View source for Optical Character Recognition - OCR

=Optical Character Recognition=


== software used ==


imagemagick (convert) version 6.8.8.10-r1

tesseract version 3.03_rc1

source file: PDF made up entirely of scanned images of a book.

destination format: text file

== problem ==

I had scanned images of books that I wanted in electronic, text format.  They were manuals for a computer course I was taking.  The pwd



== solution ==

=== step 1 - break up pdf into individual files using convert ===

The convert utility gets installed along with the imagemagick package.  

 $ convert -limit memory 1 -monitor -verbose -density 300 -colorspace Gray source_file.pdf output_file.png

The command above will not output just one file, but 1 file per page of the source pdf.

example...

output_file-1.png, output_file-2.png,output_file-3.png, etc...

=== step 2 - translate image text into plain text using OCR ===

tesseract will perform the OCR.  We feed it the individual files and append the text to an output file.  A bash loop will process all the output files for us.

 $ ls -tr1 output_file*.png | while read line; do tesseract $line stdout >> output.txt; done

=== step 3 - cleanup ===

Invariably, we will have mistranslations by the OCR. For example, the document I was translating had "open quote" and "close quote" characters in it.  However; ascii only has the double quote character.  When viewing the text file using less, the hex codes for the non-ascii characters should be displayed.  I would examine the file and identify patterns.  If one had a reliable traslation, I would perform it using the line editor sed.