ActsFatras/Digitization/UncorrelatedHitSmearer.hpp

0001 // This file is part of the Acts project.
0002 //
0003 // Copyright (C) 2020 CERN for the benefit of the Acts project
0004 //
0005 // This Source Code Form is subject to the terms of the Mozilla Public
0006 // License, v. 2.0. If a copy of the MPL was not distributed with this
0007 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
0008
0009 #pragma once
0010
0011 #include "Acts/Definitions/TrackParametrization.hpp"
0012 #include "Acts/EventData/TransformationHelpers.hpp"
0013 #include "Acts/Geometry/DetectorElementBase.hpp"
0014 #include "Acts/Surfaces/Surface.hpp"
0015 #include "Acts/Utilities/Result.hpp"
0016 #include "ActsFatras/EventData/Hit.hpp"
0017
0018 #include <array>
0019 #include <functional>
0020 #include <utility>
0021
0022 namespace ActsFatras {
0023
0024 /// Smearing function definition for single track parameters.
0025 ///
0026 /// The function takes the unsmeared parameter and returns the smeared value and
0027 /// a standard deviation.
0028 ///
0029 /// @tparam generator_t The type of the random generator.
0030 template <typename generator_t>
0031 using SingleParameterSmearFunction =
0032     std::function<Acts::Result<std::pair<double, double>>(double,
0033                                                           generator_t&)>;
0034
0035 /// Uncorrelated smearing algorithm for fast digitisation of bound parameters.
0036 ///
0037 /// @tparam generator_t Random number generator type
0038 /// @tparam kSize Number of smeared parameters
0039 ///
0040 /// The smearer takes a single simulated `Hit` and generates a smeared parameter
0041 /// vector and associated covariance matrix.
0042 template <typename generator_t, std::size_t kSize>
0043 struct BoundParametersSmearer {
0044   using Scalar = Acts::ActsScalar;
0045   using ParametersVector = Acts::ActsVector<kSize>;
0046   using CovarianceMatrix = Acts::ActsSquareMatrix<kSize>;
0047   using Result = Acts::Result<std::pair<ParametersVector, CovarianceMatrix>>;
0048
0049   /// Parameter indices that will be used to create the smeared measurements.
0050   std::array<Acts::BoundIndices, kSize> indices{};
0051   std::array<SingleParameterSmearFunction<generator_t>, kSize> smearFunctions{};
0052
0053   static constexpr std::size_t size() { return kSize; }
0054
0055   /// Generate smeared measured for configured parameters.
0056   ///
0057   /// @param rng Random number generator
0058   /// @param hit Simulated hit
0059   /// @param surface Local surface on which the hit is smeared
0060   /// @param geoCtx Geometry context
0061   /// @retval Smeared parameters vector and associated covariance on success
0062   /// @retval Error code for failure
0063   Result operator()(generator_t& rng, const Hit& hit,
0064                     const Acts::Surface& surface,
0065                     const Acts::GeometryContext& geoCtx) const {
0066     // We use the thickness of the detector element as tolerance, because Geant4
0067     // treats the Surfaces as volumes and thus it is not ensured, that each hit
0068     // lies exactly on the Acts::Surface
0069     const auto tolerance =
0070         surface.associatedDetectorElement() != nullptr
0071             ? surface.associatedDetectorElement()->thickness()
0072             : Acts::s_onSurfaceTolerance;
0073
0074     // construct full bound parameters. they are probably not all needed, but it
0075     // is easier to just create them all and then select the requested ones.
0076     Acts::Result<Acts::BoundVector> boundParamsRes =
0077         Acts::transformFreeToBoundParameters(hit.position(), hit.time(),
0078                                              hit.direction(), 0, surface,
0079                                              geoCtx, tolerance);
0080
0081     if (!boundParamsRes.ok()) {
0082       return boundParamsRes.error();
0083     }
0084
0085     const auto& boundParams = *boundParamsRes;
0086
0087     ParametersVector par = ParametersVector::Zero();
0088     CovarianceMatrix cov = CovarianceMatrix::Zero();
0089     for (int i = 0; i < static_cast<int>(kSize); ++i) {
0090       auto res = smearFunctions[i](boundParams[indices[i]], rng);
0091       if (!res.ok()) {
0092         return Result::failure(res.error());
0093       }
0094       auto [value, stddev] = res.value();
0095       par[i] = value;
0096       cov(i, i) = stddev * stddev;
0097     }
0098
0099     return Result::success(std::make_pair(par, cov));
0100   }
0101 };
0102
0103 /// Uncorrelated smearing algorithm for fast digitisation of free parameters.
0104 ///
0105 /// @tparam generator_t Random number generator type
0106 /// @tparam kSize Number of smeared parameters
0107 ///
0108 /// The smearer takes a single simulated `Hit` and generates a smeared parameter
0109 /// vector and associated covariance matrix.
0110 ///
0111 /// @note Uncorrelated smearing of the direction using each components
0112 ///   individually is not recommended
0113 template <typename generator_t, std::size_t kSize>
0114 struct FreeParametersSmearer {
0115   using Scalar = Acts::ActsScalar;
0116   using ParametersVector = Acts::ActsVector<kSize>;
0117   using CovarianceMatrix = Acts::ActsSquareMatrix<kSize>;
0118   using Result = Acts::Result<std::pair<ParametersVector, CovarianceMatrix>>;
0119
0120   /// Parameter indices that will be used to create the smeared measurements.
0121   std::array<Acts::FreeIndices, kSize> indices{};
0122   std::array<SingleParameterSmearFunction<generator_t>, kSize> smearFunctions;
0123
0124   static constexpr std::size_t size() { return kSize; }
0125
0126   /// Generate smeared measured for configured parameters.
0127   ///
0128   /// @param rng Random number generator
0129   /// @param hit Simulated hit
0130   /// @return Smeared free parameter set wrapped in a Result<...> object
0131   /// @retval Smeared parameters vector and associated covariance on success
0132   /// @retval Error code for failure
0133   Result operator()(generator_t& rng, const Hit& hit) const {
0134     // construct full free parameters. they are probably not all needed, but it
0135     // is easier to just create them all and then select the requested ones.
0136     Acts::FreeVector freeParams;
0137     freeParams.segment<3>(Acts::eFreePos0) = hit.position();
0138     freeParams[Acts::eFreeTime] = hit.time();
0139     freeParams.segment<3>(Acts::eFreeDir0) = hit.direction();
0140     freeParams[Acts::eFreeQOverP] = 0;
0141
0142     ParametersVector par = ParametersVector::Zero();
0143     CovarianceMatrix cov = CovarianceMatrix::Zero();
0144     for (std::size_t i = 0; i < kSize; ++i) {
0145       auto res = smearFunctions[i](freeParams[indices[i]], rng);
0146       if (!res.ok()) {
0147         return Result::failure(res.error());
0148       }
0149       auto [value, stddev] = res.value();
0150       par[i] = value;
0151       cov(i, i) = stddev * stddev;
0152     }
0153
0154     return Result::success(std::make_pair(par, cov));
0155   }
0156 };
0157
0158 }  // namespace ActsFatras