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"""
Trainable segmentation using local features and random forests
==============================================================
A pixel-based segmentation is computed here using local features based on
local intensity, edges and textures at different scales. A user-provided
mask is used to identify different regions. The pixels of the mask are used
to train a random-forest classifier [1]_ from scikit-learn. Unlabeled pixels
are then labeled from the prediction of the classifier.
This segmentation algorithm is called trainable segmentation in other software
such as ilastik [2]_ or ImageJ [3]_ (where it is also called "weka
segmentation").
.. [1] https://scikit-learn.org/stable/modules/generated/sklearn.ensemble.RandomForestClassifier.html
.. [2] https://www.ilastik.org/documentation/pixelclassification/pixelclassification
.. [3] https://imagej.net/Trainable_Weka_Segmentation#Training_features_.282D.29
"""
import numpy as np
import matplotlib.pyplot as plt
from skimage import data, segmentation, feature, future
from sklearn.ensemble import RandomForestClassifier
from functools import partial
full_img = data.skin()
img = full_img[:900, :900]
# Build an array of labels for training the segmentation.
# Here we use rectangles but visualization libraries such as plotly
# (and napari?) can be used to draw a mask on the image.
training_labels = np.zeros(img.shape[:2], dtype=np.uint8)
training_labels[:130] = 1
training_labels[:170, :400] = 1
training_labels[600:900, 200:650] = 2
training_labels[330:430, 210:320] = 3
training_labels[260:340, 60:170] = 4
training_labels[150:200, 720:860] = 4
sigma_min = 1
sigma_max = 16
features_func = partial(
feature.multiscale_basic_features,
intensity=True,
edges=False,
texture=True,
sigma_min=sigma_min,
sigma_max=sigma_max,
channel_axis=-1,
)
features = features_func(img)
clf = RandomForestClassifier(n_estimators=50, n_jobs=-1, max_depth=10, max_samples=0.05)
clf = future.fit_segmenter(training_labels, features, clf)
result = future.predict_segmenter(features, clf)
fig, ax = plt.subplots(1, 2, sharex=True, sharey=True, figsize=(9, 4))
ax[0].imshow(segmentation.mark_boundaries(img, result, mode='thick'))
ax[0].contour(training_labels)
ax[0].set_title('Image, mask and segmentation boundaries')
ax[1].imshow(result)
ax[1].set_title('Segmentation')
fig.tight_layout()
##############################################################################
# Feature importance
# ------------------
#
# We inspect below the importance of the different features, as computed by
# scikit-learn. Intensity features have a much higher importance than texture
# features. It can be tempting to use this information to reduce the number of
# features given to the classifier, in order to reduce the computing time.
# However, this can lead to overfitting and a degraded result at the boundary
# between regions.
fig, ax = plt.subplots(1, 2, figsize=(9, 4))
l = len(clf.feature_importances_)
feature_importance = (
clf.feature_importances_[: l // 3],
clf.feature_importances_[l // 3 : 2 * l // 3],
clf.feature_importances_[2 * l // 3 :],
)
sigmas = np.logspace(
np.log2(sigma_min),
np.log2(sigma_max),
num=int(np.log2(sigma_max) - np.log2(sigma_min) + 1),
base=2,
endpoint=True,
)
for ch, color in zip(range(3), ['r', 'g', 'b']):
ax[0].plot(sigmas, feature_importance[ch][::3], 'o', color=color)
ax[0].set_title("Intensity features")
ax[0].set_xlabel("$\\sigma$")
for ch, color in zip(range(3), ['r', 'g', 'b']):
ax[1].plot(sigmas, feature_importance[ch][1::3], 'o', color=color)
ax[1].plot(sigmas, feature_importance[ch][2::3], 's', color=color)
ax[1].set_title("Texture features")
ax[1].set_xlabel("$\\sigma$")
fig.tight_layout()
##############################################################################
# Fitting new images
# ------------------
#
# If you have several images of similar objects acquired in similar conditions,
# you can use the classifier trained with `fit_segmenter` to segment other
# images. In the example below we just use a different part of the image.
img_new = full_img[:700, 900:]
features_new = features_func(img_new)
result_new = future.predict_segmenter(features_new, clf)
fig, ax = plt.subplots(1, 2, sharex=True, sharey=True, figsize=(6, 4))
ax[0].imshow(segmentation.mark_boundaries(img_new, result_new, mode='thick'))
ax[0].set_title('Image')
ax[1].imshow(result_new)
ax[1].set_title('Segmentation')
fig.tight_layout()
plt.show()
|
