hough 0.2.7

Skew detection and correction in scanned images

hough - Skew detection in scanned images

Hough finds skew angles in scanned document pages, using the Hough transform.

It is oriented to batch processing, and can make use of multiple cores. (You'll want this - analysis and image processing is very CPU intensive!)

Installation and usage

Installation

pip install -U pip
pip install hough

The first line is required to update pip to a new enough version to be compatible with manylinux wheel packaging, required for PyMuPDF.

Older versions of pip are fine, but you'll have to install MuPDF, its headers, and a compiler first, so PyMuPDF can be compiled locally.

Usage

To get started right away, here's some examples.

Generate angles (in CSV form) for a bunch of TIFF page images, one page per file:

hough --csv in/*.tif

The same, but for a PDF file, and display a histogram at the end:

hough --histogram Able_Attach_Sep83.pdf

The same, but show progress while running:

hough -v --histogram Able_Attach_Sep83.pdf

The deskewing results are placed in the results.csv file. Example:

"Input File","Page Number","Computed angle","Variance of computed angles","Image width (px)","Image height (px)"
"/home/toby/my-pages/orig/a--0000.pgm.tif",,-0.07699791151672428,0.001073874144832815,5014,6659
"/home/toby/my-pages/orig/a--0001.pgm.tif",,,,5018,6630
"/home/toby/my-pages/orig/a--0002.pgm.tif",,0.24936351676615068,0.005137031681286154,5021,6629
"/home/toby/my-pages/orig/a--0003.pgm.tif",,,,5020,6608
"/home/toby/my-pages/orig/a--0004.pgm.tif",,-0.037485115754500545,0.025945115897015238,5021,6616

The program should work on various image input formats, and with both grey scale and RGB images. Hough works best with images ≥300dpi.

Here's a histogram sample:

=== Skew statistics ===
0.00° - 0.10°  [57]  ████████████████████████████████████████
0.10° - 0.20°  [39]  ███████████████████████████▍
0.20° - 0.30°  [30]  █████████████████████
0.30° - 0.40°  [30]  █████████████████████
0.40° - 0.50°  [11]  ███████▊
0.50° - 0.60°  [11]  ███████▊
0.60° - 0.70°  [ 3]  ██▏
0.70° - 0.80°  [ 4]  ██▊
0.80° - 0.90°  [ 0]
0.90° - 1.00°  [ 1]  ▊
1.00° - 1.10°  [ 1]  ▊
1.10° - 1.20°  [ 0]
1.20° - 1.30°  [ 1]  ▊
1.30° - 1.40°  [ 1]  ▊
1.40° - 1.50°  [ 1]  ▊
1.50° - 1.60°  [ 2]  █▍
1.60° - 1.70°  [ 0]
1.70° - 1.80°  [ 1]  ▊
1.80° - 1.90°  [ 2]  █▍
1.90° - 2.00°  [ 0]
Samples: 195
50th percentile: 0.20°
90th percentile: 0.55°
99th percentile: 1.77°

Command line options

You can list them by running hough --help:

hough - straighten scanned pages using the Hough transform.

Usage:
  hough (-h | --help)
  hough [options] [FILE] ...
  hough [options] [--results=<file>] [FILE] ...
  hough (-r | --rotate) [options] [--results=<file>]
  hough (-r | --rotate) [options] [--results=<file>] [FILE] ...

Arguments:
  FILE                          input files to analyze/rotate

Options:
  -h --help                     display this help and exit
  --version                     display the version number and exit
  -v --verbose                  print status messages
  -d --debug                    retain debug image output in debug/ dir
                                (also enables --verbose)
  --histogram                   display rotation angle histogram summary
  -o DIR, --out=DIR             store output results/images in named
                                directory. Directory is created if it
                                does not exist [default: out/TIMESTAMP]
  --results=<file>              save results in FILE under output path,
                                or specify path to results file for
                                rotation [default: results.csv]
  -w <workers> --workers=<#>    specify the number of workers to run
                                simultaneously. Default: total # of CPUs
  -r --rotate                   rotates the files passed on the command
                                line, or if none given, those listed
                                in the results file.

Examples

Just about all of these files have been deskewed this way.

Getting the best results

NOTE: This is a beta product!

There's a few guidelines you should follow to get the best deskewing results from your document scans:

1. Bilevel (black-and-white) bitmaps will produce lower quality results. For best results, scan to greyscale or RGB first, deskew with Hough, then reduce the colour depth to bilevel.
2. Hough deskewing is an inexact process, with many heuristics discovered by trial and error. Hough may not work well on your material without tuning and further modification. (We'd love your pull requests!)

Debugging output

You can spy on Hough's attempts to perform deskewing by passing the --debug flag on the command line. The generated images, and any detected lines in them, are placed in the debug/<datetime>/ directory.

Note that Hough cannot always determine a skew for a page (e.g. blank pages in particular), and will very occasionally get the skew wrong (depending on source material). It's worth reviewing these images if Hough makes a bad decision on your scans. Please submit these files along with the original image when filing an issue!

Recommended scanners

The authors have tested this software with output from the following scanners:

• Fujitsu fi-4530C, USB
  • Fast
  • Cheap on eBay
  • Requires a Windows XP VirtualBox for drivers
• Brother ADS-2700W, USB + Ethernet + WiFi
  • Fast
  • Can scan directly to the network or to a memory stick
  • Factory reconditioned models stilll available (March 2020)
  • Very low skew out of the box
• Epson WF-7610, USB + Ethernet + WiFi
  • 11"x17" and duplex capable
  • Can scan directly to the network or to a memory stick

Developing

First, clone this repo.

You'll need to install Poetry, then run:

poetry run pip install -U pip setuptools
poetry install
poetry shell

Do some work, then run the tests with

pytest
pre-commit run

License notice

This file is part of "hough", which detects skew angles in scanned images
Copyright (C) 2016-2020 Toby Thain <[e-mail]>
Copyright (C) 2020 Joan Touzet <[e-mail]>

This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA