Field.cpp

// Gmsh - Copyright (C) 1997-2009 C. Geuzaine, J.-F. Remacle
//
// See the LICENSE.txt file for license information. Please report all
// bugs and problems to <gmsh@geuz.org>.
//
// Contributor(s):
//   Jonathan Lambrechts
//

#include <list>
#include <math.h>
#include <fstream>
#include <string>
#include <string.h>
#include <sstream>
#include "GmshConfig.h"

#if defined(HAVE_MATH_EVAL)
#include "matheval.h"
#endif
#if defined(HAVE_ANN)
#include "ANN/ANN.h"
#endif

#include "Context.h"
#include "Field.h"
#include "GeoInterpolation.h"
#include "GModel.h"
#include "GmshMessage.h"
#include "Numeric.h"

#if !defined(HAVE_NO_POST)
#include "OctreePost.h"
#include "PViewDataList.h"
#include "MVertex.h"
#endif

class FieldOptionDouble : public FieldOption
{
 public:
  double &val;
  FieldOptionType get_type(){ return FIELD_OPTION_DOUBLE; }
  FieldOptionDouble(double &_val, std::string _help, bool *_status=0)
    : FieldOption(_help, _status), val(_val){}
  double numerical_value() const { return val; }
  void numerical_value(double v){ modified(); val = v; }
  void get_text_representation(std::string &v_str)
  {
    std::ostringstream sstream;
    sstream.precision(16);
    sstream << val;
    v_str = sstream.str();
  }
};

class FieldOptionInt : public FieldOption
{
 public:
  int &val;
  FieldOptionType get_type(){ return FIELD_OPTION_INT; }
  FieldOptionInt(int &_val, std::string _help, bool *_status=0) 
    : FieldOption(_help, _status), val(_val){}
  double numerical_value() const { return val; }
  void numerical_value(double v){ modified(); val = (int)v; }
  void get_text_representation(std::string & v_str)
  {
    std::ostringstream sstream;
    sstream << val;
    v_str = sstream.str();
  }
};

class FieldOptionList : public FieldOption
{
 public:
  std::list<int> &val;
  FieldOptionType get_type(){ return FIELD_OPTION_LIST; }
  FieldOptionList(std::list<int> &_val, std::string _help, bool *_status=0) 
    : FieldOption(_help, _status), val(_val) {}
  std::list<int> &list(){ modified(); return val; }
  const std::list<int>& list() const { return val; }
  void get_text_representation(std::string & v_str)
  {
    std::ostringstream sstream;
    sstream << "{";
    for(std::list<int>::iterator it = val.begin(); it != val.end(); it++) {
      if(it != val.begin())
        sstream << ", ";
      sstream << *it;
    }
    sstream << "}";
    v_str = sstream.str();
  }
};

class FieldOptionString : public FieldOption
{
 public:
  std::string & val;
  virtual FieldOptionType get_type(){ return FIELD_OPTION_STRING; }
  FieldOptionString(std::string &_val, std::string _help, bool *_status=0)
    : FieldOption(_help, _status), val(_val) {}
  std::string &string() { modified(); return val; }
  const std::string &string() const { return val; }
  void get_text_representation(std::string &v_str)
  {
    std::ostringstream sstream;
    sstream << "\"" << val << "\"";
    v_str = sstream.str();
  }
};

class FieldOptionPath : public FieldOptionString
{
 public:
  virtual FieldOptionType get_type(){ return FIELD_OPTION_PATH; }
  FieldOptionPath(std::string &_val, std::string _help, bool *_status=0)
    : FieldOptionString(_val, _help, _status) {}
};

class FieldOptionBool : public FieldOption
{
 public:
  bool & val;
  FieldOptionType get_type(){ return FIELD_OPTION_BOOL; }
  FieldOptionBool(bool & _val, std::string _help, bool *_status=0)
    : FieldOption(_help, _status), val(_val) {}
  double numerical_value() const { return val; }
  void numerical_value(double v){ modified(); val = v; }
  void get_text_representation(std::string & v_str)
  {
    std::ostringstream sstream;
    sstream << val;
    v_str = sstream.str();
  }
};

void FieldManager::reset()
{
  for(std::map<int, Field *>::iterator it = begin(); it != end(); it++) {
    delete it->second;
  }
  clear();
}

Field *FieldManager::get(int id)
{
  iterator it = find(id);
  if(it == end()) return NULL;
  return it->second;
}

Field *FieldManager::new_field(int id, std::string type_name)
{
  if(find(id) != end()) {
    Msg::Error("Field id %i is already defined.", id);
    return 0;
  }
  if(map_type_name.find(type_name) == map_type_name.end()) {
    Msg::Error("Unknown field type \"%s\".", type_name.c_str());
    return 0;
  }
  Field *f = (*map_type_name[type_name]) ();
  if(!f)
    return 0;
  f->id = id;
  (*this)[id] = f;
  return f;
}

int FieldManager::new_id()
{
  int i = 0;
  iterator it = begin();
  while(1) {
    i++;
    while(it != end() && it->first < i)
      it++;
    if(it == end() || it->first != i)
      break;
  }
  return std::max(i, 1);
}

int FieldManager::max_id()
{
  if(!empty())
    return rbegin()->first;
  else
    return 0;
}

void FieldManager::delete_field(int id)
{
  iterator it = find(id);
  if(it == end()) {
    Msg::Error("Cannot delete field id %i, it does not exist.", id);
    return;
  }
  delete it->second;
  erase(it);
}

// StructuredField
class StructuredField : public Field
{
  double o[3], d[3];
  int n[3];
  double *data;
  bool error_status;
  bool text_format;
  std::string file_name;
 public:
  StructuredField()
  {
    options["FileName"] = new FieldOptionPath
      (file_name, "Name of the input file", &update_needed);
    text_format = false;
    options["TextFormat"] = new FieldOptionBool
      (text_format, "True for ASCII input files, false for binary files (4 bite\n"
       "signed integers for n, double precision floating points for v, D and O)", 
       &update_needed);
    data = 0;
  }
  std::string get_description()
  {
    return "Linearly interpolate between data provided on a 3D rectangular\n"
      "structured grid.\n\n"
      "The format of the input file is:\n\n"
      "  Ox Oy Oz \n"
      "  Dx Dy Dz \n"
      "  nx ny nz \n"
      "  v(0,0,0) v(0,0,1) v(0,0,2) ... \n"
      "  v(0,1,0) v(0,1,1) v(0,1,2) ... \n"
      "  v(0,2,0) v(0,2,1) v(0,2,2) ... \n"
      "  ...      ...      ... \n"
      "  v(1,0,0) ...      ... \n\n"
      "where O are the coordinates of the first node, D are the distances\n"
      "between nodes in each direction, n are the numbers of nodes in each\n"
      "direction, and v are the values on each node.";
  }
  const char *get_name()
  {
    return "Structured";
  }
  virtual ~StructuredField()
  {
    if(data) delete[]data;
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    if(update_needed) {
      error_status = false;
      try {
        std::ifstream input;
        if(text_format)
          input.open(file_name.c_str());
        else
          input.open(file_name.c_str(),std::ios::binary);
        if(!input.is_open())
          throw(1);
        input.
          exceptions(std::ifstream::eofbit | std::ifstream::failbit | std::
                     ifstream::badbit);
        if(!text_format) {
          input.read((char *)o, 3 * sizeof(double));
          input.read((char *)d, 3 * sizeof(double));
          input.read((char *)n, 3 * sizeof(int));
          int nt = n[0] * n[1] * n[2];
          if(data)
            delete[]data;
          data = new double[nt];
          input.read((char *)data, nt * sizeof(double));
        }
        else {
          input >> o[0] >> o[1] >> o[2] >> d[0] >> d[1] >> d[2] >> n[0] >>
            n[1] >> n[2];
          int nt = n[0] * n[1] * n[2];
          if(data)
            delete[]data;
          data = new double[nt];
          for(int i = 0; i < nt; i++)
            input >> data[i];
        }
        input.close();
      }
      catch(...) {
        error_status = true;
        Msg::Error("Field %i : error reading file %s", this->id, file_name.c_str());
      }
      update_needed = false;
    }
    if(error_status)
      return MAX_LC;
    //tri-linear
    int id[2][3];
    double xi[3];
    double xyz[3] = { x, y, z };
    for(int i = 0; i < 3; i++) {
      id[0][i] = (int)floor((xyz[i] - o[i]) / d[i]);
      id[1][i] = id[0][i] + 1;
      id[0][i] = std::max(std::min(id[0][i], n[i] - 1), 0);
      id[1][i] = std::max(std::min(id[1][i], n[i] - 1), 0);
      xi[i] = (xyz[i] - (o[i] + id[0][i] * d[i])) / d[i];
      xi[i] = std::max(std::min(xi[i], 1.), 0.);
    }
    double v = 0;
    for(int i = 0; i < 2; i++)
      for(int j = 0; j < 2; j++)
        for(int k = 0; k < 2; k++) {
          v += data[id[i][0] * n[1] * n[2] + id[j][1] * n[2] + id[k][2]]
            * (i * xi[0] + (1 - i) * (1 - xi[0]))
            * (j * xi[1] + (1 - j) * (1 - xi[1]))
            * (k * xi[2] + (1 - k) * (1 - xi[2]));
        }
    return v;
  }
};

class UTMField : public Field
{
  int field_id, zone;
  double a, b, n, n2, n3, n4, n5, e, e2, e1, e12, e13, e14, J1, J2, J3, J4,
    Ap, Bp, Cp, Dp, Ep, e4, e6, ep, ep2, ep4, k0, mu_fact;
 public:
  std::string get_description()
  {
    return "Evaluate Field[IField] in Universal Transverse Mercator coordinates.\n"
      "The formulas for the coordinates transformation are taken from:\n\n"
      "  http://www.uwgb.edu/dutchs/UsefulData/UTMFormulas.HTM";
  }
  UTMField()
  {
    field_id = 1;
    zone = 0;
    options["IField"] = new FieldOptionInt
      (field_id, "Index of the field to evaluate");
    options["Zone"] = new FieldOptionInt
      (zone, "Zone of the UTM projection");
    a = 6378137; // Equatorial Radius
    b = 6356752.3142; // Rayon Polar Radius
    // see http://www.uwgb.edu/dutchs/UsefulData/UTMFormulas.HTM
    n = (a - b) / (a + b);
    n2 = n * n;
    n3 = n * n * n;
    n4 = n * n * n * n;
    n5 = n * n * n * n * n;
    e = sqrt(1 - b * b / a / a);
    e2 = e * e;
    e1 = (1 - sqrt(1 - e2)) / (1 + sqrt(1 - e2));
    e12 = e1 * e1;
    e13 = e1 * e1 * e1;
    e14 = e1 * e1 * e1 * e1;
    J1 = (3 * e1 / 2 - 27 * e13 / 32);
    J2 = (21 * e12 / 16 - 55 * e14 / 32);
    J3 = 151 * e13 / 96;
    J4 = 1097 * e14 / 512;
    Ap = a * (1 - n + (5. / 4.) * (n2 - n3) + (81. / 64.) * (n4 - n5));
    Bp = -3 * a * n / 2 * (1 - n + (7. / 8.) * (n2 - n3) +
                           (55. / 64.) * (n4 - n5));
    Cp = 14 * a * n2 / 16 * (1 - n + (3. / 4) * (n2 - n3));
    Dp = -35 * a * n3 / 48 * (1 - n + 11. / 16. * (n2 - n3));
    Ep = +315 * a * n4 / 51 * (1 - n);
    e4 = e2 * e2;
    e6 = e2 * e2 * e2;
    ep = e * a / b;
    ep2 = ep * ep;
    ep4 = ep2 * ep2;
    k0 = 0.9996;
    mu_fact = 1 / (k0 * a * (1 - e2 / 4 - 3 * e4 / 64 - 5 * e6 / 256));
  }
  const char *get_name()
  {
    return "UTM";
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    double r = sqrt(x * x + y * y + z * z);
    double lon = atan2(y, x);
    double lat = asin(z / r);
    double meridionalarc = Ap * lat + Bp * sin(2 * lat)
      + Cp * sin(4 * lat) + Dp * sin(6 * lat) + Ep;
    double slat = sin(lat);
    double clat = cos(lat);
    double slat2 = slat * slat;
    double clat2 = clat * clat;
    double clat3 = clat2 * clat;
    double clat4 = clat3 * clat;
    double tlat2 = slat2 / clat2;
    double nu = a / sqrt(1 - e * e * slat2);
    double p = lon - ((zone - 0.5) / 30 - 1) * M_PI;
    double p2 = p * p;
    double p3 = p * p2;
    double p4 = p2 * p2;
    double utm_x =
      k0 * nu * clat * p + (k0 * nu * clat3 / 6) * (1 - tlat2 +
                                                    ep2 * clat2) * p3 + 5e5;
    double utm_y =
      meridionalarc * k0 + k0 * nu * slat * clat / 2 * p2 +
      k0 * nu * slat * clat3 / 24 * (5 - tlat2 + 9 * ep2 * clat2 +
                                     4 * ep4 * clat4) * p4;
    Field *field = GModel::current()->getFields()->get(field_id);
    if(!field) return MAX_LC;
    return (*field)(utm_x, utm_y, 0);
  }
};

class LonLatField : public Field
{
  int field_id;
 public:
  std::string get_description()
  {
    return "Evaluate Field[IField] in geographic coordinates (longitude, latitude):\n\n"
      "  F = Field[IField](atan(y/x), asin(z/sqrt(x^2+y^2+z^2))";
  }
  LonLatField()
  {
    field_id = 1;
    options["IField"] = new FieldOptionInt
      (field_id, "Index of the field to evaluate.");
  }
  const char *get_name()
  {
    return "LonLat";
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(field_id);
    if(!field) return MAX_LC;
    return (*field)(atan2(y, x), asin(z / sqrt(x * x + y * y + z * z)), 0);
  }
};

class BoxField : public Field
{
  double v_in, v_out, x_min, x_max, y_min, y_max, z_min, z_max;
 public:
  std::string get_description()
  {
    return "The value of this field is VIn inside the box, VOut outside the box.\n"
      "The box is given by\n\n"
      "  Xmin <= x <= XMax &&\n"
      "  YMin <= y <= YMax &&\n"
      "  ZMin <= z <= ZMax";
  }
  BoxField()
  {
    v_in = v_out = x_min = x_max = y_min = y_max = z_min = z_max = 0;
    options["VIn"] = new FieldOptionDouble
      (v_in, "Value inside the box");
    options["VOut"] = new FieldOptionDouble
      (v_out, "Value outside the box");
    options["XMin"] = new FieldOptionDouble
      (x_min, "Minimum X coordinate of the box");
    options["XMax"] = new FieldOptionDouble
      (x_max, "Maximum X coordinate of the box");
    options["YMin"] = new FieldOptionDouble
      (y_min, "Minimum Y coordinate of the box");
    options["YMax"] = new FieldOptionDouble
      (y_max, "Maximum Y coordinate of the box");
    options["ZMin"] = new FieldOptionDouble
      (z_min, "Minimum Z coordinate of the box");
    options["ZMax"] = new FieldOptionDouble
      (z_max, "Maximum Z coordinate of the box");
  }
  const char *get_name()
  {
    return "Box";
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    return (x <= x_max && x >= x_min && y <= y_max && y >= y_min && z <= z_max
            && z >= z_min) ? v_in : v_out;
  }
};

class ThresholdField : public Field
{
  int iField;
  double dmin, dmax, lcmin, lcmax;
  bool sigmoid, stopAtDistMax;
 public:
  const char *get_name()
  {
    return "Threshold";
  }
  std::string get_description()
  {
    return "F = LCMin if Field[IField] <= DistMin,\n"
      "F = LCMax if Field[IField] >= DistMax,\n"
      "F = interpolation between LcMin and LcMax if DistMin < Field[IField] < DistMax";
  }
  ThresholdField()
  {
    iField = 0;
    dmin = 1;
    dmax = 10;
    lcmin = 0.1;
    lcmax = 1;
    sigmoid = false;
    stopAtDistMax = false;
    options["IField"] = new FieldOptionInt
      (iField, "Index of the field to evaluate");
    options["DistMin"] = new FieldOptionDouble
      (dmin, "Distance from entity up to which element size will be LcMin");
    options["DistMax"] = new FieldOptionDouble
      (dmax, "Distance from entity after which element size will be LcMax");
    options["LcMin"] = new FieldOptionDouble
      (lcmin, "Element size inside DistMin");
    options["LcMax"] = new FieldOptionDouble
      (lcmax, "Element size outside DistMax");
    options["Sigmoid"] = new FieldOptionBool
      (sigmoid, "True to interpolate between LcMin and LcMax using a sigmoid,\n"
       "false to interpolate linearly");
    options["StopAtDistMax"] = new FieldOptionBool
      (stopAtDistMax, "True to not impose element size outside DistMax (i.e.,\n"
       "F = a very big value if Field[IField] > DistMax)");
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    double r = ((*field) (x, y, z) - dmin) / (dmax - dmin);
    r = std::max(std::min(r, 1.), 0.);
    double lc;
    if(stopAtDistMax && r >= 1.){
      lc = MAX_LC;
    }
    else if(sigmoid){
      double s = exp(12. * r - 6.) / (1. + exp(12. * r - 6.));
      lc = lcmin * (1. - s) + lcmax * s;
    }
    else{ // linear
      lc = lcmin * (1 - r) + lcmax * r;
    }
    return lc;
  }
};

class GradientField : public Field
{
  int iField, kind;
  double delta;
 public:
  const char *get_name()
  {
    return "Gradient";
  }
  std::string get_description()
  {
    return "Compute the finite difference gradient of Field[IField]:\n\n"
      "  F = (Field[IField](X + Delta/2) -\n"
      "       Field[IField](X - Delta/2)) / Delta";
  }
  GradientField() : iField(0), kind(3), delta(CTX::instance()->lc / 1e4)
  {
    iField = 1;
    kind = 0;
    delta = 0.;
    options["IField"] = new FieldOptionInt
      (iField, "Field index");
    options["Kind"] = new FieldOptionInt
      (kind, "Component of the gradient to evaluate: 0 for X, 1 for Y, 2 for Z,\n"
       "3 for the norm");
    options["Delta"] = new FieldOptionDouble
      (delta, "Finite difference step");
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    double gx, gy, gz;
    switch (kind) {
    case 0:    /* x */
      return ((*field) (x + delta / 2, y, z) -
              (*field) (x - delta / 2, y, z)) / delta;
    case 1:    /* y */
      return ((*field) (x, y + delta / 2, z) -
              (*field) (x, y - delta / 2, z)) / delta;
    case 2:    /* z */
      return ((*field) (x, y, z + delta / 2) -
              (*field) (x, y, z - delta / 2)) / delta;
    case 3:    /* norm */
      gx =
        ((*field) (x + delta / 2, y, z) -
         (*field) (x - delta / 2, y, z)) / delta;
      gy =
        ((*field) (x, y + delta / 2, z) -
         (*field) (x, y - delta / 2, z)) / delta;
      gz =
        ((*field) (x, y, z + delta / 2) -
         (*field) (x, y, z - delta / 2)) / delta;
      return sqrt(gx * gx + gy * gy + gz * gz);
    default:
      Msg::Error("Field %i : Unknown kind (%i) of gradient.", this->id,
		 kind);
      return MAX_LC;
    }
  }
};

class CurvatureField : public Field
{
  int iField;
  double delta;
 public:
  const char *get_name()
  {
    return "Curvature";
  }
  std::string get_description()
  {
    return "Compute the curvature of Field[IField]:\n\n"
      "  F = div(norm(grad(Field[IField])))";
  }
  CurvatureField() : iField(0), delta(CTX::instance()->lc / 1e4)
  {
    iField = 1;
    delta = 0.;
    options["IField"] = new FieldOptionInt
      (iField, "Field index");
    options["Delta"] = new FieldOptionDouble
      (delta, "Step of the finite differences");
  }
  void grad_norm(Field &f, double x, double y, double z, double *g)
  {
    g[0] = f(x + delta / 2, y, z) - f(x - delta / 2, y, z);
    g[1] = f(x, y + delta / 2, z) - f(x, y - delta / 2, z);
    g[2] = f(x, y, z + delta / 2) - f(x, y, z - delta / 2);
    double n=sqrt(g[0] * g[0] + g[1] * g[1] + g[2] * g[2]);
    g[0] /= n;
    g[1] /= n;
    g[2] /= n;
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    double grad[6][3];
    grad_norm(*field, x + delta / 2, y, z, grad[0]);
    grad_norm(*field, x - delta / 2, y, z, grad[1]);
    grad_norm(*field, x, y + delta / 2, z, grad[2]);
    grad_norm(*field, x, y - delta / 2, z, grad[3]);
    grad_norm(*field, x, y, z + delta / 2, grad[4]);
    grad_norm(*field, x, y, z - delta / 2, grad[5]);
    return (grad[0][0] - grad[1][0] + grad[2][1] - 
	    grad[3][1] + grad[4][2] - grad[5][2]) / delta;
  }
};

class MaxEigenHessianField : public Field
{
  int iField;
  double delta;
 public:
  const char *get_name()
  {
    return "MaxEigenHessian";
  }
  std::string get_description()
  {
    return "Compute the maximum eigenvalue of the Hessian matrix of\n"
      "Field[IField], with the gradients evaluated by finite differences:\n\n"
      "  F = max(eig(grad(grad(Field[IField]))))";
  }
  MaxEigenHessianField() : iField(0), delta(CTX::instance()->lc / 1e4)
  {
    iField = 1;
    delta = 0.;
    options["IField"] = new FieldOptionInt
      (iField, "Field index");
    options["Delta"] = new FieldOptionDouble
      (delta, "Step used for the finite differences");
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    double mat[3][3], eig[3];
    mat[1][0] = mat[0][1] = (*field) (x + delta / 2 , y + delta / 2, z)
      + (*field) (x - delta / 2 , y - delta / 2, z)
      - (*field) (x - delta / 2 , y + delta / 2, z)
      - (*field) (x + delta / 2 , y - delta / 2, z);
    mat[2][0] = mat[0][2] = (*field) (x + delta/2 , y, z + delta / 2)
      + (*field) (x - delta / 2 , y, z - delta / 2)
      - (*field) (x - delta / 2 , y, z + delta / 2)
      - (*field) (x + delta / 2 , y, z - delta / 2);
    mat[2][1] = mat[1][2] = (*field) (x, y + delta/2 , z + delta / 2)
      + (*field) (x, y - delta / 2 , z - delta / 2)
      - (*field) (x, y - delta / 2 , z + delta / 2)
      - (*field) (x, y + delta / 2 , z - delta / 2);
    double f = (*field)(x, y, z);
    mat[0][0] = (*field)(x + delta, y, z) + (*field)(x - delta, y, z) - 2 * f;
    mat[1][1] = (*field)(x, y + delta, z) + (*field)(x, y - delta, z) - 2 * f;
    mat[2][2] = (*field)(x, y, z + delta) + (*field)(x, y, z - delta) - 2 * f;
    eigenvalue(mat, eig);
    return eig[0] / (delta * delta);
  }
};

class LaplacianField : public Field
{
  int iField;
  double delta;
 public:
  const char *get_name()
  {
    return "Laplacian";
  }
  std::string get_description()
  {
    return "Compute finite difference the Laplacian of Field[IField]:\n\n"
      "  F = G(x+d,y,z) + G(x-d,y,z) +\n"
      "      G(x,y+d,z) + G(x,y-d,z) +\n"
      "      G(x,y,z+d) + G(x,y,z-d) - 6 * G(x,y,z),\n\n"
      "where G=Field[IField] and d=Delta";
  }
  LaplacianField() : iField(0), delta(CTX::instance()->lc / 1e4)
  {
    iField = 1;
    delta = 0.1;
    options["IField"] = new FieldOptionInt
      (iField, "Field index");
    options["Delta"] = new FieldOptionDouble
      (delta, "Finite difference step");
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    return ((*field) (x + delta , y, z)+ (*field) (x - delta , y, z)
	    +(*field) (x, y + delta , z)+ (*field) (x, y - delta , z)
	    +(*field) (x, y, z + delta )+ (*field) (x, y, z - delta )
	    -6* (*field) (x , y, z)) / (delta*delta);
  }
};

class MeanField : public Field
{
  int iField;
  double delta;
 public:
  const char *get_name()
  {
    return "Mean";
  }
  std::string get_description()
  {
    return "Simple smoother:\n\n"
      "  F = (G(x+delta,y,z) + G(x-delta,y,z) +\n"
      "       G(x,y+delta,z) + G(x,y-delta,z) +\n"
      "       G(x,y,z+delta) + G(x,y,z-delta) +\n"
      "       G(x,y,z)) / 7,\n\n"
      "where G=Field[IField]";
  }
  MeanField() : iField(0), delta(CTX::instance()->lc / 1e4)
  {
    options["IField"] = new FieldOptionInt
      (iField, "Field index");
    options["Delta"] = new FieldOptionDouble
      (delta, "Distance used to compute the mean value");
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    Field *field = GModel::current()->getFields()->get(iField);
    if(!field) return MAX_LC;
    return ((*field) (x + delta , y, z) + (*field) (x - delta, y, z)
	    + (*field) (x, y + delta, z) + (*field) (x, y - delta, z)
	    + (*field) (x, y, z + delta) + (*field) (x, y, z - delta)
	    + (*field) (x, y, z)) / 7;
  }
};

#if defined(HAVE_MATH_EVAL)
class MathEvalExpression
{
  bool error_status;
  std::list<Field*> *list;
  int nvalues;
  char **names;
  double *values;
  void *eval;
  int *evaluators_id;
  std::string function;
  char *c_str_function;
 public:
  double evaluate(double x, double y, double z)
  {
    if(error_status)
      return MAX_LC;
    for(int i = 0; i < nvalues; i++){
      switch (evaluators_id[i]) {
      case -1:
        values[i] = x;
        break;
      case -2: 
	values[i] = y;
        break;
      case -3: 
	values[i] = z;
        break;
      default:
        {
          Field *f = GModel::current()->getFields()->get(evaluators_id[i]);
          values[i] = f ? (*f) (x, y, z) : MAX_LC;
        }
      }
    }
    return evaluator_evaluate(eval, nvalues, names, values);
  }
  MathEvalExpression()
  {
    eval = 0;
    values = 0;
    c_str_function = 0;
    evaluators_id = 0;
  }
  bool set_function(const std::string & f)
  {
    free_members();
    error_status = false;
    c_str_function = strdup(f.c_str());
    eval = evaluator_create(c_str_function);
    if(!eval) {
      error_status = true;
      return false;
    }
    evaluator_get_variables(eval, &names, &nvalues);
    values = new double[nvalues];
    evaluators_id = new int[nvalues];
    for(int i = 0; i < nvalues; i++) {
      int id;
      if(!strcmp("x", names[i]))
        evaluators_id[i] = -1;
      else if(!strcmp("y", names[i]))
        evaluators_id[i] = -2;
      else if(!strcmp("z", names[i]))
        evaluators_id[i] = -3;
      else if(sscanf(names[i], "F%i", &id) == 1)
        evaluators_id[i] = id;
      else {
        Msg::Error("Unknown matheval argument \"%s\"\n", names[i]);
        error_status = true;
        return false;
      }
    }
    return true;
  }
  void free_members()
  {
    if(c_str_function)
      free(c_str_function);
    if(eval)
      evaluator_destroy(eval);
    if(values)
      delete[]values;
    if(evaluators_id)
      delete evaluators_id;
  }
  ~MathEvalExpression()
  {
    free_members();
  }
};

class MathEvalField : public Field
{
  MathEvalExpression expr;
  std::string f;
 public:
  MathEvalField()
  {
    options["F"] = new FieldOptionString
      (f, "Mathematical function to evaluate.", &update_needed);
    f = "F2 + Sin(z)";
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    if(update_needed) {
      if(!expr.set_function(f))
        Msg::Error("Field %i: Invalid matheval expression \"%s\"",
		   this->id, f.c_str());
      update_needed = false;
    }
    return expr.evaluate(x, y, z);
  }
  const char *get_name()
  {
    return "MathEval";
  }
  std::string get_description()
  {
    return "Evaluate a mathematical expression. The expression can contain\n"
      "x, y, z for spatial coordinates, F0, F1, ... for field values, and\n"
      "and mathematical functions.";
  }
};

class ParametricField : public Field
{
  MathEvalExpression expr[3];
  std::string f[3];
  int ifield;
 public:
  ParametricField()
  {
    ifield = 1;
    options["IField"] = new FieldOptionInt
      (ifield, "Field index");
    options["FX"] = new FieldOptionString
      (f[0], "X component of parametric function", &update_needed);
    options["FY"] = new FieldOptionString
      (f[1], "Y component of parametric function", &update_needed);
    options["FZ"] = new FieldOptionString
      (f[2], "Z component of parametric function", &update_needed);
  }
  std::string get_description()
  {
    return "Evaluate Field IField in parametric coordinates:\n\n"
      "  F = Field[IField](FX,FY,FZ)\n\n"
      "See the MathEval Field help to get a description of valid FX, FY\n"
      "and FZ expressions.";
  }
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    if(update_needed) {
      for(int i = 0; i < 3; i++) {
        if(!expr[i].set_function(f[i]))
          Msg::Error("Field %i : Invalid matheval expression \"%s\"",
		     this->id, f[i].c_str());
      }
      update_needed = false;
    }
    Field *field = GModel::current()->getFields()->get(ifield);
    if(!field) return MAX_LC;
    return (*field)(expr[0].evaluate(x, y, z),
		    expr[1].evaluate(x, y, z),
		    expr[2].evaluate(x, y, z));
  }
  const char *get_name()
  {
    return "Param";
  }
};
#endif

#if !defined(HAVE_NO_POST)
class PostViewField : public Field
{
  OctreePost *octree;
 public:
  int view_index;
  bool crop_negative_values;
  double operator() (double x, double y, double z, GEntity *ge=0)
  {
    // FIXME: should test unique view num instead, but that would be slower
    if(view_index < 0 || view_index >= (int)PView::list.size())
      return MAX_LC;
    if(update_needed){
      if(octree) delete octree;
      octree = new OctreePost(PView::list[view_index]);
      update_needed = false;
    }
    double l = 0.;
    if(!octree->searchScalarWithTol(x, y, z, &l, 0, 0, 10.))
      Msg::Info("No element found containing point (%g,%g,%g)", x, y, z);
    if(l <= 0 && crop_negative_values) return MAX_LC;
    return l;
  }
  const char *get_name()
  {
    return "PostView";
  }
  std::string get_description()
  {
    return "Evaluate the post processing view IView.";
  }
  PostViewField()
  {
    octree = 0;
    view_index = 0;
    options["IView"] = new FieldOptionInt
      (view_index, "Post-processing view index", &update_needed);
    crop_negative_values = true;
    options["CropNegativeValues"] = new FieldOptionBool
      (crop_negative_values, "return LC_MAX instead of a negative value (this\n"
       "option is needed for backward compatibility with the BackgroundMesh option",
       &update_needed);
  }
  ~PostViewField()
  {
    if(octree) delete octree;
  }
};
#endif

class MinField : public Field
{
  std::list<int> idlist;