# Installing the JBIG2 encoder

Most Linux distributions do not include a JBIG2 encoder since JBIG2 encoding was patented for a long time. All known JBIG2 US patents have expired as of 2017, but it is possible that unknown patents exist.

JBIG2 encoding is recommended for OCRmyPDF and is used to losslessly create smaller PDFs. If JBIG2 encoding is not available, lower quality CCITT encoding will be used for monochrome images.

JBIG2 decoding is not patented and is performed automatically by most PDF viewers. It is widely supported and has been part of the PDF specification since 2001.

JBIG encoding is automatically provided by these OCRmyPDF packages: - Docker image (both Ubuntu and Alpine) - Snap package - ArchLinux AUR package - Alpine Linux package - Homebrew on macOS

For all other platforms, you would need to build the JBIG2 encoder from source:

    git clone https://github.com/agl/jbig2enc
    cd jbig2enc
    ./autogen.sh
    ./configure && make
    [sudo] make install

Dependencies include libtoolize and libleptonica, which on Ubuntu systems are packaged as libtool and libleptonica-dev. On Fedora (35) they are packaged as libtool and leptonica-devel. For this to work, please make sure to install `autotools`, `automake`, `libtool`, `pkg-config` and `leptonica` first if not already installed. Other dependencies might be required depending on your system.

    [sudo] apt install autotools-dev automake libtool libleptonica-dev pkg-config

## Lossy mode JBIG2

OCRmyPDF provides lossy mode JBIG2 as an advanced and potentially dangerous feature. Users should review the technical concerns with JBIG2 in lossy mode and decide if this feature is acceptable for their use case. In general, this mode should not be used for archival purposes, should not be used when the original document is not available or will be destroyed, and should not be used when numbers present in the document are important, because there is a risk of 6/8 and 8/6 substitution errors.

JBIG2 lossy mode does achieve higher compression ratios than any other monochrome (bitonal) compression technology; for large text documents the savings are considerable. JBIG2 lossless still gives great compression ratios and is a major improvement over the older CCITT G4 standard.

To turn on JBIG2 lossy mode, add the argument `--jbig2-lossy`. `--optimize {1,2,3}` are necessary for the argument to take effect also required. Also, a JBIG2 encoder must be installed as described in the previous section.

You can adjust the threshold for JBIG2 compression with the `--jbig2-threshold`. The default is 0.85, meaning that if two symbols are 85% similar, they will be compressed together.

Due to an oversight, ocrmypdf v7.0 and v7.1 used lossy mode by default.