From 3342528c080bfa129660a8c2370e65b8043f3a3f Mon Sep 17 00:00:00 2001
From: Fabio Cossutti <fabio.cossutti@ts.infn.it>
Date: Tue, 8 Nov 2022 10:33:52 +0100
Subject: [PATCH] Move cluster error to weighted mean position error, based on
 cell size

---
 DataFormats/FTLRecHit/interface/FTLCluster.h         | 12 ++++++++++++
 .../FTLClusterizer/interface/MTDCPEBase.h            |  2 ++
 RecoLocalFastTime/FTLClusterizer/src/MTDCPEBase.cc   |  7 ++++---
 3 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/DataFormats/FTLRecHit/interface/FTLCluster.h b/DataFormats/FTLRecHit/interface/FTLCluster.h
index 2a45a93fee89a..51e0e26c0f9f6 100644
--- a/DataFormats/FTLRecHit/interface/FTLCluster.h
+++ b/DataFormats/FTLRecHit/interface/FTLCluster.h
@@ -127,6 +127,18 @@ class FTLCluster {
     return weighted_mean(this->theHitENERGY, y_pos);
   }
 
+  inline float positionError(const float sigmaPos) const {
+    float sumW2(0.f), sumW(0.f);
+    for (const auto& hitW : theHitENERGY) {
+      sumW2 += hitW * hitW;
+      sumW += hitW;
+    }
+    if (sumW > 0)
+      return sigmaPos * std::sqrt(sumW2) / sumW;
+    else
+      return -999.f;
+  }
+
   inline float time() const {
     auto t = [this](unsigned int i) { return this->theHitTIME[i]; };
     return weighted_mean(this->theHitENERGY, t);
diff --git a/RecoLocalFastTime/FTLClusterizer/interface/MTDCPEBase.h b/RecoLocalFastTime/FTLClusterizer/interface/MTDCPEBase.h
index 4330904c84224..125abb8fa5779 100644
--- a/RecoLocalFastTime/FTLClusterizer/interface/MTDCPEBase.h
+++ b/RecoLocalFastTime/FTLClusterizer/interface/MTDCPEBase.h
@@ -80,6 +80,8 @@ class MTDCPEBase : public MTDClusterParameterEstimator {
   virtual TimeValue clusterTime(DetParam const& dp, ClusterParam& cp) const;
   virtual TimeValueError clusterTimeError(DetParam const& dp, ClusterParam& cp) const;
 
+  static constexpr float sigma_flat = 1.f / std::sqrt(12.f);
+
 protected:
   //---------------------------------------------------------------------------
   //  Data members
diff --git a/RecoLocalFastTime/FTLClusterizer/src/MTDCPEBase.cc b/RecoLocalFastTime/FTLClusterizer/src/MTDCPEBase.cc
index 85d7aec9a379c..8f0c6b63af812 100644
--- a/RecoLocalFastTime/FTLClusterizer/src/MTDCPEBase.cc
+++ b/RecoLocalFastTime/FTLClusterizer/src/MTDCPEBase.cc
@@ -66,10 +66,11 @@ LocalPoint MTDCPEBase::localPosition(DetParam const& dp, ClusterParam& cp) const
 }
 
 LocalError MTDCPEBase::localError(DetParam const& dp, ClusterParam& cp) const {
-  constexpr double one_over_twelve = 1. / 12.;
   MeasurementPoint pos(cp.theCluster->x(), cp.theCluster->y());
-  MeasurementError simpleRect(one_over_twelve, 0, one_over_twelve);
-  return dp.theTopol->localError(pos, simpleRect);
+  float sigma2 = cp.theCluster->positionError(sigma_flat);
+  sigma2 *= sigma2;
+  MeasurementError posErr(sigma2, 0, sigma2);
+  return dp.theTopol->localError(pos, posErr);
 }
 
 MTDCPEBase::TimeValue MTDCPEBase::clusterTime(DetParam const& dp, ClusterParam& cp) const {