# File BFieldMapUtils.hpp¶

namespace Acts

Note

This file is foreseen for the `Geometry` module to replace `Extent`

Functions

Acts::InterpolatedBFieldMap<Acts::detail::Grid<Acts::Vector2, Acts::detail::EquidistantAxis, Acts::detail::EquidistantAxis>> fieldMapRZ(const std::function<size_t(std::array<size_t, 2> binsRZ, std::array<size_t, 2> nBinsRZ)> &localToGlobalBin, std::vector<double> rPos, std::vector<double> zPos, std::vector<Acts::Vector2> bField, double lengthUnit = UnitConstants::mm, double BFieldUnit = UnitConstants::T, bool firstQuadrant = false)

Method to setup the FieldMap.

e.g.: we have small grid with the values: r={2,3}, z ={4,5}, the corresponding indices are i (belonging to r) and j (belonging to z), the globalIndex is M (belonging to the value of the magnetic field B(r,z)) and the field map is:

r

i

z

j

B(r,z)

M

2

0

4

0

2.323

0

2

0

5

1

2.334

1

3

1

4

0

2.325

2

3

1

5

1

2.331

3

In this case the function would look like:
```[](std::array<size_t, 2> binsRZ, std::array<size_t, 2> nBinsRZ) {
return (binsRZ.at(0) * nBinsRZ.at(1) + binsRZ.at(1));
}
```

Note

The values do not need to be sorted or unique (this will be done inside the function)

Note

The values do not need to be sorted or unique (this will be done inside the function)

Note

The function localToGlobalBin determines how the magnetic field was stored in the vector in respect to the grid values

Note

If `firstQuadrant` is true will create a field that is symmetric for all quadrants. e.g. we have the grid values r={0,1} with BFieldValues={2,3} on the r axis. If the flag is set to true the r-axis grid values will be set to {-1,0,1} and the BFieldValues will be set to {3,2,3}.

Parameters
• localToGlobalBin – Function mapping the local bins of r,z to the global bin of the map magnetic field value

• rPos[in] Values of the grid points in r

• zPos[in] Values of the grid points in z

• bField[in] The magnetic field values inr r and z for all given grid points stored in a vector

• lengthUnit[in] The unit of the grid points

• BFieldUnit[in] The unit of the magnetic field

• firstQuadrant[in] Flag if set to true indicating that only the first quadrant of the grid points and the BField values has been given.

Returns

A field map instance for use in interpolation.

Acts::InterpolatedBFieldMap<Acts::detail::Grid<Acts::Vector3, Acts::detail::EquidistantAxis, Acts::detail::EquidistantAxis, Acts::detail::EquidistantAxis>> fieldMapXYZ(const std::function<size_t(std::array<size_t, 3> binsXYZ, std::array<size_t, 3> nBinsXYZ)> &localToGlobalBin, std::vector<double> xPos, std::vector<double> yPos, std::vector<double> zPos, std::vector<Acts::Vector3> bField, double lengthUnit = UnitConstants::mm, double BFieldUnit = UnitConstants::T, bool firstOctant = false)

Method to setup the FieldMap.

e.g.: we have small grid with the values: x={2,3}, y={3,4}, z ={4,5}, the corresponding indices are i (belonging to x), j (belonging to y) and k (belonging to z), the globalIndex is M (belonging to the value of the magnetic field B(x,y,z)) and the field map is:

x

i

y

j

z

k

B(x,y,z)

M

2

0

3

0

4

0

2.323

0

2

0

3

0

5

1

2.334

1

2

0

4

1

4

0

2.325

2

2

0

4

1

5

1

2.331

3

3

1

3

0

4

0

2.323

4

3

1

3

0

5

1

2.334

5

3

1

4

1

4

0

2.325

6

3

1

4

1

5

1

2.331

7

In this case the function would look like:
```[](std::array<size_t, 3> binsXYZ, std::array<size_t, 3> nBinsXYZ) {
return (binsXYZ.at(0) * (nBinsXYZ.at(1) * nBinsXYZ.at(2))
+ binsXYZ.at(1) * nBinsXYZ.at(2)
+ binsXYZ.at(2));
}
```

Note

The grid point values `xPos`, `yPos` and `zPos` do not need to be sorted or unique (this will be done inside the function)

Note

The function `localToGlobalBin` determines how the magnetic field was stored in the vector in respect to the grid values

Note

If `firstOctant` is true, the function will assume a symmetrical field for all quadrants. e.g. we have the grid values z={0,1} with BFieldValues={2,3} on the r axis. If the flag is set to true the z-axis grid values will be set to {-1,0,1} and the BFieldValues will be set to {3,2,3}.

Parameters
• localToGlobalBin – Function mapping the local bins of x,y,z to the global bin of the map magnetic field value

• xPos[in] Values of the grid points in x

• yPos[in] Values of the grid points in y

• zPos[in] Values of the grid points in z

• bField[in] The magnetic field values inr r and z for all given grid points stored in a vector

• lengthUnit[in] The unit of the grid points

• BFieldUnit[in] The unit of the magnetic field

• firstOctant[in] Flag if set to true indicating that only the first octant of the grid points and the BField values has been given.

Returns

A field map instance for use in interpolation.

Acts::InterpolatedBFieldMap<Acts::detail::Grid<Acts::Vector2, Acts::detail::EquidistantAxis, Acts::detail::EquidistantAxis>> solenoidFieldMap(std::pair<double, double> rlim, std::pair<double, double> zlim, std::pair<size_t, size_t> nbins, const SolenoidBField &field)

Function which takes an existing SolenoidBField instance and creates a field mapper by sampling grid points from the analytical solenoid field.

Parameters
• rlim – pair of r bounds

• zlim – pair of z bounds

• nbins – pair of bin counts

• field – the solenoid field instance

Returns

A field map instance for use in interpolation.