Image Denoising Methods. A New Nonlocal Principle

The search for efficient image denoising methods is still a valid challenge at the crossing of functional analysis and statistics. In spite of the sophistication of the recently proposed methods, most algorithms have not yet attained a desirable level of applicability. All show an outstanding performance when the image model corresponds to the algorithm assumptions but fail in general and create artifacts or remove image fine structures. The main focus of this paper is, first, to define a general mathematical and experimental methodology to compare and classify classical image denoising algorithms and, second, to propose a nonlocal means (NL-means) algorithm addressing the preservation of structure in a digital image. The mathematical analysis is based on the analysis of the “method noise,” defined as the difference between a digital image and its denoised version. The NL-means algorithm is proven to be asymptotically optimal under a generic statistical image model. The denoising performance of all considered methods are compared in four ways; mathematical: asymptotic order of magnitude of the method noise under regularity assumptions; perceptual-mathematical: the algorithms artifacts and their explanation as a violation of the image model; quantitative experimental: by tables of L2 distances of the denoised version to the original image. The most powerful evaluation method seems, however, to be the visualization of the method noise on natural images. The more this method noise looks like a real white noise, the better the method. Note to the reader: The present paper is an updated version of “A review of image denoising algorithms, with a new one” [21]. The text and structure of the original paper have been preserved. However, several spurious comparisons, technical proofs, and appendices have been adapted or removed. At the request of the editor-in-chief, the controversial benchmark image Lena has been replaced. The new section 6 reviews the abundant literature on “nonlocal image processing” stemming from the original paper. The denoising algorithm NL-means can be tested on line: http://mw.cmla.ens-cachan.fr/megawave/demo/ .