This example shows how Otsu’s threshold [1] method can be applied locally. For each pixel, an “optimal” threshold is determined by maximizing the variance between two classes of pixels of the local neighborhood defined by a structuring element.
The example compares the local threshold with the global threshold.
| [1] | http://en.wikipedia.org/wiki/Otsu’s_method |
Source
from skimage import data
from skimage.morphology import disk
from skimage.filters import threshold_otsu, rank
from skimage.util import img_as_ubyte
import matplotlib
import matplotlib.pyplot as plt
matplotlib.rcParams['font.size'] = 9
img = img_as_ubyte(data.page())
radius = 15
selem = disk(radius)
local_otsu = rank.otsu(img, selem)
threshold_global_otsu = threshold_otsu(img)
global_otsu = img >= threshold_global_otsu
fig, ax = plt.subplots(2, 2, figsize=(8, 5), sharex=True, sharey=True,
subplot_kw={'adjustable': 'box-forced'})
ax0, ax1, ax2, ax3 = ax.ravel()
plt.tight_layout()
fig.colorbar(ax0.imshow(img, cmap=plt.cm.gray),
ax=ax0, orientation='horizontal')
ax0.set_title('Original')
ax0.axis('off')
fig.colorbar(ax1.imshow(local_otsu, cmap=plt.cm.gray),
ax=ax1, orientation='horizontal')
ax1.set_title('Local Otsu (radius=%d)' % radius)
ax1.axis('off')
ax2.imshow(img >= local_otsu, cmap=plt.cm.gray)
ax2.set_title('Original >= Local Otsu' % threshold_global_otsu)
ax2.axis('off')
ax3.imshow(global_otsu, cmap=plt.cm.gray)
ax3.set_title('Global Otsu (threshold = %d)' % threshold_global_otsu)
ax3.axis('off')
plt.show()
Python source code: download
(generated using skimage 0.12.3)
IPython Notebook: download
(generated using skimage 0.12.3)