Batch-processing images of documents to look like a fax

faxgradientimagemagickscanner

Let's say I have a photo or scan of a text document, possibly with
some low-contrast watermark background. If it is a photo, in addition
to the watermark there will be brightness gradients from the lighting
and possibly from the sheet of paper not laying flat due to folds.

I want to postprocess these photos with imagemagick to look like a
fax, i.e. converting the image to monochrome black/white correcting
local variations of brightness. The normal -threshold option will
not work, since

It doesn't automatically detect the required brightness level for
each photo.
Due to the brightness gradients the text on one part of the image
may be brighter than the background on another part such that for
any given global threshold some text will be lost.

Cam-Scanner apps on smartphones usually provide a black/white-document
option that corrects such color gradients and calculates a reasonable
first guess for the threshold value, that would suffice for batch
processing.

They do not help though when I have the raw image on PC already,
though I could in theory upload them to the smartphone and import them
— it is just highly impractical, especially for large numbers of images.

Does imagemagick or some other batch-processing capable software
(preferably open-source) support such a conversion?

Best Answer

You can use Imagemagick's Mathematical Compose Methods to achieve such results. Divide_src [1] in particular as it would remove any gradients, vignettes, unwanted shadings.

Then a -normalize and a -threshold should do the rest.

convert $input -colorspace gray ( +clone -blur 15,15 ) -compose Divide_Src -composite -normalize -threshold 80% $output

Here's my result:

You may want to adujst the threshold to get the best results.

Depending ot the OS you are going to run this you might have to escape the brackets: "\(" and "\)".

As for the batch processing personally I would use a "for" loop either in bash or in Cygwin again depending on the OS:

for file in test/*; do convert $file -colorspace gray ( +clone -blur 15,15 ) -compose Divide_Src -composite -normalize -threshold 80% result/`basename $file`; done

However there is another command line tool you might want to check out called mogrify [2] for inline or specific -path batch processing.

For more information and possibly different results follow [3] and [4].

[1]: www.imagemagick.org/Usage/compose/#divide

[2]: www.imagemagick.org/script/mogrify.php

[3]: staff.washington.edu/corey/camscan/

[4]: www.imagemagick.org/Usage/photos/#color-in

Software Requirements

The following software packages are available for both Windows and Linux systems, and are required for a complete, working solution:

gvim - Used to export syntax highlighted source code to HTML.
moria - Colour scheme for syntax highlighting.
wkhtmltoimage - Used to convert HTML documents to PNG files.
gawk and sed - Text processing tools.
ImageMagick - Used to trim the PNG and add a border.

General Steps

Here is how the solution works:

Load the source code into an editor that can add splashes of colour.
Export the source code as an HTML document (with embedded FONT tags).
Strip the background attribute from the HTML document (to allow transparency).
Convert the HTML document to a PNG file.
Trim the PNG border.
Add a small, 25 pixel border around the image.
Delete temporary files.

The script generates images that are all the same width for source files containing lines that are all under 80 characters in length. Source files with lines over 80 characters long result in images as wide as necessary to retain the entire line.

Installation

Install the components into the following locations:

gvim - C:\Program Files\Vim
moria - C:\Program Files\Vim\vim73\colors
wkhtmltoimage - C:\Program Files\wkhtml
ImageMagick - C:\Program Files\ImageMagick
Gawk and Sed - C:\Program Files\GnuWin32

Note: ImageMagick has a program called convert.exe, which cannot supersede the Windows convert command. Because of this, the full path to convert.exe must be hard-coded in the batch file (as opposed to adding ImageMagick to the PATH).

Environment Variables

Set the PATH environment variable to:

"C:\Program Files\Vim\vim73";"C:\Program Files\wkhtml";"C:\Program Files\GnuWin32\bin"

Batch File

Run it using:

src2png.bat src2png.bat

Create a batch file called src2png.bat by copying the following contents:

@ECHO OFF

SET NUMBERS=-c "set number"
IF "%2" == "" SET NUMBERS=

ECHO Converting %1 to %1.html...
gvim -e %1 -c "set nobackup" %NUMBERS% -c ":colorscheme moria" ^
  -c :TOhtml -c wq -c :q

REM Remove all background-color occurrences (without being self-referential)
sed -i "s/background-color: #......; \(.*\)}$/\1 }/g" %1.html

ECHO Converting %1.html to %1.png...
wkhtmltoimage --format png --transparent --minimum-font-size 80 ^
  --quality 100 --width 3600 ^
  %1.html %1.png

move %1.png %1.orig.png

REM If the text file has lines that exceed 80 characters, don't crop the
REM resulting image. (The book automatically shrinks large images to fit.)
REM The 3950 is the 80 point font at 80 characters with padding for line
REM numbers.
SET LENGTH=0
FOR /F %%l IN ('gawk ^
  "BEGIN {x=0} {if( length($0)>x ) x=length()} END {print x;}" %1') ^
DO (
  SET LENGTH=%%l
)
SET EXTENT=-extent 3950x
IF %LENGTH% GTR 80 SET EXTENT=

REM Trim the image height, then extend the width for 80 columns, if needed.
REM The result is that all images will be resized the same amount, thus
REM making the font size the same maximum for all source listings. Source
REM files beyond the 80 character limit will be scaled as necessary.
ECHO Trimming %1.png...
"C:\programs\ImageMagick\convert.exe" -format png %1.orig.png ^
  -density 150x150 ^
  -background none -antialias -trim +repage ^
  %EXTENT% ^
  -bordercolor none -border 25 ^
  %1.png

ECHO Removing old files...
IF EXIST %1.orig.png DEL /q %1.orig.png
IF EXIST %1.html DEL /q %1.html
IF EXIST sed*. DEL /q sed*.

Improvements and optimizations welcome.

Note: The latest version of wkhtmltoimage properly handles overriding the background colour. Thus the line to remove the CSS for background colours is no longer necessary, in theory.

Windows – How to change the default folder used for documents scanned with Windows Fax and Scan

Backup all the scanned files you need to keep
Open the automatic Scanned Documents folder and click on the address bar to copy its full location. It should look like C:\Users\BillGates\Documents\Scanned Documents
Go back to the Documents folder and delete Scanned Documents
Open a command prompt window. On Windows 8.1, you can do this by using the shortcut keys ⊞ (Windows key) + S and typing cmd. On Windows 7 and Windows 10, type cmd in the start menu search bar
Enter the following command and press enter, where C:\scans is the directory you want to be used to save new scanned files:

mklink /d "C:\Users\BillGates\Documents\Scanned Documents" C:\scans

This creates a linked folder (symlink, not a shortcut). It acts like a normal folder but is actually located in a different place. When the scan software saves into its default location, the files will actually be saved into C:\scans.