A few design notes:
Leptonica's deskew is far superior to ImageMagick's convert -deskew command --
around 30-40x faster. Subjectively the output appears to this contributor to
be of higher quality as well. The difference is the algorithm: ImageMagick
uses the complex Hough transform to find the skew angle, while Leptonica uses
the simpler method, Postl's variance of differential line sums -- conceptually, shear the image and check for straight horizontal. In this case
simplicity wins. Finding the skew angle is the bulk of the work.
Leptonica's author explains the advantages of his approach here:
http://www.leptonica.com/skew-measurement.html
Leptonica is the low-level library that Tesseract depends on. Hence, this
project already depends on Leptonica. Leptonica can read and write most
common image file types on its own.
Unfortunately its error handling is poor: it seldom returns any meaningful
error codes. The best it manages is writing messages to stderr, which in
the context of a verbose script is just confusing since the error's source
is not indicated. The problem is compounded by Tesseract's use of Leptonica,
which will produce exactly the same errors in some cases. So we trap stderr
between calls to Leptonica and parse it for a few different types of error
message.
leptonica.py is Python 2/3 compatible and set up to provide access to other
Leptonica functions as needed. Of particular interest are its orientation
detection (including flip and rotation errors) which it does by comparing
text ascenders to descenders.
There is a PyPI "pylepthonica" package, however it is out of date by a few
years, and it implements all of Leptonica with Python wrappers -- so it is
massive, with one .py file at 2.5 MB. This module is loosely inspired by
pyleptonica but more modern, up to date, and contains only limited
functionality.
better way of checking if the tesseract version is compatible with the
script.
If the required tess version is 3.02.02, and the actual version is 3.03,
the script would have told before that the version is too old, because
303<30202, now it compares 3.03>3.0202
mktemp: consider both FreeBSD/OSX and Linux OS having incompatible
syntax
From now on temporary files are saved in the folder specified by the
environment variable $TMPDIR
self.width remains undefined unless hOCR finds text. It might not, if
a page contains only an image for example.
Full error message is:
AttributeError: ‘hocrTransform’ object has no attribute ‘width’
First, the regular expression matches everything after the first period
in a filename. Adding the $ make it match the last, so that filenames
such as “Report.1.pdf” get trimmed to “Report.1”.
Next use mktemp to get the OS to create a temporary folder. It will
guarantee a unique directory name beginning with prefix, even if parallel
processes are at work.
If deskew and/or cleanup is not requested, do not copy the files, but
just create symbolic link.
This saves disk place and makes the script slightly quicker
Jim Barlow
fritz-hh
Mansour Behabadi