Optical character recognition with Tesseract

install

Tesseract (with languages you will be using)

• Mac brew install tesseract --all-languages
• Debian/Ubuntu: sudo aptitude install tesseract-ocr
  • See what language packages are available with: sudo aptitude search tesseract-ocr-
  • install language packages: sudo aptitude install tesseract-ocr-ara tesseract-ocr-port tesseract-ocr-spa here I am installing Arabic, Portuguese, Spanish

poppler-utils whic include tools such as pdftotext and pdftohtml

• Mac brew install poppler-utils
• Debian/Ubuntu: sudo aptitude install poppler-utils

imagemagick

• Mac brew install imagemagick
• Debian/Ubuntu: sudo aptitude install imagemagick

pdftk

• Mac brew install pdftk
• Debian/Ubuntu: sudo aptitude install pdftk

PDF

• with text layer
• without text layer

To find out the difference you can try to select the PDF's text in a PDF viewer. Only if the text layer is present will you be able to select it.

If it contains a text layer you can use pdftotext command-line application (from poppler-utils) to convert the PDF to text

Tesseract

Tesseract was originally developed at Hewlett-Packard Laboratories Bristol and at Hewlett-Packard Co, Greeley Colorado between 1985 and 1994, with some more changes made in 1996 to port to Windows, and some C++izing in 1998. In 2005 Tesseract was open sourced by HP. Since 2006 it is developed by Google.

Tesseract is a Free software OCR package

one page prototype

Getting 1 page from PDF file with PDFTK burst

pdftk yourfile.pdf burst 

Chose page you want to convert

Convert PDF to bit-map using imagemagick, with some options to optimize OCR

convert -density 300 page.pdf -depth 8 -strip -background white -alpha off ouput.tiff

• -density 300 resolution 300DPI. Lower resolutions will create errors :)
• -depth 8number of bits for color. 8bit depth == grey-scale
• -strip -background white -alpha off removes alpha channel (opacity), and makes the background white
• output.tiffin previous versions Tesseract only accepted images as tiffs, but currently more bitmap formats are accepted

OCR

tesseract output.tiff -l eng output

Will generate the file output.txt

• -l is the option for language (English is the default)

Advanced

language

Lists all tesseract languages available in your system.

tesseract --list-langs

If OCRing a document with more than one language Tesseract can use also more than one

tesseract output.tiff -l eng+spa output

multipages

Tiff files can be multi-page images. Hence if we use the prevoious IM command to convert a PDF to a TIFF, if the PDF is multi page, so will be it TIFF. Which Tesseract should handle.

$ tesseract TypewriterArt.tiff TypewriterArt
Tesseract Open Source OCR Engine v3.03 with Leptonica
Page 1 of 8
Page 2 of 8
Page 3 of 8

Another option is providing Tesseract with a text file containing the path/filename to each image in sequence:

list.txt:

p001.tiff
p002.tiff
p003.png

tesseract list.txt output

segmentation

Page Segmentation Mode (-psm) directs the layout analysis that Tesseract performs on the page.

By default, Tesseract automates the page segmentation, but does not perform orientation and script detection.

From Tesseract man page:

       -psm N
           Set Tesseract to only run a subset of layout analysis and assume a certain form of image. The options for N are:

               0 = Orientation and script detection (OSD) only.
               1 = Automatic page segmentation with OSD.
               2 = Automatic page segmentation, but no OSD, or OCR.
               3 = Fully automatic page segmentation, but no OSD. (Default)
               4 = Assume a single column of text of variable sizes.
               5 = Assume a single uniform block of vertically aligned text.
               6 = Assume a single uniform block of text.
               7 = Treat the image as a single text line.
               8 = Treat the image as a single word.
               9 = Treat the image as a single word in a circle.
               10 = Treat the image as a single character.

searchable PDF

tesseract input.tiff output -l eng pdf

hocr

Tesseract 3.0x supports a hocr option, which creates horc file.

HOCR is an HTML+XML (XHTML) file consisting of recognized words and their coordinates.

The HOCR file contains all pages as ocr_page elements. with attribute that contains the following fields :

• ppageno: The physical page number
• image: The relative path (from the HOCR file) to the page image
• bbox: The dimensions of the image

class='ocr_page

The OCRed text is atomized into text elements of different magnitude, such as:

• paragraph "ocr_par"
• line "ocr_line"
• word "ocrx_word"

HOCR tools

• hocrviewer-mirador
• Python-based box file editor: moshPyTT for Tesseract v.3.0

Training

Documentation on Training Tessearct

Tesseract needs to know about different shapes of the same character by having different fonts separated explicitly.

box

In cases where the input is a standard text, with a standard font, the result are not bad.

But when dealing with unusual fonts or hand-written scripts Tesseract has the possibility to train it.

Tesseract needs a 'box' file to go with each training image. The box file is a text file that lists the characters in the training image, in order, one per line, with the coordinates of the bounding box around the image.

tesseract input.tiff output -l nld makebox

See more Tesseract add-on in https://github.com/tesseract-ocr/tesseract/wiki/AddOns

Artistic research

"Reverse OCR"

Kindle Scanner Peter Purgathofer