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FIX: Add clip limits to Nightshade floating point calculations #1989

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Jan 21, 2022
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FIX: Add clip limits to Nightshade floating point calculations #1989

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8 changes: 6 additions & 2 deletions lib/cartopy/feature/nightshade.py
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Original file line number Diff line number Diff line change
Expand Up @@ -77,8 +77,12 @@ def __init__(self, date=None, delta=0.1, refraction=-0.83,

# Solve the generalized equation for omega0, which is the
# angle of sunrise/sunset from solar noon
omega0 = np.rad2deg(np.arccos(np.sin(np.deg2rad(refraction)) /
np.cos(np.deg2rad(y))))
# We need to clip the input to arccos to [-1, 1] due to floating
# point precision and arccos creating nans for values outside
# of the domain
arccos_tmp = np.clip(np.sin(np.deg2rad(refraction)) /
np.cos(np.deg2rad(y)), -1, 1)
omega0 = np.rad2deg(np.arccos(arccos_tmp))

# Fill the longitude values from the offset for midnight.
# This needs to be a closed loop to fill the polygon.
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11 changes: 10 additions & 1 deletion lib/cartopy/tests/feature/test_nightshade.py
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Expand Up @@ -8,7 +8,7 @@

import pytest

from cartopy.feature.nightshade import _julian_day, _solar_position
from cartopy.feature.nightshade import _julian_day, _solar_position, Nightshade


def test_julian_day():
Expand Down Expand Up @@ -44,3 +44,12 @@ def test_solar_position(dt, true_lat, true_lon):
lat, lon = _solar_position(dt)
assert pytest.approx(true_lat, 0.1) == lat
assert pytest.approx(true_lon, 0.1) == lon


def test_nightshade_floating_point():
# Smoke test for clipping nightshade floating point values
date = datetime(1999, 12, 31, 12)

# This can cause an error with floating point precision if it is
# set to exactly -6 and arccos input is not clipped to [-1, 1]
Nightshade(date, refraction=-6.0, color='none')