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I am a huge fan of Ben Marwick. He has so many useful pieces of code for the programming archaeologist or historian!

Edit July 17 1.20 pm: Mea culpa: I originally titled this post, ‘Doing OCR within R’. But, what I’m describing below – that’s not OCR. That’s extracting text from pdfs. It’s very fast and efficient, but it’s not OCR. So, brain fart. But I leave the remainder of the post as it was. For command line OCR (really, actual OCR) on a Mac, see the link to Ben Schmidt’s piece at the bottom. Sorry.

Edit July 17 10 pm: I am now an even bigger fan of Ben’s. He’s updated his script to either a) perform OCR by calling Tesseract from within R or b) grab the text layer from a pdf image. So this post no longer misleads. Thank you Ben!

Object Character Recognition, or OCR, is something that most historians will need to use at some point when working with digital documents. That is, you will often encounter pdf files of texts that you wish to work with in more detail (digitized newspapers, for instance). Often, there is a layer within the pdf image containing the text already: if you can highlight text by clicking and dragging over the image, you can copy and paste the text from the image. But this is often not the case, or worse, you have tens or hundreds or even thousands of documents to examine. There is commercial software that can do this for you, but it can be quite expensive

One way of doing OCR on your own machine with free tools, is to use Ben Marwick’s pdf-2-text-or-csv.r script for the R programming language. Marwick’s script uses R as wrapper for the Xpdf programme from Foolabs. Xpdf is a pdf viewer, much like Adobe Acrobat. Using Xpdf on its own can be quite tricky, so Marwick’s script will feed your pdf files to Xpdf, and have Xpdf perform the text extraction. For OCR, the script acts as a wrapper for Tesseract, which is not an easy piece of software to work with. There’s a final part to Marwick’s script that will pre-process the resulting text files for various kinds of text analysis, but you can ignore that part for now.

1. Make sure you have R downloaded and installed on your machine (available from http://www.r-project.org/)
2. Make sure you have Xpdf downloaded and installed (available from ftp://ftp.foolabs.com/pub/xpdf/xpdfbin-win-3.04.zip ). Make a note of where you unzipped it. In particular, you are looking for the location of the file ‘pdftotext.exe’. Also, make sure you know where ‘pdftoppm’ is located too (it’s in that download).
3. Download and install Tesseract https://code.google.com/p/tesseract-ocr/ 
4. Download and install Imagemagick http://www.imagemagick.org/
5. Have a folder with the pdfs you wish to extract text from.
6. Open R, and paste Marwick’s script into the script editor window.
7. Make sure you adjust the path for “dest” and the path to “pdftotext.exe” to the correct location
8. Run the script! But read the script carefully and make sure you run the bits you need. Ben has commented out the code very well, so it should be fairly straightforward.

Obviously, the above is framed for Windows users. For Mac users, the steps are all the same, except that you use the version of Xpdf, Tesseract, and Imagemagick built for IOS, and your paths to the other software are going to be different. And of course you’re using R for Mac, which means the ‘shell’ commands have to be swapped to ‘system’! (As of July 2014, the Xpdf file for Mac that you want is at ftp://ftp.foolabs.com/pub/xpdf/xpdfbin-mac-3.04.tar.gz ) I’m not 100% certain of any other Mac/PC differences in the R script – these should only exist at those points where R is calling on other resources (rather than on R packages). Caveat lector, eh?

The full R script may be found athttps://gist.github.com/benmarwick/11333467. So here is the section that does the text extraction from pdf images (ie, you can copy and highlight text in the pdf):

And here’s the bit that does the OCR

Besides showing how to do your own OCR, Marwick’s script shows some of the power of R for doing more than statistics. Mac users might be interested in Ben Schmidt’s tutorial ‘Command-line OCR on a Mac’ from his digital history graduate seminar at Northeastern University, online athttp://benschmidt.org/dighist13/?page_id=129.

출처 : Extracting Text from PDFs; Doing OCR; all within R