FastCircle.cc

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#include "RecoTracker/TkSeedGenerator/interface/FastCircle.h"
#include "DataFormats/Math/interface/AlgebraicROOTObjects.h"

FastCircle::FastCircle(const GlobalPoint& outerHit, const GlobalPoint& middleHit, const GlobalPoint& aVertex)
    : theOuterPoint(outerHit),
      theInnerPoint(middleHit),
      theVertexPoint(aVertex),
      theNorm(128.),
      theX0(0.),
      theY0(0.),
      theRho(0.),
      theN1(0.),
      theN2(0.),
      theC(0.),
      theValid(true),
      theIsLine(false) {
  createCircleParameters();
}

FastCircle::FastCircle(const GlobalPoint& outerHit,
                       const GlobalPoint& middleHit,
                       const GlobalPoint& aVertex,
                       double norm)
    : theOuterPoint(outerHit),
      theInnerPoint(middleHit),
      theVertexPoint(aVertex),
      theNorm(norm),
      theX0(0.),
      theY0(0.),
      theRho(0.),
      theN1(0.),
      theN2(0.),
      theC(0.),
      theValid(true),
      theIsLine(false) {
  createCircleParameters();
}

namespace {
  inline AlgebraicVector3 transform(const GlobalPoint& aPoint, float norm) {
    AlgebraicVector3 riemannPoint;

    auto p = aPoint.basicVector() / norm;
    float R2 = p.perp2();
    float fact = 1.f / (1.f + R2);  // let's factorize the common factor out
    riemannPoint[0] = fact * p.x();
    riemannPoint[1] = fact * p.y();
    riemannPoint[2] = fact * R2;

    return riemannPoint;
  }

}  // namespace

void FastCircle::createCircleParameters() {
  AlgebraicVector3 x = transform(theOuterPoint, theNorm);
  AlgebraicVector3 y = transform(theInnerPoint, theNorm);
  AlgebraicVector3 z = transform(theVertexPoint, theNorm);

  AlgebraicVector3 n;

  n[0] = x[1] * (y[2] - z[2]) + y[1] * (z[2] - x[2]) + z[1] * (x[2] - y[2]);
  n[1] = -(x[0] * (y[2] - z[2]) + y[0] * (z[2] - x[2]) + z[0] * (x[2] - y[2]));
  n[2] = x[0] * (y[1] - z[1]) + y[0] * (z[1] - x[1]) + z[0] * (x[1] - y[1]);

  double mag2 = n[0] * n[0] + n[1] * n[1] + n[2] * n[2];
  if (mag2 < 1.e-20) {
    theValid = false;
    return;
  }
  n.Unit();  // reduce n to a unit vector
  double c = -(n[0] * x[0] + n[1] * x[1] + n[2] * x[2]);
  //  c = -(n[0]*y[0] + n[1]*y[1] + n[2]*y[2]);
  //  c = -(n[0]*z[0] + n[1]*z[1] + n[2]*z[2]);

  theN1 = n[0];
  theN2 = n[1];
  theC = c;

  if (fabs(c + n[2]) < 1.e-5) {
    // numeric limit
    // circle is more a straight line...
    theValid = false;
    theIsLine = true;
    return;
  }

  double x0 = -n[0] / (2. * (c + n[2]));
  double y0 = -n[1] / (2. * (c + n[2]));
  double rho = sqrt((n[0] * n[0] + n[1] * n[1] - 4. * c * (c + n[2]))) / fabs(2. * (c + n[2]));

  theX0 = theNorm * x0;
  theY0 = theNorm * y0;
  theRho = theNorm * rho;
}