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"""
==================================================
Comparing edge-based and region-based segmentation
==================================================
In this example, we will see how to segment objects from a background. We use
the ``coins`` image from ``skimage.data``, which shows several coins outlined
against a darker background.
"""
import numpy as np
import matplotlib.pyplot as plt
import skimage as ski
coins = ski.data.coins()
hist, hist_centers = ski.exposure.histogram(coins)
fig, axes = plt.subplots(1, 2, figsize=(8, 3))
axes[0].imshow(coins, cmap=plt.cm.gray)
axes[0].set_axis_off()
axes[1].plot(hist_centers, hist, lw=2)
axes[1].set_title('histogram of gray values')
######################################################################
#
# Thresholding
# ============
#
# A simple way to segment the coins is to choose a threshold based on the
# histogram of gray values. Unfortunately, thresholding this image gives a
# binary image that either misses significant parts of the coins or merges
# parts of the background with the coins:
fig, axes = plt.subplots(1, 2, figsize=(8, 3), sharey=True)
axes[0].imshow(coins > 100, cmap=plt.cm.gray)
axes[0].set_title('coins > 100')
axes[1].imshow(coins > 150, cmap=plt.cm.gray)
axes[1].set_title('coins > 150')
for a in axes:
a.set_axis_off()
fig.tight_layout()
######################################################################
# Edge-based segmentation
# =======================
#
# Next, we try to delineate the contours of the coins using edge-based
# segmentation. To do this, we first get the edges of features using the
# Canny edge-detector.
edges = ski.feature.canny(coins)
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(edges, cmap=plt.cm.gray)
ax.set_title('Canny detector')
ax.set_axis_off()
######################################################################
# These contours are then filled using mathematical morphology.
from scipy import ndimage as ndi
fill_coins = ndi.binary_fill_holes(edges)
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(fill_coins, cmap=plt.cm.gray)
ax.set_title('filling the holes')
ax.set_axis_off()
######################################################################
# Small spurious objects are easily removed by setting a minimum size for
# valid objects.
coins_cleaned = ski.morphology.remove_small_objects(fill_coins, 21)
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(coins_cleaned, cmap=plt.cm.gray)
ax.set_title('removing small objects')
ax.set_axis_off()
######################################################################
# However, this method is not very robust, since contours that are not
# perfectly closed are not filled correctly, as is the case for one unfilled
# coin above.
#
# Region-based segmentation
# =========================
#
# We therefore try a region-based method using the watershed transform.
# First, we find an elevation map using the Sobel gradient of the image.
elevation_map = ski.filters.sobel(coins)
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(elevation_map, cmap=plt.cm.gray)
ax.set_title('elevation map')
ax.set_axis_off()
######################################################################
# Next we find markers of the background and the coins based on the extreme
# parts of the histogram of gray values.
markers = np.zeros_like(coins)
markers[coins < 30] = 1
markers[coins > 150] = 2
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(markers, cmap=plt.cm.nipy_spectral)
ax.set_title('markers')
ax.set_axis_off()
######################################################################
# Finally, we use the watershed transform to fill regions of the elevation
# map starting from the markers determined above:
segmentation_coins = ski.segmentation.watershed(elevation_map, markers)
fig, ax = plt.subplots(figsize=(4, 3))
ax.imshow(segmentation_coins, cmap=plt.cm.gray)
ax.set_title('segmentation')
ax.set_axis_off()
######################################################################
# This last method works even better, and the coins can be segmented and
# labeled individually.
segmentation_coins = ndi.binary_fill_holes(segmentation_coins - 1)
labeled_coins, _ = ndi.label(segmentation_coins)
image_label_overlay = ski.color.label2rgb(labeled_coins, image=coins, bg_label=0)
fig, axes = plt.subplots(1, 2, figsize=(8, 3), sharey=True)
axes[0].imshow(coins, cmap=plt.cm.gray)
axes[0].contour(segmentation_coins, [0.5], linewidths=1.2, colors='y')
axes[1].imshow(image_label_overlay)
for a in axes:
a.set_axis_off()
fig.tight_layout()
plt.show()
|
