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.