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.
Super cool!
Hi,
Interesting. Do you know how to apply a data extractor? I would like to use Ocropus to extract information which is in determined places inside the document.
Greetings
Hi,
This might be a stupid question, but why include the original scanned image in the result pdf? If there are photos in the document I’d understand…
@cputter: I’m typically not interested in the actual text except insofar as it is an aid to discovery. I’m usually working on cultural heritage digitization projects (see http://libx.bsu.edu). The goal is to provide a reasonably accurate digital surrogate for the original physical item, so people can see not just what the item said, but what it looked like (which will, often, include images or marginalia). OCRing the text and embedding it with the scanned image in a PDF provides a representation of the item that can be discovered by searching the full OCRed text.
If you replace text.textLine by text.textLines, no more “s” is prepended to each line (due to the “\n” character added at beginning and end of each line of text in output of `ocropus buildhtml mydir`).
I use a modified version of your script, because resolution and image size were not properly found in my images. I now type something like: python HocrConverter b.hocr c.pdf 300 300 2488 3507, with the syntax python HocrConverter.py inputHocrFile outputPdfFile imagedpix imagedpiy imagepixelwidth imagepixelheight
And i include the image with pdftk image.pdf background text.pdf output result.pdf
Very beautiful job
I work on a simple script using scanadf+unpaper+ocropus+HocrConverter+gs for a simple and complete chain
I have two question:
1)
do you know how/why, in a multicolumn document, the pdf browser could avoid consider contigous lines fragments as from the same text ?
in the hOcr file the columns are presented in the right order
2)
do you consider cuneiform as ocr (with hocr2pdf)
Best regards
François Elie, your bug comes from gs: I do not have your problem in my xpdf or my acroread. Try pdftops or pdf2ps instead of gs ?
xplus3, do you allow me to leave my modified version of your HocrConverter available for free download at http://bugs.gentoo.org/show_bug.cgi?id=185810#c32 ?
Can you set your HocrConverter version on GPL ?
@d: HocrConverter is © 2009 Jonathan Brinley and available under the MIT License. I really should add a copyright statement to the file. Maybe next week…
Can I run your HocrConverter script from bash as a oneliner?
HOCR=”temp.hocr”; TEXT=”temp.text”; IMAGE=”test.tiff”; OUT=”test.pdf”
python -c “from HocrConverter import HocrConverter; hocr = HocrConverter(\”"$HOCR”\”); hocr.to_text(\”"$TEXT”\”); hocr.to_pdf(\”"$IMAGE”\”, \”"$OUT”\”);”
How do I make python find your HocrConverter module?
I tried the Gentoo ebuild, but their hocrtopdf creates a new pdf with the found
ocr text written in Helvetica (or Courier) with reportlab. I just want the
original unmodified scanned image overlayed with the hidden ocr text.
I got confused between Gentoo’s modified python script hocrtopdf and
your original HocrConverter. The scripts take different commandline arguments:
$ python /usr/local/bin/HocrConverter
Usage: python HocrConverter.py inputHocrFile inputImageFile outputPdfFile
$ /usr/local/bin/hocrtopdf
Usage: hocrtopdf inputHocrFile fontHelveticaORCourier outputPdfFile imagedpix imagedpiy imagepixelwidth imagepixelheight oneifisotropic
The only problem I’m having with HocrConverter, is that I’m getting a division
by zero exception because my hocr file is missing ocrwidth/ocrheight
info:
# get dimensions of the OCR, which may not match the image
if self.hocr is not None:
for div in self.hocr.findall(“.//%sdiv”%(self.xmlns)):
if div.attrib['class'] == ‘ocr_page’:
coords = self.element_coordinates(div)
ocrwidth = coords[2]-coords[0]
ocrheight = coords[3]-coords[1]
which results in the division by zero error:
HOCR=”pg_0001.hocr”; IMAGE=”image.png”; OUT=”pg_0001-new.pdf”
$ HocrConverter $HOCR $IMAGE $OUT
DEBUG: ocrwidth= 0 , ocrheight= 0
Traceback (most recent call last):
File “/usr/local/bin/HocrConverter”, line 179, in
hocr.to_pdf(sys.argv[2], sys.argv[3])
File “/usr/local/bin/HocrConverter”, line 126, in to_pdf
ocr_dpi = (ocrwidth/width, ocrheight/height)
ZeroDivisionError: integer division or modulo by zero
I generated my $HOCR with ocropus:
$ ocropus book2pages $DIR $IMAGE
$ ocropus pages2lines $DIR
$ ocropus lines2fsts $DIR
$ ocropus fsts2bestpaths $DIR
$ ocropus fsts2text $DIR
$ ocropus buildhtml $DIR \
| tr ‘A12′ ’12A’ \
| sed ‘s% Transitional//ENA http://www%Transitional//EN“A “http://www%’ \
| tr ‘A12′ ’12A’ \
> $HOCR
$ sed -e ’2s:$:”:’ -e ’3s:h:”h:’ -i $HOCR
I re-read Reply #5.
Somehow I missed the part about using pdftk to add the hocr-text.pdf
as a background to the original image.pdf
hocrtopdf works like a charm!. Thanks.
@Jeremy, in #15 : you only need one sed command, theoretically, juste to address a bug of ocropus.
Is the HocrConverter script still available ?
The download link gives me a 404 page ;-(
Thanks
I’d like to try it out, but I’m also getting a 404.
Apologies for the downtime. HocrConverter is now available from github: http://github.com/jbrinley/HocrConverter
I tried, and i can’t use it with a .hocr file produced by cuneiform (which works way better than tesseract / ocropus for me).
The problem is that the html output produced by cuneiform is not very well formed (missing “” around the parameter of the src tag, missing last / in meta element, and so on).
And as python is (in my opinion) one of the biggest crap ever made (whatever the tool is, if it is writen in python, i’ll have a lot of troubles having everything working), i can’t use your tool (i get a lot of “xml.parsers.expat.ExpatError: not well-formed (invalid token): line 4, column 25″, and so on). So bad.
Hi,
Thanks for the great script, However this is working in Ubuntu 9.10, In 10.04 I get the following error.
Python 2.6.5 (r265:79063, Apr 16 2010, 13:57:41)
[GCC 4.4.3] on linux2
Type “help”, “copyright”, “credits” or “license” for more information.
>>> from HocrConverter import HocrConverter
>>> hocr = HocrConverter(“/tmp/Mytest-1.html”)
Traceback (most recent call last):
File “”, line 1, in
File “/usr/lib/python2.6/HocrConverter.py”, line 31, in __init__
self.parse_hocr(hocrFileName)
File “/usr/lib/python2.6/HocrConverter.py”, line 76, in parse_hocr
self.hocr.parse(hocrFileName)
File “/usr/lib/python2.6/xml/etree/ElementTree.py”, line 587, in parse
self._root = parser.close()
File “/usr/lib/python2.6/xml/etree/ElementTree.py”, line 1254, in close
self._parser.Parse(“”, 1) # end of data
xml.parsers.expat.ExpatError: no element found: line 1, column 0
Ubuntu version is 10.04 ( 64 Bit ).
Any help would be appreciated.
Regards,
Manjunath
@Manjunath:
Not sure, but it is possible your hocr is not in clean xhtml? I was able to work around the same issue by preceding execution of hocrconverter with a “tidy -imu -asxml my_hocr.html”.
Unfortunately, I ran into another problem:
File "/Users/someUser/somePath/HocrConverter.py", line 152, in to_pdf
text.setHorizScale((((float(coords[2])/ocr_dpi[0]*inch)-(float(coords[0])/ocr_dpi[0]*inch))/pdf.stringWidth(line.text.rstrip(), fontname, fontsize))*100)
AttributeError: 'NoneType' object has no attribute 'rstrip'
The text element apparently can be of “NoneType” (not a python dude yet :) which causes the script to trip up at “rstrip()” because I assume a String is expected.
Thanks! Great tool. I had to modify it slightly to get it to work with hocr produced by tesseract. At least in my hocr files, the text of each line is nested within sub elements of the one currently referenced by the to_pdf method, one sub-element for each word (a span element with a class attribute of ‘ocr_word,’ which has a child span element with a class attribute of ‘ocrx_word,’ where the actual text of the word is held), so the calls to line.text would yield “none,” as the text was in a child element. Here’s the tweak I used:
1) add as a sub-function of the to_pdf method the following function, which gathers the text from each of the sub-elements for the line element into a single string:
def get_full_line(line_elem):
result = “”
for word_unit in line_elem.findall(‘span[@class="ocr_word"]‘):
for word in word_unit.findall(‘span[@class="ocrx_word"]‘):
result = result + word.text + ‘ ‘
return result
2) change the following portion of the to_pdf method as indicated in order to call the above function and use the returned line text, rather than try to retrieve this text directly from the element corresponding to the entire line:
if self.hocr is not None:
for line in self.hocr.findall(“.//%sspan”%(self.xmlns)):
if line.attrib['class'] == ‘ocr_line’:
line_text = get_full_line(line)
coords = self.element_coordinates(line)
text = pdf.beginText()
text.setFont(fontname, fontsize)
text.setTextRenderMode(3) # invisible
# set cursor to bottom left corner of line bbox (adjust for dpi)
text.setTextOrigin((float(coords[0])/ocr_dpi[0])*inch, (height*inch)-(float(coords[3])/ocr_dpi[1])*inch)
# scale the width of the text to fill the width of the line’s bbox
text.setHorizScale((((float(coords[2])/ocr_dpi[0]*inch)-(float(coords[0])/ocr_dpi[0]*inch))/pdf.stringWidth(line_text, fontname, fontsize))*100)
# write the text to the page
text.textLine(line_text)
pdf.drawText(text)
After making these two minor changes, it worked great. Thanks for sharing this. It saved me a lot of time.