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====== Batch processing of images with GIMP ======

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.

===== The plug-in-erode filter =====

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.

===== Creating a script to call plug-in-erode =====

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**:

<file>
(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)))
</file>

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

===== Call the script from the command line =====

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:

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

===== Combining Imagemagick and GIMP =====

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:

<code bash>
#!/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"
</code>