Image denoising with dictionary learning

Posted on July 7, 2011

3


I am presenting an image denoising example that fully runs under my local scikits-learn fork. Coming soon near you!

The 400 square pixels area covering Lena’s face was distorted by additive gaussian noise with a standard deviation of 50 (pixel values are ranged 0-256.)

Lena image denoising using dictionary learning

The dictionary contains 100 atoms of shape 4×4 and was trained using 10000 random patches extracted from the undistorted image. Then, each one of the four 100 square pixel areas was reconstructed using the dictionary learning model and a different transform method.

  • OMP-1 reconstructs each patch as the closest dictionary atom, multiplied by a variable coefficient. This is similar to the idea of gain-shape vector quantization.
  • OMP-2 is like OMP-1, but it considers 2 atoms instead of just one. This takes advantage of the fact that the natural dictionary atoms are of such nature to efficiently represent random image patches when combined.
  • LARS finds a reconstruction of each image patch as a solution to a Lasso problem, solved using least angle regression.
  • Thresholding is a simple and quick non-linearity that (as it is currently implemented, based on [1], where it is not intended for reconstruction but for classification) breaks the local brightness of the image fragment. The bottom right fragment was forcefully renormalized to stretch fit into the 0-256 range, but brightness differences can be seen.