feat: extract impact parameters and their covariance matrix (#2464)

Allow extraction of the 

- impact parameters (i.e., `d0`, `z0`, and, optionally, `t`) and
- the corresponding covariance matrix

from `GenericBoundTrackParameters`

Core/include/Acts/EventData/GenericBoundTrackParameters.hpp

@@ -116,10 +116,46 @@ class GenericBoundTrackParameters {
   ParametersVector& parameters() { return m_params; }
   /// Parameters vector.
   const ParametersVector& parameters() const { return m_params; }
+  /// Vector of spatial impact parameters (i.e., d0 and z0)
+  ActsVector<2> spatialImpactParameters() const { return m_params.head<2>(); }
+  /// Vector of spatial and temporal impact parameters (i.e., d0, z0, and t)
+  ActsVector<3> impactParameters() const {
+    ActsVector<3> ip;
+    ip.template head<2>() = m_params.template head<2>();
+    ip(2) = m_params(eBoundTime);
+    return ip;
+  }
+
   /// Optional covariance matrix.
   std::optional<CovarianceMatrix>& covariance() { return m_cov; }
   /// Optional covariance matrix.
   const std::optional<CovarianceMatrix>& covariance() const { return m_cov; }
+  /// Covariance matrix of the spatial impact parameters (i.e., of d0 and z0)
+  std::optional<ActsSquareMatrix<2>> spatialImpactParameterCovariance() const {
+    if (not m_cov.has_value()) {
+      return std::nullopt;
+    }
+
+    return m_cov.value().template topLeftCorner<2, 2>();
+  }
+
+  /// Covariance matrix of the spatial and temporal impact parameters (i.e., of
+  /// d0, z0, and t)
+  std::optional<ActsSquareMatrix<3>> impactParameterCovariance() const {
+    if (not m_cov.has_value()) {
+      return std::nullopt;
+    }
+
+    ActsSquareMatrix<3> ipCov;
+    ipCov.template topLeftCorner<2, 2>() =
+        m_cov.value().template topLeftCorner<2, 2>();
+    ipCov.template block<2, 1>(0, 2) =
+        m_cov.value().template block<2, 1>(0, eBoundTime);
+    ipCov.template block<1, 2>(2, 0) =
+        m_cov.value().template block<1, 2>(eBoundTime, 0);
+    ipCov(2, 2) = m_cov.value()(eBoundTime, eBoundTime);
+    return ipCov;
+  }
 
   /// Access a single parameter value identified by its index.
   ///

Core/include/Acts/Vertexing/AdaptiveGridTrackDensity.ipp

@@ -78,7 +78,7 @@ template <int trkGridSize>
 typename Acts::AdaptiveGridTrackDensity<trkGridSize>::DensityMap
 Acts::AdaptiveGridTrackDensity<trkGridSize>::addTrack(
     const Acts::BoundTrackParameters& trk, DensityMap& mainDensityMap) const {
-  SquareMatrix2 cov = trk.covariance().value().block<2, 2>(0, 0);
+  SquareMatrix2 cov = trk.spatialImpactParameterCovariance().value();
   float d0 = trk.parameters()[0];
   float z0 = trk.parameters()[1];
 

Core/include/Acts/Vertexing/GaussianGridTrackDensity.ipp

@@ -58,7 +58,7 @@ std::pair<int, typename Acts::GaussianGridTrackDensity<
                    mainGridSize, trkGridSize>::TrackGridVector>
 Acts::GaussianGridTrackDensity<mainGridSize, trkGridSize>::addTrack(
     const Acts::BoundTrackParameters& trk, MainGridVector& mainGrid) const {
-  SquareMatrix2 cov = trk.covariance().value().block<2, 2>(0, 0);
+  SquareMatrix2 cov = trk.spatialImpactParameterCovariance().value();
   float d0 = trk.parameters()[0];
   float z0 = trk.parameters()[1];