Pixel classification using Scikit-learn#

Pixel classification is a technique for assigning pixels to multiple classes. If there are two classes (object and background), we are talking about binarization. In this example we use a random forest classifier for pixel classification.

Generating a feature stack#

Pixel classifiers such as the random forest classifier takes multiple images as input. We typically call these images a feature stack because for every pixel exist now multiple values (features). In the following example we create a feature stack containing three features:

The original pixel value
The pixel value after a Gaussian blur
The pixel value of the Gaussian blurred image processed through a Sobel operator.

Thus, we denoise the image and detect edges. All three images serve the pixel classifier to differentiate positive an negative pixels.

from skimage import filters

def generate_feature_stack(image):
    # determine features
    blurred = filters.gaussian(image, sigma=2)
    edges = filters.sobel(blurred)

    # collect features in a stack
    # The ravel() function turns a nD image into a 1-D image.
    # We need to use it because scikit-learn expects values in a 1-D format here. 
    feature_stack = [
        image.ravel(),
        blurred.ravel(),
        edges.ravel()
    ]
    
    # return stack as numpy-array
    return np.asarray(feature_stack)

feature_stack = generate_feature_stack(image)

# show feature images
import matplotlib.pyplot as plt
fig, axes = plt.subplots(1, 3, figsize=(10,10))

# reshape(image.shape) is the opposite of ravel() here. We just need it for visualization.
axes[0].imshow(feature_stack[0].reshape(image.shape), cmap=plt.cm.gray)
axes[1].imshow(feature_stack[1].reshape(image.shape), cmap=plt.cm.gray)
axes[2].imshow(feature_stack[2].reshape(image.shape), cmap=plt.cm.gray)

<matplotlib.image.AxesImage at 0x18eb407e760>

../_images/02_scikit_learn_random_forest_pixel_classifier_7_1.png

Formating data#

We now need to format the input data so that it fits to what scikit learn expects. Scikit-learn asks for an array of shape (n, m) as input data and (n) annotations. n corresponds to number of pixels and m to number of features. In our case m = 3.

def format_data(feature_stack, annotation):
    # reformat the data to match what scikit-learn expects
    # transpose the feature stack
    X = feature_stack.T
    # make the annotation 1-dimensional
    y = annotation.ravel()
    
    # remove all pixels from the feature and annotations which have not been annotated
    mask = y > 0
    X = X[mask]
    y = y[mask]

    return X, y

X, y = format_data(feature_stack, annotation)

print("input shape", X.shape)
print("annotation shape", y.shape)

input shape (200, 3)
annotation shape (200,)

Training the random forest classifier#

We now train the random forest classifier by providing the feature stack X and the annotations y.

classifier = RandomForestClassifier(max_depth=2, random_state=0)
classifier.fit(X, y)

RandomForestClassifier(max_depth=2, random_state=0)

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Predicting pixel classes#

After the classifier has been trained, we can use it to predict pixel classes for whole images. Note in the following code, we provide feature_stack.T which are more pixels then X in the commands above, because it also contains the pixels which were not annotated before.

res = classifier.predict(feature_stack.T) - 1 # we subtract 1 to make background = 0
imshow(res.reshape(image.shape))

<matplotlib.image.AxesImage at 0x18eb4136fa0>

../_images/02_scikit_learn_random_forest_pixel_classifier_13_1.png

Interactive segmentation#

We can also use napari to annotate some regions as negative (label = 1) and positive (label = 2).

# start napari
viewer = napari.Viewer()

# add image
viewer.add_image(image)

# add an empty labels layer and keet it in a variable
labels = viewer.add_labels(np.zeros(image.shape).astype(int))

Go ahead after annotating at least two regions with labels 1 and 2.

Take a screenshot of the annotation:

napari.utils.nbscreenshot(viewer)

Retrieve the annotations from the napari layer:

manual_annotations = labels.data

imshow(manual_annotations, vmin=0, vmax=2)

C:\Users\rober\miniconda3\envs\bio_39\lib\site-packages\skimage\io\_plugins\matplotlib_plugin.py:150: UserWarning: Low image data range; displaying image with stretched contrast.
  lo, hi, cmap = _get_display_range(image)

<matplotlib.image.AxesImage at 0x18ebb635d60>

../_images/02_scikit_learn_random_forest_pixel_classifier_19_2.png

As we have used functions in the example above, we can just repeat the same procedure with the manual annotations.

# generate features (that's actually not necessary, 
# as the variable is still there and the image is the same. 
# but we do it for completeness)
feature_stack = generate_feature_stack(image)
X, y = format_data(feature_stack, manual_annotations)

# train classifier
classifier = RandomForestClassifier(max_depth=2, random_state=0)
classifier.fit(X, y)

# process the whole image and show result
result_1d = classifier.predict(feature_stack.T)
result_2d = result_1d.reshape(image.shape)
imshow(result_2d)

C:\Users\rober\miniconda3\envs\bio_39\lib\site-packages\skimage\io\_plugins\matplotlib_plugin.py:150: UserWarning: Low image data range; displaying image with stretched contrast.
  lo, hi, cmap = _get_display_range(image)

<matplotlib.image.AxesImage at 0x18eb3ea1760>

../_images/02_scikit_learn_random_forest_pixel_classifier_21_2.png

Also we add the result to napari.

viewer.add_labels(result_2d)

<Labels layer 'result_2d' at 0x18eb3ef4bb0>

napari.utils.nbscreenshot(viewer)

Exercise#

Change the code so that you can annotate three different regions:

Nuclei
Background
The edges between blobs and background

Image data science with Python and Napari @EPFL

Pixel classification using Scikit-learn

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