# coding: utf-8
"""
=============================
Drawing and using a Great Arc
=============================

How to define and draw a great arc on an image of the
Sun, and to extract intensity values along that arc.
"""
import matplotlib.pyplot as plt
import numpy as np

import astropy.units as u
from astropy.coordinates import SkyCoord

import sunpy.map
from sunpy.coordinates.utils import GreatArc
from sunpy.data.sample import AIA_171_IMAGE

###############################################################################
# We start with the sample data
m = sunpy.map.Map(AIA_171_IMAGE)

###############################################################################
# Let's define the start and end coordinates of the arc.
start = SkyCoord(735 * u.arcsec, -471 * u.arcsec, frame=m.coordinate_frame)
end = SkyCoord(-100 * u.arcsec, 800 * u.arcsec, frame=m.coordinate_frame)

###############################################################################
# Create the great arc between the start and end points.
great_arc = GreatArc(start, end)

###############################################################################
# Plot the great arc on the Sun.
fig = plt.figure()
ax = plt.subplot(projection=m)
m.plot(axes=ax, clip_interval=(1, 99.99)*u.percent)
ax.plot_coord(great_arc.coordinates(), color='c')
plt.show()

###############################################################################
# Now we can calculate the nearest integer pixels of the data that correspond
# to the location of arc.
pixels = np.asarray(np.rint(m.world_to_pixel(great_arc.coordinates())), dtype=int)
x = pixels[0, :]
y = pixels[1, :]

###############################################################################
# Get the intensity along the arc from the start to the end point.
intensity_along_arc = m.data[y, x]

###############################################################################
# Define the angular location of each pixel along the arc from the start point
# to the end.
angles = great_arc.inner_angles().to(u.deg)

###############################################################################
# Plot the intensity along the arc from the start to the end point.
fig, ax = plt.subplots()
ax.plot(angles, intensity_along_arc)
ax.set_xlabel('degrees of arc from start')
ax.set_ylabel('intensity')
ax.grid(linestyle='dotted')
plt.show()