Checking for points inside regions¶
Let’s continue with sky and pixel regions defined in the Getting started section:
>>> from astropy.coordinates import Angle, SkyCoord
>>> from regions import CircleSkyRegion, PixCoord, CirclePixelRegion
>>> sky_center = SkyCoord(42, 43, unit='deg')
>>> sky_radius = Angle(25, 'deg')
>>> sky_region = CircleSkyRegion(sky_center, sky_radius)
>>> pixel_center = PixCoord(x=42, y=43)
>>> pixel_radius = 42
>>> pixel_region = CirclePixelRegion(pixel_center, pixel_radius)
>>> print(sky_region)
Region: CircleSkyRegion
center: <SkyCoord (ICRS): (ra, dec) in deg
(42., 43.)>
radius: 25.0 deg
>>> print(pixel_region)
Region: CirclePixelRegion
center: PixCoord(x=42, y=43)
radius: 42
To test if a given point is inside or outside the regions, the Python in
operator
can be called, which calls the special __contains__
method defined on the region classes:
>>> from regions import PixCoord
>>> PixCoord(55, 40) in pixel_region
True
>>> PixCoord(55, 200) in pixel_region
False
The in
operator only works for scalar coordinates, because Python requires
the return value to be a scalar bool, and only works for pixel regions. If you
try to use in
for non-scalar coordinates, you’ll get a ValueError
:
>>> pixcoord = PixCoord([50, 50], [10, 60])
>>> pixcoord in pixel_region
Traceback (most recent call last):
...
ValueError: coord must be scalar. coord=PixCoord(x=[50 50], y=[10 60])
If you have arrays of coordinates, use the regions.SkyRegion.contains
or
regions.PixelRegion.contains
methods:
>>> pixcoords = PixCoord.from_sky(sky_center, wcs)
>>> pixel_region.contains(pixcoords)
True
Note that regions.SkyRegion.contains
requires a WCS to be passed:
>>> skycoord = SkyCoord([50, 50], [10, 60], unit='deg')
>>> sky_region.contains(skycoord, wcs)
array([False, True])