Lyssandra

A collection of Python tools for feature extraction and image classification with Sparse Coding algorithms.

features

Sparse Coding algorithms

Sparse coding is a class of unsupervised methods for learning sets of over-complete dictionaries to represent data efficiently. Each signal can be expressed as a sparse linear combination of the atoms in the dictionary:

To encode a set of datapoints X over a dictionary D:

from lyssa.sparse_coding import sparse_encoder

# ...
se = sparse_encoder(algorithm='bomp', params={'n_nonzero_coefs': 5}, n_jobs=8)
Z = se.encode(X, D)

Some of the supported solvers include:

• Orthogonal Matching Pursuit (OMP)
• Batch OMP [1]
• Group OMP [2]
• Non-Negative OMP [3]
• Iterative Hard Thresholding

Dictionary Learning algorithms

Learning the dictionary from the data involves solving the following objective

A dictionary learned from image patched of natural images looks like

Supported solvers:

• K-SVD and its approximate variant [4]
• Online Dictionary Learning [5]
• Projected Gradient Descent

Feature Extraction

• Spatial Pyramid Matching using Sparse Coding [6]
• Convolutional Feature Encoders [8]
• Dense SIFT extraction

Classification

• Label Consistent K-SVD [9]
• Sparse Representation based Classification [10]

Installation

Install the dependencies using:

pip install -r requirements.txt

For LASSO problems, the Python version of SPAMS http://spams-devel.gforge.inria.fr/index.html must be installed in your system.

First edit config.yml to specify the

• workspace path, the location of the directory in which outputs of feature extraction tasks will be saved
• path to OpenBLAS in your system (optional)

and then do:

pip install .

For best performance, configuring numpy with OpenBLAS is recommended (see Dockerfile).

Usage

Have a look at the lyssa/examples folder for some usage examples, and typical workflows.

References

[1] R. Rubinstein, M. Zibulevsky and M. Elad: Efficient Implementation of the K-SVD Algorithm and the Batch-OMP Method.

[2] A. Lozano, G. Swirszcz, N. Abe: Group Orthogonal Matching Pursuit for Variable Selection and Prediction.

[3] A. Bruckstein, M. Elad and, M. Zibulevsky: On the uniqueness of nonnegative sparse solutions to underdetermined systems of equations. IEEE Trans. Inform. Theory, 54(11):4813–4820, 2008.

[4] M. Aharon, M. Elad, and A. Bruckstein: K-SVD: An Algorithm for Designing Overcomplete Dictionaries for Sparse Representation.

[5] J. Mairal, F. Bach, J. Ponce, and G. Sapiro: Online Dictionary Learning for Sparse Coding.

[6] J. Yang, K. Yu, Y. Gong, and T. Huang: Linear spatial pyramid matching using sparse coding for image classification, CVPR (2009).

[7] L. Bo, X. Ren, and D. Fox: Hierarchical Matching Pursuit for Image Classification: Architecture and Fast Algorithms. In NIPS, 2011.

[8] A. Coates and, A. Y. Ng: The Importance of Encoding Versus Training with Sparse Coding and Vector Quantization.

[9] Z. Jiang, Z. Lin, and L. S. Davis: Learning a discriminative dictionary for sparse coding via label consistent k-svd. CVPR, 2011.

[10] J. Wright, A. Yang, A. Ganesh, S. Sastry, and Y. Ma: Robust face recognition via sparse representation, PAMI (2009).