Delaunay2D.cxx

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// @(#)root/mathcore:$Id: Delaunay2D.h,v 1.00
// Authors: Daniel Funke, Lorenzo Moneta, Olivier Couet
 
/*************************************************************************
 * Copyright (C) 2015 ROOT Math Team                                     *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/
 
// Implementation file for class Delaunay2D
 
#include "Math/Delaunay2D.h"
#include "Rtypes.h"
#include "TError.h"
 
// workaround for https://github.com/artem-ogre/CDT/issues/228
#if defined(__GNUC__)
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wshadow"
#endif
#include "CDT/CDT.h"
#if defined(__GNUC__)
#pragma GCC diagnostic pop
#endif
 
#include <algorithm>
#include <cstdlib>
 
#include <iostream>
#include <limits>
 
 
namespace ROOT {
 
   namespace Math {
 
 
/// class constructor from array of data points
Delaunay2D::Delaunay2D(int n, const double * x, const double * y, const double * z,
                       double xmin, double xmax, double ymin, double ymax)
{
   // Delaunay2D normal constructor
 
   fX            = x;
   fY            = y;
   fZ            = z;
   fZout         = 0.;
   fNpoints      = n;
   fOffsetX      = 0;
   fOffsetY      = 0;
   fScaleFactorX = 0;
   fScaleFactorY = 0;
   fNdt          = 0;
   fXNmax        = 0;
   fXNmin        = 0;
   fYNmin        = 0;
   fYNmax        = 0;
 
   SetInputPoints(n,x,y,z,xmin,xmax,ymin,ymax);
 
}
 
/// set the input points
void Delaunay2D::SetInputPoints(int n, const double * x, const double * y, const double * z,
                           double xmin, double xmax, double ymin, double ymax) {
 
 
   fInit         = kFALSE;
 
   if (n == 0 || !x || !y || !z ) return;
 
   if (xmin >= xmax) {
      xmin = *std::min_element(x, x+n);
      xmax = *std::max_element(x, x+n);
 
      ymin = *std::min_element(y, y+n);
      ymax = *std::max_element(y, y+n);
   }
 
   fOffsetX      = -(xmax+xmin)/2.;
   fOffsetY      = -(ymax+ymin)/2.;
 
   if ( (xmax-xmin) != 0 ) {
      fScaleFactorX = 1./(xmax-xmin);
      fXNmax        = Linear_transform(xmax, fOffsetX, fScaleFactorX); //xTransformer(xmax);
      fXNmin        = Linear_transform(xmin, fOffsetX, fScaleFactorX); //xTransformer(xmin);
   } else {
      // we can't find triangle in this case
      fInit = true;
   }
 
   if (ymax-ymin != 0) {
      fScaleFactorY = 1./(ymax-ymin);
      fYNmax        = Linear_transform(ymax, fOffsetY, fScaleFactorY); //yTransformer(ymax);
      fYNmin        = Linear_transform(ymin, fOffsetY, fScaleFactorY); //yTransformer(ymin);
   } else {
      fInit = true;
   }
 
   fXCellStep    = 0.;
   fYCellStep    = 0.;
}
 
//______________________________________________________________________________
double Delaunay2D::Interpolate(double x, double y)
{
   // Return the interpolated z value corresponding to the given (x,y) point
 
   // Initialise the Delaunay algorithm if needed.
   // CreateTrianglesDataStructure computes fXoffset, fYoffset,
   // fXScaleFactor and fYScaleFactor;
   // needed in this function.
   FindAllTriangles();
 
   // handle case no triangles are found  return default value (i.e. 0)
   // to do: if points are aligned in one direction we could return in
   // some case the 1d interpolated value
   if (fNdt == 0) {
      return fZout;
   }
 
   // Find the z value corresponding to the point (x,y).
   double xx, yy;
   xx = Linear_transform(x, fOffsetX, fScaleFactorX); //xx = xTransformer(x);
   yy = Linear_transform(y, fOffsetY, fScaleFactorY); //yy = yTransformer(y);
   double zz = DoInterpolateNormalized(xx, yy);
 
   // the case of points on a regular grid (i.e. points on triangle edges) it is now handles in
   // DoInterpolateNormalized
 
   return zz;
}
 
//______________________________________________________________________________
void Delaunay2D::FindAllTriangles()
{
 
   if (fInit)
      return;
   else
      fInit = kTRUE;
 
   // Function used internally only. It creates the data structures needed to
   // compute the Delaunay triangles.
 
   // Offset fX and fY so they average zero, and scale so the average
   // of the X and Y ranges is one. The normalized version of fX and fY used
   // in Interpolate.
 
   DoNormalizePoints(); // call backend specific point normalization
 
   DoFindTriangles(); // call backend specific triangle finding
 
   fNdt = fTriangles.size();
}
 
// backend specific implementations
 
// CDT implementation (default case)
 
/// CDT implementation for points normalization
void Delaunay2D::DoNormalizePoints() {
   for (Int_t n = 0; n < fNpoints; n++) {
      fXN.push_back(Linear_transform(fX[n], fOffsetX, fScaleFactorX));
      fYN.push_back(Linear_transform(fY[n], fOffsetY, fScaleFactorY));
   }
 
   //also initialize fXCellStep and FYCellStep
   fXCellStep = fNCells / (fXNmax - fXNmin);
   fYCellStep = fNCells / (fYNmax - fYNmin);
}
 
/// CDT implementation for finding all the triangles
void Delaunay2D::DoFindTriangles() {
 
   int i;
   std::vector<CDT::V2d<double>> points(fNpoints);
   for (i = 0; i < fNpoints; ++i) points[i] = CDT::V2d<double>(fXN[i], fYN[i]);
   CDT::RemoveDuplicates(points);
   if (fNpoints-points.size() > 0)
      Warning("DoFindTriangles",
              "This TGraph2D has duplicated vertices. To remove them call RemoveDuplicates before drawing");
 
   CDT::Triangulation<double> cdt;
   cdt.insertVertices(points);
   cdt.eraseSuperTriangle();
 
   auto AllTriangles      = cdt.triangles;
   auto AllVertices       = cdt.vertices;
   int  NumberOfTriangles = cdt.triangles.size();
 
   fTriangles.resize(NumberOfTriangles);
 
   for(i = 0; i < NumberOfTriangles; i++){
      Triangle tri;
      const auto& t = AllTriangles[i];
 
      const auto& v0  = AllVertices[t.vertices[0]];
      tri.x[0]   = v0.x;
      tri.y[0]   = v0.y;
      tri.idx[0] = t.vertices[0];
 
      const auto& v1  = AllVertices[t.vertices[1]];
      tri.x[1]   = v1.x;
      tri.y[1]   = v1.y;
      tri.idx[1] = t.vertices[1];
 
      const auto& v2  = AllVertices[t.vertices[2]];
      tri.x[2]   = v2.x;
      tri.y[2]   = v2.y;
      tri.idx[2] = t.vertices[2];
 
      tri.invDenom = 1 / ( (tri.y[1] - tri.y[2])*(tri.x[0] - tri.x[2]) + (tri.x[2] - tri.x[1])*(tri.y[0] - tri.y[2]) );
 
      fTriangles[i] = tri;
 
      auto bx = std::minmax({tri.x[0], tri.x[1], tri.x[2]});
      auto by = std::minmax({tri.y[0], tri.y[1], tri.y[2]});
 
      unsigned int cellXmin = CellX(bx.first);
      unsigned int cellXmax = CellX(bx.second);
 
      unsigned int cellYmin = CellY(by.first);
      unsigned int cellYmax = CellY(by.second);
 
      for(unsigned int j = cellXmin; j <= cellXmax; j++) {
         for(unsigned int k = cellYmin; k <= cellYmax; k++) {
            fCells[Cell(j,k)].insert(i);
         }
      }
   }
}
 
/// Triangle implementation for interpolation
/// Finds the Delaunay triangle that the point (xi,yi) sits in (if any) and
/// calculate a z-value for it by linearly interpolating the z-values that
/// make up that triangle.
/// Relay that all the triangles have been found before
/// see comment in class description (in Delaunay2D.h) for implementation details:
/// finding barycentric coordinates and computing the interpolation
double Delaunay2D::DoInterpolateNormalized(double xx, double yy)
{
 
   // compute barycentric coordinates of a point P(xx,yy,zz)
   auto bayCoords = [&](const unsigned int t) -> std::tuple<double, double, double> {
      double la = ((fTriangles[t].y[1] - fTriangles[t].y[2]) * (xx - fTriangles[t].x[2]) +
                   (fTriangles[t].x[2] - fTriangles[t].x[1]) * (yy - fTriangles[t].y[2])) *
                  fTriangles[t].invDenom;
      double lb = ((fTriangles[t].y[2] - fTriangles[t].y[0]) * (xx - fTriangles[t].x[2]) +
                   (fTriangles[t].x[0] - fTriangles[t].x[2]) * (yy - fTriangles[t].y[2])) *
                  fTriangles[t].invDenom;
 
      return std::make_tuple(la, lb, (1 - la - lb));
   };
 
   // function to test if a point with barycentric coordinates (a,b,c) is inside the triangle
   // If the point is outside one or more of the coordinate are negative.
   // If the point is on a triangle edge, one of the coordinate (the one not part of the edge) is zero.
   // Due to numerical error, it can happen that if the point is at the edge the result is a small negative value.
   // Use then a tolerance (of - eps) to still consider the point within the triangle
   auto inTriangle = [](const std::tuple<double, double, double> &coords) -> bool {
      constexpr double eps = -4 * std::numeric_limits<double>::epsilon();
      return std::get<0>(coords) > eps && std::get<1>(coords) > eps && std::get<2>(coords) > eps;
   };
 
   int cX = CellX(xx);
   int cY = CellY(yy);
 
   if (cX < 0 || cX > fNCells || cY < 0 || cY > fNCells)
      return fZout; // TODO some more fancy interpolation here
 
   for (unsigned int t : fCells[Cell(cX, cY)]) {
 
      auto coords = bayCoords(t);
 
      // std::cout << "result of bayCoords " << std::get<0>(coords) <<
      //     "  " << std::get<1>(coords)  << "   " << std::get<2>(coords) << std::endl;
 
      if (inTriangle(coords)) {
 
         // we found the triangle -> interpolate using the barycentric interpolation
 
         return std::get<0>(coords) * fZ[fTriangles[t].idx[0]] + std::get<1>(coords) * fZ[fTriangles[t].idx[1]] +
                std::get<2>(coords) * fZ[fTriangles[t].idx[2]];
      }
   }
 
   // no triangle found return standard value
   return fZout;
}
 
} // namespace Math
} // namespace ROOT