I had a set of many scanned images from magazine pages, the sizes were varying and I wanted to apply the GIMP erode filter to enhance the text readability. Generally for image batch processing I use Imagemagick from the command line, but I was not able to reproduce the erode filter with Imagemagick, so I searched the recipe to do batch processing with the GIMP.

You can apply the erode filter in GIMP from menu Filters ⇒ Generic ⇒ Erode. There is no dialog box to control the filter parameters, but it turned out that when you want to call the plug-in-erode function from a script you have to pass several parameters.

To inspect the required parameters go to menu Filters ⇒ Script-Fu ⇒ Console ⇒ Browse. Here you are into the Script-Fu Procedure Browser and you can search for the plug-in-erode procedure. For each parameter you have the type (e.g. float or integer) and a brief description:

• run-mode
• image
• drawable
• propagate-mode
• propagating-channel
• propagating-rate
• direction-mask
• lower-limit
• upper-limit

It is not simple to have an explanation of each paramter, so I proceeded by trials. It turned out that the wanted result is obtaiuned by passing propagate-mode = O:white. I set to zero the other parameters too.

The next step was to create a script which will simply call the plug-in, this is the file that I saved as simple-myerode.scm:

(define (simple-myerode
            filename
            propagate-mode
            propagating-channel
            propagating-rate
            direction-mask
            lower-limit
            upper-limit)
    (let* ((image (car (gimp-file-load RUN-NONINTERACTIVE filename filename)))
           (drawable (car (gimp-image-get-active-layer image))))
    (plug-in-erode  RUN-NONINTERACTIVE
                    image drawable propagate-mode propagating-channel
                    propagating-rate direction-mask lower-limit upper-limit)
    (gimp-file-save RUN-NONINTERACTIVE image drawable filename filename)
    (gimp-image-delete image)))

Copy the script into the $HOME/.config/GIMP/2.10/scripts/ directory, no particular action is required to enable it.

Now it is possibile to execute the GIMP in batch mode, using the -b option. Calling the custom function simple-myerode you can pass the paramteres:

#!/bin/bash
gimp -i -b "(simple-myerode \"IMAGE.JPG\" 0 0 0 0 0 0)" -b '(gimp-quit 0)'

The final recipe works as follow:

• Enlarge the image at 250% with Imagemagick. This proved to be the best choice to enhance the text readability via the erode filter. This is because the erode filter works by adding a single pixel around the dark parts.
• Apply the GIMP erode filter.
• Use Imagemagick to apply the -unsharp filter. Resize to the original size and crop/resize the image to a fixed size, even if the input image is slightly wider or narrower. Force the resolution to 200 dpi ignoring that of the original image.

This is the full script, which receives the filename to be processed and saves the output file into the ./outdir/ directory:

#!/bin/bash
 
BASENAME="$(basename "$1")"
TMP="tmp.$$.png"
 
convert "$1" -scale '250%' "$TMP"
cp -p simple-myerode.scm /home/niccolo/.config/GIMP/2.10/scripts/
gimp -i -b "(simple-myerode \"$TMP\" 0 0 0 0 0 0)" -b '(gimp-quit 0)'
convert "$TMP" -unsharp '0x3+1.0+0' \
    -scale 'x2000' -gravity center -crop '1400:2000' \
    -background white -extent '1400x2000' -density 200 -units PixelsPerInch \
    -quality 90 \
    "./outdir/$BASENAME"
rm "$TMP"