GREYCstoration is an image regularization algorithm that processes an image by locally removing small variations of pixel intensities while preserving significant global image features, such as sharp edges and corners. The most direct application is denoising. By extension, it can also be used to inpaint or resize images. It is based on methods using nonlinear multi-valued diffusion PDEs (Partial Differential Equations) for image regularization. This kind of method generally outperforms basic image filtering techniques (such as convolution, median filtering, etc.) classically encountered in painting programs. Other image denoising plugins are available, but are not free, and the corresponding algorithms are kept secret.