fix the calculation of a mc position at a given z

In CbmStsPoint.cxx the GetX(z) calculation produces unexpected output when z is outside of the (fX, fX_out) interval.

For example, when z==fZ or z==fZ_out, the method is expected to return fX and fX_out respectively, but it returns the mean value (fX_out+fX)/2 instead.

The MR fixes the calculation.

// -----   Point x coordinate from linear extrapolation   ------------------
double CbmStsPoint::GetX(double z) const
{
  //  LOG(info) << fZ << " " << z << " " << fZ_out;
  if ((fZ_out - z) * (fZ - z) >= 0.) return (fX_out + fX) / 2.;
  double dz = fZ_out - fZ;
  return (fX + (z - fZ) / dz * (fX_out - fX));
}
// -------------------------------------------------------------------------

Dear @v.friese,

you have been identified as code owner of at least one file which was changed with this merge request.

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• fc525224...6dabb4fa - 4 commits from branch computing:master
• f853ff05 - fix the calculation of a mc position at a given z

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The method is meant for interpolation of the trajectory within the sensor, so the valid z range is [fZ, fZ_out]. Using it outside of the sensor, i.e. for z < fZ or z > fZ_out is questionable, but of course mathematically possible.

The current implementation returns the coordinates at the mid-plane of the sensor if z is outside. This is probably not a good recipe, apart from that, as you rightly pointed out, it also happens at the sensor surface.

The possible options are:

1. Throw an error for illegal z arguments;
2. Allow extrapolation outside of the sensor. This is what you propose. However, your implementation is not correct; it should read if (fabs(dz) < 1.e-4) return fX_out;

resolved all threads

added 5 commits

• f853ff05...e283f472 - 4 commits from branch computing:master
• ac16e4ac - fix the calculation of a mc position at a given z

Compare with previous version

enabled an automatic merge when the pipeline for ac16e4ac succeeds

merged