unit_vectors.hpp

// This file is part of the ACTS project.
//
// Copyright (C) 2016 CERN for the benefit of the ACTS project
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this
// file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
#pragma once
 
// Project include(s).
#include "detray/definitions/algebra.hpp"
#include "detray/definitions/containers.hpp"
#include "detray/definitions/detail/qualifiers.hpp"
 
namespace detray {

template <concepts::vector3D vector3_t>
struct unit_vectors {
  DETRAY_HOST_DEVICE inline vector3_t make_curvilinear_unit_u(
      const vector3_t& dir) {
    vector3_t unit_u{0.f, 0.f, 0.f};
    // explicit version of U = Z x T
    unit_u[0] = -dir[1];
    unit_u[1] = dir[0];
 
    // if the absolute scale is tiny, the initial direction vector is
    // aligned with the z-axis. the ZxT product is ill-defined since any
    // vector in the x-y plane would be orthogonal to the direction. fix the
    // U unit vector along the x-axis to avoid this numerical instability.
    if (const auto scale = vector::norm(unit_u); scale < 1e-6f) {
      unit_u[0] = 1;
      unit_u[1] = 0;
    } else {
      unit_u = unit_u * (1.f / scale);
    }
 
    return unit_u;
  }

  DETRAY_HOST_DEVICE inline darray<vector3_t, 2> make_curvilinear_unit_vectors(
      const vector3_t& dir) {
    darray<vector3_t, 2> uv;
    uv[0] = make_curvilinear_unit_u(dir);
    uv[1] = vector::normalize(vector::cross(dir, uv[0]));
 
    return uv;
  }
};
 
}  // namespace detray