Comparing Map Rotation Functions

This example will compare between the current library implementations for sunpy.map.GenericMap.rotate.

import matplotlib.pyplot as plt

import astropy.units as u

import sunpy.data.sample
import sunpy.map

Rotating a map in sunpy has a choice between three libraries: scipy (the default), scikit-image and opencv. Furthermore, one can also create a custom rotation function and register it for use with rotate(), see Adding a new rotation method.

Defining an appropriate metric to compare different algorithms is challenging. This example will just compare the raw value differences.

Using an HMI sample data, we will do a rotation to align the image to the north. By default, the order of rotation is 3.

hmi_map = sunpy.map.Map(sunpy.data.sample.HMI_LOS_IMAGE)

scipy_map = hmi_map.rotate(method='scipy')
skimage_map = hmi_map.rotate(method='scikit-image')
cv2_map = hmi_map.rotate(method='opencv')

Now for a visual comparison, the raw differences, that should highlight the differences. Note that only two comparisons are shown. Note the scale here is ± 10.

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(12, 6))

img1 = ax1.imshow(scipy_map.data - skimage_map.data, cmap='RdBu_r', vmin=-10, vmax=10)
ax1.set_title("HMI Difference: scipy vs scikit-image")
fig.colorbar(img1, ax=ax1)

img2 = ax2.imshow(scipy_map.data - cv2_map.data, cmap='RdBu_r', vmin=-10, vmax=10)
ax2.set_title("HMI Difference: scipy vs opencv")
fig.colorbar(img2, ax=ax2)

plt.show()

We can repeat this but for AIA data, using a 171 sample image. We will rotate it by the large amount of 30 degrees.

aia_map = sunpy.map.Map(sunpy.data.sample.AIA_171_IMAGE)

scipy_map = aia_map.rotate(30*u.deg, method='scipy')
skimage_map = aia_map.rotate(30*u.deg, method='scikit-image')
cv2_map = aia_map.rotate(30*u.deg, method='opencv')

Now for a visual comparison, the raw differences, that should highlight the differences. Note that only two comparisons are shown. Note the scale here is ± 75.

fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(12, 6))

img1 = ax1.imshow(scipy_map.data - skimage_map.data, cmap='RdBu_r', vmin=-75, vmax=75)
ax1.set_title("AIA Difference: scipy vs scikit-image")
fig.colorbar(img1, ax=ax1)

img2 = ax2.imshow(scipy_map.data - cv2_map.data, cmap='RdBu_r', vmin=-75, vmax=75)
ax2.set_title("AIA Difference: scipy vs opencv2")
fig.colorbar(img2, ax=ax2)

plt.show()