"""
=======================
Fading between two maps
=======================

Often it is useful to plot two maps of the Sun on top of each other, so
features observed in one map (e.g. strong magnetic fields in a magnetogram)
can be identified with features in the same place in another map
(e.g. active regions in a EUV image).

This example shows how to plot two maps on top of each other, with a slider to
fade between them.
"""
import matplotlib.pyplot as plt
from matplotlib.widgets import Slider

import astropy.units as u

import sunpy.map
from sunpy.data.sample import AIA_171_IMAGE, AIA_1600_IMAGE

###############################################################################
# Start by loading two AIA maps from the sample data.

map_171 = sunpy.map.Map(AIA_171_IMAGE)
map_1600 = sunpy.map.Map(AIA_1600_IMAGE)

###############################################################################
# First we create a figure, and add the axes that will show the maps. Then
# plot both of the images on the same axes. Finally, we add another axes that
# contains the slider.

fig = plt.figure()
# Add the main axes. Note this is resized to leave room for the slider axes
ax = fig.add_axes([0.1, 0.2, 0.9, 0.7], projection=map_171)

im_1600 = map_1600.plot(axes=ax)
im_171 = map_171.plot(axes=ax, alpha=0.5, clip_interval=(1, 99.99)*u.percent)
ax.set_title('AIA 171 + 1600')

# Add the slider axes
ax_slider = fig.add_axes([0.25, 0.05, 0.65, 0.03])
slider = Slider(ax_slider, 'Alpha', 0, 1, valinit=0.5)

###############################################################################
# Finally, define what happens when the slider is changed and link this to the
# slider we set up above. In this case we just set the alpha (ie. transparency)
# of the 171 image.


def update(val):
    alpha = slider.val
    im_171.set_alpha(alpha)


slider.on_changed(update)

plt.show()