pdfalto

pdfalto is a command line executable for parsing PDF files and producing structured XML representations of the PDF content in ALTO format.

pdfalto is initially a fork of pdf2xml, developed at XRCE, with modifications for robustness, addition of features, improved layout element detections, and output enhanced format in ALTO (including in particular space information, useful for instance for further machine learning processing). It is based on the Xpdf library.

The latest stable version is 0.4. Working version (master) is 0.5.

An Archlinux package for pdfalto is available here, thanks to @andreasbaumann. The build process described below will create a portable standalone pdfalto executable that can be packaged with other tools without further installation requirements for the end-user.

Requirements

• compilers : clang 3.6 or gcc 4.9
• makefile generator : cmake 3.12.0
• fetching dependencies : wget

Usage

General usage is as follow:

Usage: pdfalto [options] <PDF-file> [<xml-file>]
  -f <int>                      : first page to convert
  -l <int>                      : last page to convert
  -verbose                      : display pdf attributes
  -noImage                      : do not extract Images (Bitmap and Vectorial)
  -outline                      : create an outline file xml
  -annotation                   : create an annotations file xml
  -noLineNumbers                : do not output line numbers added in manuscript-style textual documents
  -readingOrder                 : blocks follow the reading order
  -noText                       : do not extract textual objects (might be useful, but non-valid ALTO)
  -charReadingOrderAttr         : include TYPE attribute to String elements to indicate right-to-left reading order (might be useful, but non-valid ALTO)
  -fullFontName                 : fonts names are not normalized
  -nsURI <string>               : add the specified namespace URI
  -opw <string>                 : owner password (for encrypted files)
  -upw <string>                 : user password (for encrypted files)
  -filesLimit <int>             : limit of asset files be extracted
  -q                            : don't print any messages or errors
  -v                            : print version info
  -h                            : print usage information
  -help                         : print usage information
  --help                        : print usage information
  -?                            : print usage information

In addition to the ALTO file describing the PDF content, the following files are generated:

• _metadata.xml file containing a pdf file metadata (generate metadata information in a separate XML file as ALTO schema does not support that).

• _annot.xml file containing a description of the annotations in the PDF (e.g. GOTO, external http links, ...) obtained with -annotation option

• _outline.xml file containing a possible PDF-embedded table of content (aka outline) obtained with -outline option

• .xml_data/ subdirectory containing the vectorial (.vec) and bitmap images (.png) embedded in the PDF, this is generated by default - when the option -noImage is not present. This extraction slows down the process very significantly, so if no image is required, use the option -noImage. When the images are not extracted, image elements with layout properties still appear in the ALTO file, but they reference no extracted image files.

Dependencies

Dependencies can be recompiled by running this script

./install_deps.sh

The script will download and build the dependencies unders libs/ and the additional language support packages for xpdf under languages/.

If necessary, see compiling dependencies procedures for further details.

Known issues

(issue 41) might occur while building, in this case you'll need to compile the dependencies before building pdflato.

Build

• NOTE for windows : it's recommended to use Cygwin and install standard libraries (either for cland or gcc)

git clone https://github.com/kermitt2/pdfalto.git && cd pdfalto

• Xpdf-4.03 is shipped as git submodule, to download it:

git submodule update --init --recursive

• Build pdfalto:

cmake .

make

The executable pdfalto is generated in the root directory. Additionally, this will create a static library for xpdf-4.03 at the following path xpdf-4.03/build/xpdf/lib/libxpdf.a and all the libraries and their respective subdirectory.

To use the additional xpdf language support packages, the executable pdfalto comes with a config file xpdfrc and language resources installed under languages/. Both xpdfrc and languages/ must be alongside the executable pdfalto to be used. To add pdfalto with these additional resources to a third party application (e.g. GROBID), move the executation together with these files:

lopez@work:~$ ls my_pdfalto/
languages  pdfalto  xpdfrc

Known issues

(issue #135) on macOS "fontconfig.h file not found" might occur while building, see described workaround.

Future work

• Text like containing block element characters (https://unicode.org/charts/PDF/U2B00.pdf) are used as placeholders for unknown character unicodes, instead of what would be expected when visually inspecting the text. The reason for these unsolved character unicode values is that the actual characters are glyphs that are embedded in the PDF document which use free unicode range for embedded fonts, not the right unicode. The only way to extract the valid text for those special characters is to use OCR at glyph level . This is our targeted main future enhancement, relying on a custom Deep Learning approach.

• map special characters in secondary fonts to their expected unicode

• try to optimize speed and memory

• see the issue tracker for further tasks

Changes

New in version 0.4 (apart various bug fixes):

• support for xpdf language support package for language-specific fonts like Arabic, Chinese-simplified, Japanese, etc. they are pre-installed locally and portable

• refined line number detection and fixing a bug which could result in random missing numbers in the ALTO output

• update to xpdf-4.03

• fix issue with character spacing due to invalid rotation condition

• update dependencies and dependency install script

New in version 0.3 (apart various bug fixes):

• line number detection: line numbers (typically added for review in manuscripts/preprints) are specifically identified and not anymore mixed with the rest of text content, they will be grouped in a separate block or, optionally, not outputted in the ALTO file (noLineNumbers option)

• removal of -blocks option, the block information are always returned for ensuring ALTO validation (<TextBlock> element)

• bug fixing on reading order

• fix possible incorrect XMax and YMax values at 0 on block coordinates having only one line

New in version 0.2 (apart various bug fixes):

• support Unicode composition of characters

• generalize reading order to all blocks (it was limited to the blocks of the first page)

• detect subscript/superscript text font style attribute

• use SVG as a format for vectorial images

• propagate unsolved character Unicode value (free Unicode range for embedded fonts) as encoded special character in ALTO (so-called "placeholder" approach)

• generate metadata information in a separate XML file (as ALTO schema does not support that)

• use the latest version of xpdf, version 4.00

• add cmake

• ALTO output is replacing custom Xerox XML format

• Note: this released version was used for Grobid release 0.5.6

New in version 0.1 (apart various bug fixes):

• encode URI (using xmlURIEscape from libxml2) for the @href attribute content to avoid blocking XML wellformedness issues. From our experiments, this problem happens in average for 2-3 scholar PDF out of one thousand.

• output coordinates attributes for the BLOCK elements when the -block option is selected,

• add a parameter -readingOrder which re-order the blocks following the reading order when the -block option is selected. By default in pdf2xml, the elements followed the PDF content stream (the so-called raw order). In xpdf, several text flow orders are available including the raw order and the reading order. Note that, with this modification and this new option, only the blocks are re-ordered.

  From our experiments, the raw order can diverge quite significantly from the order of elements according to the visual/reading layout in 2-4% of scholar PDF (e.g. title element is introduced at the end of the page element, while visually present at the top of the page), and minor changes can be present in up to 100% of PDF for some scientific publishers (e.g. headnote introduced at the end of the page content). This additional mode can be thus quite useful for information/structure extraction applications exploiting pdfalto output.

• use the latest version of xpdf, version 3.04.

Contributors

Contact: Patrice Lopez ([email protected])

pdfalto is developed by Patrice Lopez ([email protected]) and Achraf Azhar ([email protected]).

pdf2xml is orignally written by Hervé Déjean, Sophie Andrieu, Jean-Yves Vion-Dury and Emmanuel Giguet (XRCE) under GPL2 license.

Xpdf is developed by Glyph & Cog, LLC (1996-2017) and distributed under GPL2 or GPL3 license.

The windows version has been built originally by @pboumenot and ported on windows 7 for 64 bit, then for windows (native and cygwin) by @lfoppiano and @flydutch.

License

As the original pdf2xml and main dependency Xpdf, pdfalto is distributed under GPL2 license.

Useful links

Some tools for converting ALTO into other formats:

• https://github.com/filak/hOCR-to-ALTO
• https://github.com/UB-Mannheim/ocr-fileformat