Image-Processing Utilities

The GIMP

Beyond a doubt, the best image-processing program available for Linux is called the GIMP, or GNU Image Manipulation Program. It is released under the GNU General Public License, so source code is available, and it is built to allow easy integration of third-party extensions. It is often compared to Adobe's Photoshop in the breadth of its features. Like Photoshop, the GIMP also has a considerable learning curve, but the time investment is well worth it if you need to process images on a regular basis. The GIMP has been in development for many years now, and plentiful reference guides and tutorials are available on the Web. There is even a site oriented toward artists , rather than techies, who would like to investigate the creative possibilities of this package and other Linux-based image-processing tools.

ImageMagick

For command line image processing, one of the best tools is ImageMagick. It is provided as a shared library and a set of binaries, and supports even more image formats than the GIMP (if that's possible!). The commands can be combined with shell or another scripting language to perform batch processing on large numbers of images.

The convert command can be used to convert between image formats, scale, crop, rotate, or merge images, add borders to an image, adjust brightness and contrast, and perform many other operations. Not surprisingly, you can check the man page to get an idea of all the available options. ImageMagick also includes commands to display images to the screen ( display ), create photo montages ( montage ), and create animations ( animate; can you see a pattern in the command names ?).

The general form of the convert command is:

convert [options] input-file output-file

As a simple example, suppose you have a number of BMP images that were scanned from a set of photos. The sizes of the images vary, and the brightness and contrast vary moderately from photo to photo. The following command line will convert all the photos to approximately 250 x 250 pixels while preserving aspect ratio, adjusting the brightness, and adding a 10-pixel white border:

for file in *.bmp; do

 convert -border 10x10 -bordercolor #FFFFFF -equalize -geometry 250 \

 $file 'echo $file | sed 's/bmp/jpg/'';

done

What's aspect ratio? Simply put, it's the ratio of width to height. If we didn't preserve this value, the image would look squashed horizontally or vertically, and we don't want that.

As usual, a Linux software environment shines at automating a task that could potentially be highly repetitive. Try retouching 50 images using a traditional application; each image might take 10 or 15 mouse clicks. That would definitely not be a well-spent hour , and that estimate assumes you're highly proficient with a mouse! In this situation, a command line solution is far quicker.