As I mentioned recently, OCRopus OCR software output an hOCR file. What is hOCR? hOCR is an open standard for representing OCR results in an HTML document (not to be confused with HOCR). It is basically a microformat using div and span tags’ class and title attributes to convey information from the OCR process, as you can see from this example of a basic hOCR document:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta content="ocr_line ocr_page" name="ocr-capabilities"/> <meta content="en" name="ocr-langs"/> <meta content="Latn" name="ocr-scripts"/> <meta content="" name="ocr-microformats"/> <title>OCR Output</title> </head> <body> <div class="ocr_page" title="bbox 0 0 2548 3300; image /path/to/scanned/image.png"> <span class="ocr_line" title="bbox 659 143 863 177">Some Text</span> <span class="ocr_line" title="bbox 723 275 916 324">More Text</span> </div> </body> </html>
For the more details, see Thomas Breuel’s complete hOCR specification draft. The example, though, shows all that I need to know. From the div[@class='ocr_page'], I can learn the dimensions (in pixels) of the image that was OCRed (as well as the path to that image on my machine). From a span[@class='ocr_line'], I can learn the location and dimensions of the bounding box around a line of recognized text, as well as the content of that line.
That information, along with a copy of the original image (or a sufficiently similar image) is enough to create a PDF of the image with selectable text.
Creating a PDF
A while back, Florian Hackenberger created a basic hOCR to PDF converter in Java. It does its job reasonably well, but it’s somewhat rough around the edges. Since I’m more familiar with Python than Java, it seemed like a good idea to rewrite the code in Python, so I could more comfortably hack it.
And so I present to you, dear reader, the hOCR Converter Python script (note: this script depends on the ReportLab PDF Library, which, in turn, depends on the FreeType 2 Font Engine). Right now, it has two primary functions:
- Given an hOCR file, create a text-only (i.e., no HTML) document
- Given an hOCR file and an image, create a PDF
Usage is pretty simple:
from HocrConverter import HocrConverter hocr = HocrConverter("myHocrFile.html") hocr.to_text("output.txt") hocr.to_pdf("myImageFile.png", "output.pdf")
You end up with a text file at output.txt containing the contents of the body of the hOCR document and a PDF file at output.pdf containing your image superimposed on top of the text.
Note that the image you use to create the PDF need not be the same image used for the OCR. If, for example, you used a 300 or 400 DPI image for the OCR, but you want a smaller file for the PDF, you can create a 72 DPI version of the image and feed that through the script instead of the original.
With a simple script that iterates through a directory of images, calling OCRopus and this script for each image (maybe with some ImageMagick and pdftk thrown in for good measure), you can quickly OCR a large batch of images and convert them to searchable PDF documents.
Future Development
I’d like to add the capability to convert the output of various OCR programs into hOCR format. For example, OmniPage, which is much better than OCRopus when it comes to layout analysis for complex images, can output documents in its proprietary XML schema. I should be able to transform that into an hOCR document and then use this script to create a PDF from there.
I would welcome any other suggestions or contributions.
