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Lossy JBIG2 has been removed due to well-documented risks of character substitution errors (e.g., 6/8 confusion). The --jbig2-lossy and --jbig2-page-group-size arguments are now deprecated and ignored with a warning. Changes: - Remove jbig2_lossy and jbig2_page_group_size from OCROptions - Simplify optimize.py to use single-image JBIG2 encoding only (no symbol dictionaries/JBIG2Globals) - Remove convert_group() from jbig2enc.py - Deprecate CLI args with warnings for backward compatibility - Update documentation to explain lossless-only JBIG2
64 lines
2.3 KiB
Markdown
64 lines
2.3 KiB
Markdown
% SPDX-FileCopyrightText: 2022 James R. Barlow
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% SPDX-License-Identifier: CC-BY-SA-4.0
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{#jbig2}
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# Installing the JBIG2 encoder
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Most Linux distributions do not include a JBIG2 encoder since JBIG2
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encoding was patented for a long time. All known JBIG2 US patents have
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expired as of 2017, but it is possible that unknown patents exist.
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JBIG2 encoding is recommended for OCRmyPDF and is used to losslessly
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create smaller PDFs. If JBIG2 encoding is not available, lower quality
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CCITT encoding will be used for monochrome images.
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JBIG2 decoding is not patented and is performed automatically by most
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PDF viewers. It is widely supported and has been part of the PDF
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specification since 2001.
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JBIG encoding is automatically provided by these OCRmyPDF packages: -
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Docker image (both Ubuntu and Alpine) - Snap package - ArchLinux AUR
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package - Alpine Linux package - Homebrew on macOS
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For all other platforms, you would need to build the JBIG2 encoder from
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source:
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:::{code} bash
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git clone https://github.com/agl/jbig2enc
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cd jbig2enc
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./autogen.sh
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./configure && make
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[sudo] make install
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:::
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Dependencies include libtoolize and libleptonica, which on Ubuntu
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systems are packaged as libtool and libleptonica-dev. On Fedora (35)
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they are packaged as libtool and leptonica-devel. For this to work,
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please make sure to install `autotools`, `automake`, `libtool`, `pkg-config`
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and `leptonica` first if not already installed. Other dependencies might
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be required depending on your system.
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:::{code} bash
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[sudo] apt install autotools-dev automake libtool libleptonica-dev pkg-config
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:::
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## JBIG2 Compression
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OCRmyPDF uses JBIG2 lossless compression for bitonal (black and white)
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images. This provides excellent compression ratios compared to the older
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CCITT G4 standard, while preserving the exact pixel content of the
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original image.
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You can adjust the threshold for JBIG2 compression with
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`--jbig2-threshold`. The default is 0.85.
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:::{note}
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Previous versions of OCRmyPDF supported a lossy JBIG2 mode
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(`--jbig2-lossy`). This feature has been removed due to the well-known
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risk of character substitution errors (e.g., 6/8 confusion). See
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[JBIG2 disadvantages](https://en.wikipedia.org/wiki/JBIG2#Disadvantages)
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for more information on why lossy JBIG2 is problematic. The `--jbig2-lossy`
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and `--jbig2-page-group-size` arguments are now ignored with a warning.
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:::
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