Using an Image Viewer as a Presentation Program,
Image Correction in Batch Mode (Using The GIMP)
November 27, 2008,
Lisp
Last edited on November 28, 2008
As already mentioned in the German article Public
Speech: History of Typesetting and Letterpress Printing I have given
that speech without any slides. Instead I've made use of a simple
image viewer and a LCD projector to present the eighty
pictures. In this article I describe the preparations.
Color Correction of Scanned Images
Eighty pictures, and most of them I had to scan by myself from the various
books that I have used. That means that the colors had to be
altered a tad, at least we need to correct the input level of the
colors (e.g. using The GIMP's
Adjust Color Levels tool). We want well balanced images, the levels
should be distributed all over the whole color range, and it'd be best
if the text of the reverse side could not be seen through (remember that
often book pages are quite thin).
If your picture is just composed of
pen strokes (for example pencil drawings or woodcuts) it's a nice
trick to propagate the lower value pixels before using the Levels tool
(Filters → Distorts → Value Propagate, More Black),
so that afterwards you can apply the Levels tool more harshly.
This method is described quite nicely in Liam Quin's How to Clean Up Scanned Engravings and Old Photographs,
(second) step 8. Strengthen Those Lines. He aptly writes:
"The point of this is that when you make the image brighter the edges
of the dark lines get eaten away, so you need to strengthen them
first."
Automate the Color Correction
We certainly do not want to do this for some dozen images! Luckily
The GIMP has a nice batch mode to automate this process. At the
same time it is (in my opinion) a bit easier to write a batch script
instead of a filter script that is embedded into The GIMP with a
special dialog box (though that is not that hard, either).
How this is done is described in the compact tutorial GIMP Batch
Mode. Thank you, anonymous writer, for having written that nice page! Remember
to consult The GIMP's helpful Procedure Browser,
(GIMP toolbox → Xtns → Procedure Browser) when
writing your own scripts. It gives you a list of all commands and
describes all their parameters in detail (although, sadly, it describes
not everything you need to know…). Here is my script:
(define (batch-vpropagate-levels pattern)
(let∗ ((filelist (cadr (file-glob pattern 1))))
(while (not (null? filelist))
(let∗ ((filename (car filelist))
(image (car (gimp-file-load RUN-NONINTERACTIVE
filename filename)))
(drawable (car (gimp-image-get-active-layer image))))
(plug-in-vpropagate RUN-NONINTERACTIVE
image drawable
1 ;; more black
1 ;; channel
0.18 ;; rate
15 ;; direction mask
0 ;; lower-limit
255 ;; upper-limit
)
(gimp-levels drawable
0 ;; value channel
125 242
1.5
0 255)
(gimp-file-save RUN-NONINTERACTIVE
image drawable filename filename)
(gimp-image-delete image))
(set! filelist (cdr filelist)))))
(define (batch-levels pattern)
(let∗ ((filelist (cadr (file-glob pattern 1))))
(while (not (null? filelist))
(let∗ ((filename (car filelist))
(image (car (gimp-file-load RUN-NONINTERACTIVE
filename filename)))
(drawable (car (gimp-image-get-active-layer image))))
(gimp-levels drawable
0 ;; value channel
75 245
1.0
0 255)
(gimp-file-save RUN-NONINTERACTIVE
image drawable filename filename)
(gimp-image-delete image))
(set! filelist (cdr filelist)))))
Save it to a file named adjust-scan.scm in the directory
~/.gimp-2.x/scripts/.
The first function is an example script for line drawings; the second
one is the function that I've used for all scanned images of said speech.
I wanted a quick'n'dirty solution, so I've just determined reasonable
values using one or two images. Of course, we have to scan all images
with the same scanning settings! Then I have copied all original
images to a new directory—as the functions change the images
in place—, and fired up:
gimp -i -b '(batch-levels "∗.png")' -b '(gimp-quit 0)'
Sorting the Images
Now I've put all images—the scanned and post-processed ones as
well as selected images from the Web—in a new directory called
Selection. I've generated a list of all image files using the
command:
ls -1 > files
Open it and sort the files as you need them during your
speech. Prepend whatever you want to lines of files that you do not
need after all and insert blank lines as you want. Create a
subdirectory Sorted and call this BASH script:
IFS=$'\n';
num=1;
for i in $(<files);do
# echo $i | grep "__handiwork\."
handifile=${i:0:${#i}-4}__handiwork${i: -4}
ls $handifile >/dev/null 2>&1
if [ $? -eq 0 ] ; then
i=$handifile
fi
newname=$(printf "%03d" $num)__$i;
echo $newname;
cp -l $i Sorted/$newname ;
let num=$num+1;
done
It generates numbered hard links in the subdirectory
to all files that you have listed in the file files.
If you need to manually adjust an image append "__handiwork"
to its file name just before the (three letter) extension; it will be
selected instead of the original file.
A nice thing about this solution is the fact that it is independent
of the particular image viewer program, as long as it can show images
in the alphabetical order of their file names. If your notebook has a
problem just before your speech, it is more easy to use another
person's computer: even Microsoft Windows's built-in picture preview,
the Picture and Fax Viewer, is enough.
Please send me an email if you liked this article!
You even do not need to mention the colour of your socks and/or
include a picture of your ankles, as Liam Quin
asks you to do! :-)
