source: gs2-extensions/tesseract/trunk/README.txt@ 34186

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CONTENTS
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In this file:

A. COMPILING TESSERACT GS2-EXTENSION
& CREATING THE CUT-DOWN BINARY-ONLY TARBALL

B. GETTING TIKA AND TESSERACT TO OCR A PDF


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A. COMPILING TESSERACT GS2-EXTENSION
& CREATING THE CUT-DOWN BINARY-ONLY TARBALL
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To compile the Tesseract gs2-extension and then create the "binary" tarball needed to run
Tesseract, we follow an equivalent version of the instructions for the imagemagick gs2-extension
at http://trac.greenstone.org/browser/gs2-extensions/imagemagick/trunk/README

1. Find a location on your machine

2. Check out the tesseract extension from gs2-extensions
   svn co http://trac.greenstone.org/browser/gs2-extensions/tesseract/trunk tesseract

3. Compile it all up (tesseract and dependencies):
   cd tesseract
   ./CASCADE-MAKE.sh

4. Open a fresh terminal and check that the tesseract now installed in src/linux/bin works:

   cd src
   source ./setup.bash

This should have set up env vars like GEXTTESS, GEXTTESS_INSTALLED, and TESSDATA_PREFIX
which Tesseract needs to have set

   tesseract --list-langs
   tesseract sample.tif out
   
OCRs sample.tif and generates out.txt from it.

   cat out.txt

5. If successful,

a. create a folder at the same level as src called tesseract
   cd src
   cd ..
   mkdir tesseract

b. COPY the setup files and MOVE the installed folder (src/linux) into the new cut-down tesseract folder:

   cp src/setup.ba* tesseract/.
   mv src/linux tesseract/.

c. COPY the TESSERACT-APACHE-LICENSE and LEPTONICA-LICENSE txt files (note it uses
American spelling!) from src/packages into the cut-down tesseract/linux:

   cp src/packages/*LICENSE.txt tesseract/linux/.

d. REMOVE folder "man" from tesseract/linux:
   rm -rf tesseract/linux/man

6. Create a tarball of the cut down tesseract folder named tesseract-<os>-<arch>.tar.gz:
   tar -cvzf tesseract-linux-x64.tar.gz tesseract

7. (Add/SVN up and) commit that to svn:
   svn up
   svn add tesseract-linux-x64.tar.gz
   (or svn diff tesseract-linux-x64.tar.gz if there was an earlier version to confirm modified)
   svn commit -m "MESSAGE" tesseract-linux-x64.tar.gz


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B. GETTING TIKA AND TESSERACT TO OCR A PDF
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Tesseract does not OCR PDFs (https://github.com/tesseract-ocr/tesseract/issues/1476).
Trying to do so, you'll see:
       tesseract pdf05-notext.pdf notext
       Tesseract Open Source OCR Engine v5.0.0-alpha-694-g6ee3 with Leptonica
       Error in pixReadStream: Pdf reading is not supported
       Error in pixRead: pix not read
       Error during processing.

Tesseract can OCR the individual images constituting a page of the PDF, but to OCR PDFs
with Tesseract, you need an additional tool to split PDFs into its pages and extract images
from them, feed each page's image into Tesseract to get it OCR-ed and then create an html or
txt file collating all the individual OCR-ed page content.

Tika does this.

By default if Tika is on the environment and TESSDATA_PREFIX is set to the tessdata folder
containing the language files, Tika is able to get Tesseract to OCR images out of the box.
Yet not PDFs. Tika will output empty OCR for (x)html/txt, extracting only extractable text
from PDFs and no OCR until the following is correct.

To get Tika (app v1.24.x) and Tesseract (v5.0.0) set up to OCR PDFs, needed to do 2 more
things:
1. Have to run the tika-app-*.jar with in --config=/path/to/a/tika-config.xml file
configured correctly for the TesseractOCRParser and PDFParser
2. The <tika-config.xml> file passed to tika-app-*.jar should configure the "outputType"
param's configuration of the TesseractOCRParser as follows:
   a. Set the "outputType" param to "txt" so Tesseract produces the OCR in .txt format, and
      Tesseract will produce .txt as OCR output which Tika will intercept and process,
   b. OR if you want the OCR generated by Tesseract to be in hocr (html ocr) format, then set
   the "outputType" param value to "hocr" AND ensure a config file also called hocr exists in
   $TESSDATA_PREFIX/configs containing the following (taken from
   https://github.com/tesseract-ocr/tessconfigs/blob/3decf1c8252ba6dbeef0bf908f4b0aab7f18d113/configs/hocr):
               tessedit_create_hocr 1
               hocr_font_info 0
           
2. In order to have a tessdata/configs/hocr, needed to correct the Tesseract we were
cascade-making to get it to put the "configs" subfolder inside the installed Tessereact's
tessdata folder. The source version of tesseract has this folder, but it wasn't getting
included in the built version despite us exporting TESSDATA_PREFIX before CASCADE-MAKing.