libdmtx is open source software for reading and writing Data Matrix barcodes. Data Matrix barcodes are two-dimensional symbols that hold a dense pattern of data with built-in error correction. The Data Matrix symbology (sometimes casually referred to as DataMatrix) was invented and released into the public domain by RVSI Acuity CiMatrix. Wikipedia has a good article on the symbology and its characteristics.

WWW: https://github.com/dmtx/libdmtx
