Density Forest

Code Repository of the EPFL SIE Master Project, Spring Semester 2018. The goal of this project is to perform error detection and novelty detection in Convolutional Neural Networks (CNNs) using Density Forests. Applications to the MNIST dataset and a dataset for semantic segmentation of land cover classes in Zurich are visualized in Code/ and Zurich/.

Installation

The package can be simply installed from pip:

pip install density_forest

📈 Results

Density trees maximize Gaussianity at each split level. In 2D this might look as follows:

A density forest is a collection of density trees each trained on a random subset of all data.

The above example shows the t-SNE of the pre-softmax activations of a network trained for semantic segmentation of the Zurich dataset, leaving out one class during training. Density trees were trained on bootstrap samples of all classes but the unseen one.

Confidence of each data point in the test set, the probability is calculated as the average Gaussian likelihood to come from the leaf node clusters.

Darker points represent regions of lower certainty and crosses represent activations of unseen classes.

📖 Usage of the DensityForest class:

Fitting a Density Forest

Suppose you have your training data X_train and test data X_test, in [N, D] with N data points in D dimensions:

from density_forest.density_forest import DensityForest

clf_df = DensityForest(**params)         # create new class instance, put hyperparameters here
clf_df.fit(X_train)                      # fit to a training set
conf = clf_df.decision_function(X_test)  # get confidence values for test set
outliers = clf_df.predict(X_test)        # predict whether a point is an outlier (-1 for outliers, 1 for inliers)

Hyperparameters are documented in the docstring. To find the optimal hyperparameters, consider the section below.

Finding Hyperparameters

To find the optimal hyperparameters, use the ParameterSearch from helpers.cross_validator, which allows CV, and hyperparameter search.

from helpers.cross_validator import ParameterSearch

# define hyperparameters to test
tuned_params = [{'max_depth':[2, 3, 4], 'n_trees': [10, 20]}] # optionally add non-default arguments as single-element arrays
default_params = [{'verbose':0, ...}]  # other default parameters 
# do parameter search
ps = ParameterSearch(DensityForest, tuned_parameters, X_train, X_train_all, y_true_tr, f_scorer, n_iter=2, verbosity=0, n_jobs=1, default_params=default_params)
ps.fit()

# get model with the best parameters, as above
clf_df = DensityForest(**ps.best_params, **default_params)  # create new class instance with best hyperparameters
...  # continue as above

Check the docstrings for more detailed documentation af the ParameterSearch class.

🗂 File Structure

👾 Code

All libraries for density forests, helper libraries for semantic segmentation and for baselines.

density_forest/density_forest/

Package for implementation of Decision Trees, Random Forests, Density Trees and Density Forests

• create_data.py: functions for generating labelled and unlabelled data
• decision_tree.py: data structure for decision tree nodes
• decision_tree_create.py: functions for generating decision trees
• decision_tree_traverse.py: functions for traversing a decision tree and predicting labels
• density_forest.py: functions for creating density forests
• density_tree.py: data struture for density tree nodes
• density_tree_create.py: functions for generating a density tree
• density_tree_traverse.py: functions for descending a density tree and retrieving its cluster parameters
• helper.py: various helper functions
• random_forests.py: functions for creating random forests

density_forest/helpers:

General helpers library for semantic segmentation

• data_augment.py: custom data augmentation methods applied to both the image and the ground truth
• data_loader.py: PyTorch data loader for Zurich dataset
• helpers.py: functions for importing, cropping, padding images and other related image tranformations
• parameter_search.py: functions for finding optimal hyperparameters for Density Forest, OC-SVM and GMM (explained above)
• plots.py: Generic plotter functions for labelled and unlabelled 2D and 3D plots, used for t-SNE and PCA plots

density_forest/baselines:

Helper functions for confidence estimation baselines MSR, margin, entropy and MC-Dropout

Zurich Land Cover/keras_helpers

Helper functions for Keras

• helpers.py: get activations
• callbacks.py: callbacks to be evaluated after each epoch
• unet.py: UNET model for training of network on Zurich dataset

🗾 Visualizations

density_forest/:

Visualizations of basic decision tree and density tree

• Decision Forest.ipynb: Decision Trees and Random Forest on randomly generated labelled data
• Density Forest.ipynb: Density Trees on randomly generated unlabelled data

MNIST/:

• MNIST Novelty Detection.ipynb: Training of a CNN leaving out one class, baselines and DF for novelty detection
• MNIST Error Detection.ipynb: Training of a CNN, baselines and DF for error detection

Zurich/

• Zurich Dataset Novelty Detection.ipynb: Training of CNN, baselines and DF for novelty detection
• Zurich Dataset Error Detection.ipynb: Training of CNN, baselines and DF for error detection

🎓 Supervisors:

• Prof. Devis Tuia, University of Wageningen
• Diego Marcos González, University of Wageningen
• Prof. François Golay, EPFL

Cyril Wendl, 2018