords[0] > 1.0 && pcoords[1] < 0.0 )
        {
        dist2 = vtkMath::Distance2BetweenPoints(x,pt2);
        for (i=0; i<3; i++)
          {
          closestPoint[i] = pt2[i];
          }
        }
      else if ( pcoords[0] > 1.0 && pcoords[1] > 1.0 )
        {
        dist2 = vtkMath::Distance2BetweenPoints(x,pt3);
        for (i=0; i<3; i++)
          {
          closestPoint[i] = pt3[i];
          }
        }
      else if ( pcoords[0] < 0.0 && pcoords[1] > 1.0 )
        {
        dist2 = vtkMath::Distance2BetweenPoints(x,pt4);
        for (i=0; i<3; i++)
          {
          closestPoint[i] = pt4[i];
          }
        }
      else if ( pcoords[0] < 0.0 )
        {
        dist2 = vtkLine::DistanceToLine(x,pt1,pt4,t,closestPoint);
        }
      else if ( pcoords[0] > 1.0 )
        {
        dist2 = vtkLine::DistanceToLine(x,pt2,pt3,t,closestPoint);
        }
      else if ( pcoords[1] < 0.0 )
        {
        dist2 = vtkLine::DistanceToLine(x,pt1,pt2,t,closestPoint);
        }
      else if ( pcoords[1] > 1.0 )
        {
        dist2 = vtkLine::DistanceToLine(x,pt3,pt4,t,closestPoint);
        }
      }
    return 0;
    }
}

//----------------------------------------------------------------------------
void vtkQuad::EvaluateLocation(int& vtkNotUsed(subId), double pcoords[3], 
                               double x[3], double *weights)
{
  int i, j;
  double pt[3];

  this->InterpolationFunctions(pcoords, weights);

  x[0] = x[1] = x[2] = 0.0;
  for (i=0; i<4; i++)
    {
    this->Points->GetPoint(i, pt);
    for (j=0; j<3; j++)
      {
      x[j] += pt[j] * weights[i];
      }
    }
}

//----------------------------------------------------------------------------
// Compute iso-parametric interpolation functions
//
void vtkQuad::InterpolationFunctions(double pcoords[3], double sf[4])
{
  double rm, sm;

  rm = 1. - pcoords[0];
  sm = 1. - pcoords[1];

  sf[0] = rm * sm;
  sf[1] = pcoords[0] * sm;
  sf[2] = pcoords[0] * pcoords[1];
  sf[3] = rm * pcoords[1];
}

//----------------------------------------------------------------------------
void vtkQuad::InterpolationDerivs(double pcoords[3], double derivs[8])
{
  double rm, sm;

  rm = 1. - pcoords[0];
  sm = 1. - pcoords[1];

  derivs[0] = -sm;
  derivs[1] = sm;
  derivs[2] = pcoords[1];
  derivs[3] = -pcoords[1];
  derivs[4] = -rm;
  derivs[5] = -pcoords[0];
  derivs[6] = pcoords[0];
  derivs[7] = rm;
}

//----------------------------------------------------------------------------
int vtkQuad::CellBoundary(int vtkNotUsed(subId), double pcoords[3], 
                          vtkIdList *pts)
{
  double t1=pcoords[0]-pcoords[1];
  double t2=1.0-pcoords[0]-pcoords[1];

  pts->SetNumberOfIds(2);

  // compare against two lines in parametric space that divide element
  // into four pieces.
  if ( t1 >= 0.0 && t2 >= 0.0 )
    {
    pts->SetId(0,this->PointIds->GetId(0));
    pts->SetId(1,this->PointIds->GetId(1));
    }

  else if ( t1 >= 0.0 && t2 < 0.0 )
    {
    pts->SetId(0,this->PointIds->GetId(1));
    pts->SetId(1,this->PointIds->GetId(2));
    }

  else if ( t1 < 0.0 && t2 < 0.0 )
    {
    pts->SetId(0,this->PointIds->GetId(2));
    pts->SetId(1,this->PointIds->GetId(3));
    }

  else //( t1 < 0.0 && t2 >= 0.0 )
    {
    pts->SetId(0,this->PointIds->GetId(3));
    pts->SetId(1,this->PointIds->GetId(0));
    }

  if ( pcoords[0] < 0.0 || pcoords[0] > 1.0 ||
       pcoords[1] < 0.0 || pcoords[1] > 1.0 )
    {
    return 0;
    }
  else
    {
    return 1;
    }
}

//----------------------------------------------------------------------------
// Marching (convex) quadrilaterals
//
static int edges[4][2] = { {0,1}, {1,2}, {3,2}, {0,3} };

typedef int EDGE_LIST;
typedef struct {
       EDGE_LIST edges[5];
} LINE_CASES;

static LINE_CASES lineCases[] = { 
  {{-1, -1, -1, -1, -1}},
  {{0, 3, -1, -1, -1}},
  {{1, 0, -1, -1, -1}},
  {{1, 3, -1, -1, -1}},
  {{2, 1, -1, -1, -1}},
  {{0, 3, 2, 1, -1}},
  {{2, 0, -1, -1, -1}},
  {{2, 3, -1, -1, -1}},
  {{3, 2, -1, -1, -1}},
  {{0, 2, -1, -1, -1}},
  {{1, 0, 3, 2, -1}},
  {{1, 2, -1, -1, -1}},
  {{3, 1, -1, -1, -1}},
  {{0, 1, -1, -1, -1}},
  {{3, 0, -1, -1, -1}},
  {{-1, -1, -1, -1, -1}}
};

//----------------------------------------------------------------------------
void vtkQuad::Contour(double value, vtkDataArray *cellScalars, 
                      vtkPointLocator *locator, 
                      vtkCellArray *verts, 
                      vtkCellArray *lines, 
                      vtkCellArray *vtkNotUsed(polys), 
                      vtkPointData *inPd, vtkPointData *outPd,
                      vtkCellData *inCd, vtkIdType cellId, vtkCellData *outCd)
{
  static int CASE_MASK[4] = {1,2,4,8};
  LINE_CASES *lineCase;
  EDGE_LIST  *edge;
  int i, j, index, *vert;
  int newCellId;
  vtkIdType pts[2];
  int e1, e2;
  double t, x1[3], x2[3], x[3], deltaScalar;
  vtkIdType offset = verts->GetNumberOfCells();

  // Build the case table
  for ( i=0, index = 0; i < 4; i++)
    {
    if (cellScalars->GetComponent(i,0) >= value)
      {
      index |= CASE_MASK[i];
      }
    }

  lineCase = lineCases + index;
  edge = lineCase->edges;

  for ( ; edge[0] > -1; edge += 2 )
    {
    for (i=0; i<2; i++) // insert line
      {
      vert = edges[edge[i]];
      // calculate a preferred interpolation direction
      deltaScalar = (cellScalars->GetComponent(vert[1],0) 
                     - cellScalars->GetComponent(vert[0],0));
      if (deltaScalar > 0)
        {
        e1 = vert[0]; e2 = vert[1];
        }
      else
        {
        e1 = vert[1]; e2 = vert[0];
        deltaScalar = -deltaScalar;
        }
      
      // linear interpolation
      if (deltaScalar == 0.0)
        {
        t = 0.0;
        }
      else
        {
        t = (value - cellScalars->GetComponent(e1,0)) / deltaScalar;
        }

      this->Points->GetPoint(e1, x1);
      this->Points->GetPoint(e2, x2);

      for (j=0; j<3; j++)
        {
        x[j] = x1[j] + t * (x2[j] - x1[j]);
        }
      if ( locator->InsertUniquePoint(x, pts[i]) )
        {
        if ( outPd ) 
          {
          vtkIdType p1 = this->PointIds->GetId(e1);
          vtkIdType p2 = this->PointIds->GetId(e2);
          outPd->InterpolateEdge(inPd,pts[i],p1,p2,t);
          }
        }
      }
    // check for degenerate line
    if ( pts[0] != pts[1] )
      {
      newCellId = offset + lines->InsertNextCell(2,pts);
      outCd->CopyData(inCd,cellId,newCellId);
      }
    }
}

//----------------------------------------------------------------------------
vtkCell *vtkQuad::GetEdge(int edgeId)
{
  int edgeIdPlus1 = edgeId + 1;
  
  if (edgeIdPlus1 > 3)
    {
    edgeIdPlus1 = 0;
    }

  // load point id's
  this->Line->PointIds->SetId(0,this->PointIds->GetId(edgeId));
  this->Line->PointIds->SetId(1,this->PointIds->GetId(edgeIdPlus1));

  // load coordinates
  this->Line->Points->SetPoint(0,this->Points->GetPoint(edgeId));
  this->Line->Points->SetPoint(1,this->Points->GetPoint(edgeIdPlus1));

  return this->Line;
}


//----------------------------------------------------------------------------
// Intersect plane; see whether point is in quadrilateral. This code
// splits the quad into two triangles and intersects them (because the
// quad may be non-planar).
//
int vtkQuad::IntersectWithLine(double p1[3], double p2[3], double tol, double& t,
                              double x[3], double pcoords[3], int& subId)
{
  int diagonalCase;
  double d1 = vtkMath::Distance2BetweenPoints(this->Points->GetPoint(0), 
                                             this->Points->GetPoint(2));
  double d2 = vtkMath::Distance2BetweenPoints(this->Points->GetPoint(1), 
                                             this->Points->GetPoint(3));
  subId = 0;
  
  // Figure out how to uniquely tessellate the quad. Watch out for 
  // equivalent triangulations (i.e., the triangulation is equivalent
  // no matter where the diagonal). In this case use the point ids as 
  // a tie breaker to insure unique triangulation across the quad.
  //
  if ( d1 == d2 ) //rare case; discriminate based on point id
    {
    int i, id, maxId=0, maxIdx=0;
    for (i=0; i<4; i++) //find the maximum id
      {
      if ( (id=this->PointIds->GetId(i)) > maxId )
        {
        maxId = id;
        maxIdx = i;
        }
      }
    if ( maxIdx == 0 || maxIdx == 2) diagonalCase = 0;
    else diagonalCase = 1;
    }
  else if ( d1 < d2 )
    {
    diagonalCase = 0;
    }
  else //d2 < d1
    {
    diagonalCase = 1;
    }

  // Note: in the following code the parametric coords must be adjusted to
  // reflect the use of the triangle parametric coordinate system.
  switch (diagonalCase)
    {
    case 0:
      this->Triangle->Points->SetPoint(0,this->Points->GetPoint(0));
      this->Triangle->Points->SetPoint(1,this->Points->GetPoint(1));
      this->Triangle->Points->SetPoint(2,this->Points->GetPoint(2));
      if (this->Triangle->IntersectWithLine(p1, p2, tol, t, x, pcoords, subId) )
        {
        pcoords[0] = pcoords[0] + pcoords[1];
        return 1;
        }
      this->Triangle->Points->SetPoint(0,this->Points->GetPoint(2));
      this->Triangle->Points->SetPoint(1,this->Points->GetPoint(3));
      this->Triangle->Points->SetPoint(2,this->Points->GetPoint(0));
      if (this->Triangle->IntersectWithLine(p1, p2, tol, t, x, pcoords, subId) )
        {
        pcoords[0] = 1.0 - (pcoords[0]+pcoords[1]);
        pcoords[1] = 1.0 - pcoords[1];
        return 1;
        }
      return 0;

    case 1:
      this->Triangle->Points->SetPoint(0,this->Points->GetPoint(0));
      this->Triangle->Points->SetPoint(1,this->Points->GetPoint(1));
      this->Triangle->Points->SetPoint(2,this->Points->GetPoint(3));
      if (this->Triangle->IntersectWithLine(p1, p2, tol, t, x, pcoords, subId) )
        {
        return 1;
        }
      this->Triangle->Points->SetPoint(0,this->Points->GetPoint(2));
      this->Triangle->Points->SetPoint(1,this->Points->GetPoint(3));
      this->Triangle->Points->SetPoint(2,this->Points->GetPoint(1));
      if (this->Triangle->IntersectWithLine(p1, p2, tol, t, x, pcoords, subId) )
        {
        pcoords[0] = 1.0 - pcoords[0];
        pcoords[1] = 1.0 - pcoords[1];
        return 1;
        }

      return 0;
    }

  return 0;
}

//----------------------------------------------------------------------------
int vtkQuad::Triangulate(int vtkNotUsed(index), vtkIdList *ptIds,
                         vtkPoints *pts)
{
  double d1, d2;

  pts->Reset();
  ptIds->Reset();

  // use minimum diagonal (Delaunay triangles) - assumed convex
  d1 = vtkMath::Distance2BetweenPoints(this->Points->GetPoint(0), 
                                   this->Points->GetPoint(2));
  d2 = vtkMath::Distance2BetweenPoints(this->Points->GetPoint(1), 
                                   this->Points->GetPoint(3));

  if ( d1 <= d2 )
    {
    ptIds->InsertId(0,this->PointIds->GetId(0));
    pts->InsertPoint(0,this->Points->GetPoint(0));
    ptIds->InsertId(1,this->PointIds->GetId(1));
    pts->InsertPoint(1,this->Points->GetPoint(1));
    ptIds->InsertId(2,this->PointIds->GetId(2));
    pts->InsertPoint(2,this->Points->GetPoint(2));

    ptIds->InsertId(3,this->PointIds->GetId(0));
    pts->InsertPoint(3,this->Points->GetPoint(0));
    ptIds->InsertId(4,this->PointIds->GetId(2));
    pts->InsertPoint(4,this->Points->GetPoint(2));
    ptIds->InsertId(5,this->PointIds->GetId(3));
    pts->InsertPoint(5,this->Points->GetPoint(3));
    }
  else
    {
    ptIds->InsertId(0,this->PointIds->GetId(0));
    pts->InsertPoint(0,this->Points->GetPoint(0));
    ptIds->InsertId(1,this->PointIds->GetId(1));
    pts->InsertPoint(1,this->Points->GetPoint(1));
    ptIds->InsertId(2,this->PointIds->GetId(3));
    pts->InsertPoint(2,this->Points->GetPoint(3));

    ptIds->InsertId(3,this->PointIds->GetId(1));
    pts->InsertPoint(3,this->Points->GetPoint(1));
    ptIds->InsertId(4,this->PointIds->GetId(2));
    pts->InsertPoint(4,this->Points->GetPoint(2));
    ptIds->InsertId(5,this->PointIds->GetId(3));
    pts->InsertPoint(5,this->Points->GetPoint(3));
    }

  return 1;
}

//----------------------------------------------------------------------------
void vtkQuad::Derivatives(int vtkNotUsed(subId), double pcoords[3], 
                          double *values, int dim, double *derivs)
{
  double v0[2], v1[2], v2[2], v3[2], v10[3], v20[3], lenX;
  double x0[3], x1[3], x2[3], x3[3], n[3], vec20[3], vec30[3];
  double *J[2], J0[2], J1[2];
  double *JI[2], JI0[2], JI1[2];
  double funcDerivs[8], sum[2], dBydx, dBydy;
  int i, j;

  // Project points of quad into 2D system
  this->Points->GetPoint(0, x0);
  this->Points->GetPoint(1, x1);
  this->Points->GetPoint(2, x2);
  ComputeNormal (this,x0, x1, x2, n);
  this->Points->GetPoint(3, x3);

  for (i=0; i < 3; i++) 
    {
    v10[i] = x1[i] - x0[i];
    vec20[i] = x2[i] - x0[i];
    vec30[i] = x3[i] - x0[i];
    }

  vtkMath::Cross(n,v10,v20); //creates local y' axis

  if ( (lenX=vtkMath::Normalize(v10)) <= 0.0
       || vtkMath::Normalize(v20) <= 0.0 ) //degenerate
    {
    for ( j=0; j < dim; j++ )
      {
      for ( i=0; i < 3; i++ )
        {
        derivs[j*dim + i] = 0.0;
        }
      }
    return;
    }

  v0[0] = v0[1] = 0.0; //convert points to 2D (i.e., local system)
  v1[0] = lenX; v1[1] = 0.0;
  v2[0] = vtkMath::Dot(vec20,v10);
  v2[1] = vtkMath::Dot(vec20,v20);
  v3[0] = vtkMath::Dot(vec30,v10);
  v3[1] = vtkMath::Dot(vec30,v20);

  this->InterpolationDerivs(pcoords, funcDerivs);

  // Compute Jacobian and inverse Jacobian
  J[0] = J0; J[1] = J1;
  JI[0] = JI0; JI[1] = JI1;

  J[0][0] = v0[0]*funcDerivs[0] + v1[0]*funcDerivs[1] +
            v2[0]*funcDerivs[2] + v3[0]*funcDerivs[3];
  J[0][1] = v0[1]*funcDerivs[0] + v1[1]*funcDerivs[1] +
            v2[1]*funcDerivs[2] + v3[1]*funcDerivs[3];
  J[1][0] = v0[0]*funcDerivs[4] + v1[0]*funcDerivs[5] +
            v2[0]*funcDerivs[6] + v3[0]*funcDerivs[7];
  J[1][1] = v0[1]*funcDerivs[4] + v1[1]*funcDerivs[5] +
            v2[1]*funcDerivs[6] + v3[1]*funcDerivs[7];

  // Compute inverse Jacobian, return if Jacobian is singular
  if (!vtkMath::InvertMatrix(J,JI,2))
    {
    for ( j=0; j < dim; j++ )
      {
      for ( i=0; i < 3; i++ )
        {
        derivs[j*dim + i] = 0.0;
        }
      }
    return;
    }

  // Loop over "dim" derivative values. For each set of values, 
  // compute derivatives
  // in local system and then transform into modelling system.
  // First compute derivatives in local x'-y' coordinate system
  for ( j=0; j < dim; j++ )
    {
    sum[0] = sum[1] = 0.0;
    for ( i=0; i < 4; i++) //loop over interp. function derivatives
      {
      sum[0] += funcDerivs[i] * values[dim*i + j]; 
      sum[1] += funcDerivs[4 + i] * values[dim*i + j];
      }
    dBydx = sum[0]*JI[0][0] + sum[1]*JI[0][1];
    dBydy = sum[0]*JI[1][0] + sum[1]*JI[1][1];

    // Transform into global system (dot product with global axes)
    derivs[3*j] = dBydx * v10[0] + dBydy * v20[0];
    derivs[3*j + 1] = dBydx * v10[1] + dBydy * v20[1];
    derivs[3*j + 2] = dBydx * v10[2] + dBydy * v20[2];
    }
}

//----------------------------------------------------------------------------
// support quad clipping
typedef int QUAD_EDGE_LIST;
typedef struct {
       QUAD_EDGE_LIST edges[14];
} QUAD_CASES;
 
static QUAD_CASES quadCases[] = { 
{{  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 0
{{   3, 100,   0,   3,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 1
{{   3, 101,   1,   0,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 2
{{   4, 100, 101,   1,   3,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 3
{{   3, 102,   2,   1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 4
{{   3, 100,   0,   3,   3, 102,   2,   1,   4,   0,   1,   2,   3,  -1}}, // 5
{{   4, 101, 102,   2,   0,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 6
{{   3, 100, 101,   3,   3, 101,   2,   3,   3, 101, 102,   2,  -1,  -1}}, // 7
{{   3, 103,   3,   2,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 8
{{   4, 100,   0,   2, 103,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 9
{{   3, 101,   1,   0,   3, 103,   3,   2,   4,   0,   1,   2,   3,  -1}}, // 10
{{   3, 100, 101,   1,   3, 100,   1,   2,   3, 100,   2, 103,  -1,  -1}}, // 11
{{   4, 102, 103,   3,   1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 12
{{   3, 100,   0, 103,   3,   0,   1, 103,   3,   1, 102, 103,  -1,  -1}}, // 13
{{   3,   0, 101, 102,   3,   0, 102,   3,   3, 102, 103,   3,  -1,  -1}}, // 14
{{   4, 100, 101, 102, 103,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 15
};

static QUAD_CASES quadCasesComplement[] = { 
{{  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 0
{{   3, 100,   0,   3,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 1
{{   3, 101,   1,   0,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 2
{{   4, 100, 101,   1,   3,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 3
{{   3, 102,   2,   1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 4
{{   3, 100,   0,   3,   3, 102,   2,   1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 5
{{   4, 101, 102,   2,   0,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 6
{{   3, 100, 101,   3,   3, 101,   2,   3,   3, 101, 102,   2,  -1,  -1}}, // 7
{{   3, 103,   3,   2,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 8
{{   4, 100,   0,   2, 103,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 9
{{   3, 101,   1,   0,   3, 103,   3,   2,  -1,  -1,  -1,  -1,  -1,  -1}}, // 10
{{   3, 100, 101,   1,   3, 100,   1,   2,   3, 100,   2, 103,  -1,  -1}}, // 11
{{   4, 102, 103,   3,   1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 12
{{   3, 100,   0, 103,   3,   0,   1, 103,   3,   1, 102, 103,  -1,  -1}}, // 13
{{   3,   0, 101, 102,   3,   0, 102,   3,   3, 102, 103,   3,  -1,  -1}}, // 14
{{   4, 100, 101, 102, 103,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1,  -1}}, // 15
};

//----------------------------------------------------------------------------
// Clip this quad using scalar value provided. Like contouring, except
// that it cuts the quad to produce other quads and/or triangles.
void vtkQuad::Clip(double value, vtkDataArray *cellScalars, 
                   vtkPointLocator *locator, vtkCellArray *polys,
                   vtkPointData *inPd, vtkPointData *outPd,
                   vtkCellData *inCd, vtkIdType cellId, vtkCellData *outCd,
                   int insideOut)
{
  static int CASE_MASK[4] = {1,2,4,8};
  QUAD_CASES *quadCase;
  QUAD_EDGE_LIST  *edge;
  int i, j, index, *vert;
  int e1, e2;
  int newCellId;
  vtkIdType pts[4];
  int vertexId;
  double t, x1[3], x2[3], x[3], deltaScalar;
  double scalar0, scalar1, e1Scalar;

  // Build the index into the case table
  if ( insideOut )
    {    
    for ( i=0, index = 0; i < 4; i++)
      {
      if (cellScalars->GetComponent(i,0) <= value)
        {
        index |= CASE_MASK[i];
        }
      }
    // Select case based on the index and get the list of edges for this case
    quadCase = quadCases + index;
    }    
  else
    {
    for ( i=0, index = 0; i < 4; i++)
      {
      if (cellScalars->GetComponent(i,0) > value)
        {
        index |= CASE_MASK[i];
        }
      }
    // Select case based on the index and get the list of edges for this case
    quadCase = quadCasesComplement + index;
    }

  edge = quadCase->edges;

  // generate each quad
  for ( ; edge[0] > -1; edge += edge[0]+1 )
    {
    for (i=0; i < edge[0]; i++) // insert quad or triangle
      {
      // vertex exists, and need not be interpolated
      if (edge[i+1] >= 100)
        {
        vertexId = edge[i+1] - 100;
        this->Points->GetPoint(vertexId, x);
        if ( locator->InsertUniquePoint(x, pts[i]) )
          {
          outPd->CopyData(inPd,this->PointIds->GetId(vertexId),pts[i]);
          }
        }

      else //new vertex, interpolate
        {
        vert = edges[edge[i+1]];

        // calculate a preferred interpolation direction
        scalar0 = cellScalars->GetComponent(vert[0],0);
        scalar1 = cellScalars->GetComponent(vert[1],0);
        deltaScalar = scalar1 - scalar0;

        if (deltaScalar > 0)
          {
          e1 = vert[0]; e2 = vert[1];
          e1Scalar = scalar0;
          }
        else
          {
          e1 = vert[1]; e2 = vert[0];
          e1Scalar = scalar1;
          deltaScalar = -deltaScalar;
          }

        // linear interpolation
        if (deltaScalar == 0.0)
          {
          t = 0.0;
          }
        else
          {
          t = (value - e1Scalar) / deltaScalar;
          }

        this->Points->GetPoint(e1, x1);
        this->Points->GetPoint(e2, x2);

        for (j=0; j<3; j++)
          {
          x[j] = x1[j] + t * (x2[j] - x1[j]);
          }

        if ( locator->InsertUniquePoint(x, pts[i]) )
          {
          vtkIdType p1 = this->PointIds->GetId(e1);
          vtkIdType p2 = this->PointIds->GetId(e2);
          outPd->InterpolateEdge(inPd,pts[i],p1,p2,t);
          }
        }
      }
    // check for degenerate output
    if ( edge[0] == 3 ) //i.e., a triangle
      {
      if (pts[0] == pts[1] || pts[0] == pts[2] || pts[1] == pts[2] )
        {
        continue;
        }
      }
    else // a quad
      {
      if ((pts[0] == pts[3] && pts[1] == pts[2]) || 
          (pts[0] == pts[1] && pts[3] == pts[2]) )
        {
        continue;
        }
      }

    newCellId = polys->InsertNextCell(edge[0],pts);
    outCd->CopyData(inCd,cellId,newCellId);
    }
}

//----------------------------------------------------------------------------
static double vtkQuadCellPCoords[12] = {0.0,0.0,0.0, 1.0,0.0,0.0,
                                       1.0,1.0,0.0, 0.0,1.0,0.0};
double *vtkQuad::GetParametricCoords()
{
  return vtkQuadCellPCoords;
}

//----------------------------------------------------------------------------
void vtkQuad::PrintSelf(ostream& os, vtkIndent indent)
{
  this->Superclass::PrintSelf(os,indent);
  
  os << indent << "Line:\n";
  this->Line->PrintSelf(os,indent.GetNextIndent());
  os << indent << "Triangle:\n";
  this->Triangle->PrintSelf(os,indent.GetNextIndent());
}