Custom source objects#

The class CustomSource allows users to integrate their own custom-object field computation into the Magpylib interface. For this, the argument field_func must be provided with a function that is then automatically called with getB and getH. This custom field function is treated like a core function. It must have the positional arguments field (with values 'B' or 'H'), and observers (must accept array_like, shape (n,3)) and return the B-field in units of [mT] and the H-field in units of [kA/m] with a similar shape. A fundamental example how to create a custom source object is shown below:

import numpy as np
import magpylib as magpy

# define field function
def custom_field(field, observers):
    """ user defined custom field
    position input: array_like, shape (n,3)
    returns: B-field, ndarray, shape (n,3)
    if field=='B':
        return np.array(observers)*2
    return np.array(observers)

# custom source
source = magpy.misc.CustomSource(field_func=custom_field)

# compute field with 2-pixel sensor
sensor = magpy.Sensor(pixel=((1,1,1), (2,2,2)))

B = magpy.getB(source, sensor)
H = magpy.getH(source, sensor)
[[2. 2. 2.]
 [4. 4. 4.]]
[[1. 1. 1.]
 [2. 2. 2.]]

Custom sources behave like native Magpylib objects. They can make full use of the geometry interface, have style properties, and can be part of collections and field computation together with all other source types. A custom 3D representation for display in show can be provided via the style.model3d attribute, see Custom 3D models.

In the following example we show this functionality by constructing a quadrupole collection from custom source monopoles:

import numpy as np
import matplotlib.pyplot as plt
import magpylib as magpy

# define monopole field
def monopole_field(charge, position):
    """ monopole field"""
    position = np.array(position)
    dist = np.linalg.norm(position, axis=1)
    return charge*(position.T/dist**3).T

# prepare a custom 3D model
trace_pole =

# combine four monopole custom sources into a quadrupole collection
def create_pole(charge):
    """ create a monopole object"""
    field = lambda field, observers: monopole_field(charge, observers)
    monopole = magpy.misc.CustomSource(
    return monopole

quadrupole = magpy.Collection(*[create_pole(q) for q in [1,1,-1,-1]])

# move and color the pole objects
pole_pos = np.array([(1,0,0), (-1,0,0), (0,0,1), (0,0,-1)])
for pole, pos, col in zip(quadrupole, pole_pos, 'rrbb'):
    pole.position = pos = col

# Matplotlib figure
fig = plt.figure(figsize=(12, 5))
ax1 = fig.add_subplot(121, projection="3d", azim=-80, elev=15,)
ax2 = fig.add_subplot(122,)

# show 3D model in ax1*quadrupole, canvas=ax1)

# compute B-field on xz-grid and display in ax2
ts = np.linspace(-3, 3, 30)
grid = np.array([[(x,0,z) for x in ts] for z in ts])
B = quadrupole.getB(grid)

scale = np.linalg.norm(B, axis=2)
cp = ax2.contourf(grid[:,:,0], grid[:,:,2], np.log(scale), levels=100, cmap='rainbow')
ax2.streamplot(grid[:,:,0], grid[:,:,2], B[:,:,0], B[:,:,2], density=2,
    color='k', linewidth=scale**0.3)

# display pole positions in ax2
ax2.plot(pole_pos[:,0], pole_pos[:,2], marker='o', ms=10, mfc='k', mec='w', ls='')

# plot styling
    title='3D model',
    xlabel='x-position [mm]',
    ylabel='y-position [mm]',
    zlabel='z-position [mm]',
    title='Quadrupole field',
    xlabel='x-position [mm]',
    ylabel='z-position [mm]',
fig.colorbar(cp, label='[$charge/mm^2$]', ax=ax2)