...As such, it is also subject to the GNU General Public License version 2.
Several patents apply to two very specific aspects of DjVu and DjVuLibre. The patents cover a particular aspect of the ZP-coder (the arithmetic coder used in DjVu and implemented in libdjvu/ZPCodec.cpp) and the background masking technique used in the IW44 wavelet encoder (implemented in libdjvu/IW44EncodeCodec.cpp).
Most patents are owned by AT&T. LizardTech has very broad rights to them and grants free and permanent licenses to them for the purpose of building GPL software with the DjVu Reference Library. ...