def field_add(field, vector):
    """ Add the vector to the field.

    Parameters
    ----------
    field: array (x, y, z, 3)
        an image of vectors.
    vector: array (3, )
        the vector that will be added to the field.

    Returns
    -------
    field: array (x, y, z, 3)
        the incremented image of vectors.
    """
    field[..., 0] += vector[0]
    field[..., 1] += vector[1]
    field[..., 2] += vector[2]
    return field