In this paper we introduce libcrn, a multiplatform open-source document image processing library aimed at researchers and companies. It is written in C++11 and has a non-contaminating license that makes it available for use in any project without legal constraints. The features include low-level image processing (color format conversion, binarization, convolution, PDE…), document images specific tools (connected components extraction, recursive block description, PDF export…), maths (matrix arithmetics, linear algebra, GMMs, equation solvers…), classification and clustering (kNN, k-means, HMMs…). The API is comprehensively documented and libcrn's architecture follows modern C++ guidelines to facilitate the handling of the library and enforce its safe usage. A sample OCR, which is only 30 lines long, is described to illustrate libcrn's scope of possibilities.