G'MIC is a full-featured open-source framework for digital image processing, distributed under the CeCILL free software licenses (LGPL-like and/or GPL-compatible). It provides several user interfaces to convert / process / visualize generic image datasets, ranging from 1D scalar signals to 3D+t sequences of multi-spectral volumetric images, hence including 2D color images.
Website, Windows, Linux
Ease of Use:
Scale of 1 to 5; 1 being easiest, 5 being hardest