Source code for astropy.visualization.wcsaxes.patches

# Licensed under a 3-clause BSD style license - see LICENSE.rst

import numpy as np
from matplotlib.patches import Polygon

from astropy import units as u
from astropy.coordinates.representation import UnitSphericalRepresentation
from astropy.coordinates.matrix_utilities import rotation_matrix, matrix_product

__all__ = ['SphericalCircle']

def _rotate_polygon(lon, lat, lon0, lat0):
    Given a polygon with vertices defined by (lon, lat), rotate the polygon
    such that the North pole of the spherical coordinates is now at (lon0,
    lat0). Therefore, to end up with a polygon centered on (lon0, lat0), the
    polygon should initially be drawn around the North pole.

    # Create a representation object
    polygon = UnitSphericalRepresentation(lon=lon, lat=lat)

    # Determine rotation matrix to make it so that the circle is centered
    # on the correct longitude/latitude.
    m1 = rotation_matrix(-(0.5 * np.pi * u.radian - lat0), axis='y')
    m2 = rotation_matrix(-lon0, axis='z')
    transform_matrix = matrix_product(m2, m1)

    # Apply 3D rotation
    polygon = polygon.to_cartesian()
    polygon = polygon.transform(transform_matrix)
    polygon = UnitSphericalRepresentation.from_cartesian(polygon)

    return polygon.lon,

[docs]class SphericalCircle(Polygon): """ Create a patch representing a spherical circle - that is, a circle that is formed of all the points that are within a certain angle of the central coordinates on a sphere. Here we assume that latitude goes from -90 to +90 This class is needed in cases where the user wants to add a circular patch to a celestial image, since otherwise the circle will be distorted, because a fixed interval in longitude corresponds to a different angle on the sky depending on the latitude. Parameters ---------- center : tuple or `~astropy.units.Quantity` This can be either a tuple of two `~astropy.units.Quantity` objects, or a single `~astropy.units.Quantity` array with two elements. radius : `~astropy.units.Quantity` The radius of the circle resolution : int, optional The number of points that make up the circle - increase this to get a smoother circle. vertex_unit : `~astropy.units.Unit` The units in which the resulting polygon should be defined - this should match the unit that the transformation (e.g. the WCS transformation) expects as input. Notes ----- Additional keyword arguments are passed to `~matplotlib.patches.Polygon` """ def __init__(self, center, radius, resolution=100,, **kwargs): # Extract longitude/latitude, either from a tuple of two quantities, or # a single 2-element Quantity. longitude, latitude = center # Start off by generating the circle around the North pole lon = np.linspace(0., 2 * np.pi, resolution + 1)[:-1] * u.radian lat = np.repeat(0.5 * np.pi - radius.to_value(u.radian), resolution) * u.radian lon, lat = _rotate_polygon(lon, lat, longitude, latitude) # Extract new longitude/latitude in the requested units lon = lon.to_value(vertex_unit) lat = lat.to_value(vertex_unit) # Create polygon vertices vertices = np.array([lon, lat]).transpose() super().__init__(vertices, **kwargs)