Since long, we wished to write about scripting for image creation and manipulation. There are many reason why you would spend some time to do it. To resize a lot of images by hand can be a tedious task, or your software misses a component to achieve a particular result. Or you want to turn a web application into an image editor, etc.

We will start with a modest example taken from a real life situation. We, Femke and Nicolas, are working on an illustration in Inkscape. For this illustration, we have scanned a lot of notes we have written on paper. The scanned images have been saved in tiff. We have imported them in Inkscape and started making the composition. Half-way we realise that it should be a lot more easier to work with the same images but saved as PNG with a transparent background. As there is 165 images to transform, to do it one by one in Gimp sounds just frightening. This is where the wonderful Imagemagick software enters into play.

Imagemagick is shipped with every major linux distribution or can easily be installed by the different package managers. It is also available on windows; and on MacosX via the Fink installer. Once there, Imagemagick gives you many tools to edit, resize, transform images. One of them is convert that takes a file in input and converts it into (nearly) any format. In our case, a simple conversion was not enough since we wanted also to transform the white colour into a transparent background. The following command did the trick for one image:

convert myfile.tiff -transparent white myfile.png

To apply it to a whole directory of images and keep the filenames, we had to include it in a small shell script:

#!/bin/sh
for file in `ls | grep tiff`
do
  convert "$file" -fuzz 5% -transparent white "${file}.png"
  echo "writing ${file}.png"
done

The fuzz parameter makes it possible to give transparency to ‘nearly-white’ pixels.