The CImg Library (https://github.com/dtschump/CImg) is a small, open-source, and modern C++ toolkit for image processing.
CImg stands for Cool Image : It is easy to use, efficient and is intended to be a very pleasant toolbox to design image processing algorithms in C++. Due to its generic conception, it can cover a wide range of image processing applications.
I translate the document from http://cimg.eu/reference/modules.html to Markdown file and put it on GitHub here.