This is the third article that IBM DeveloperWorks has published on the ImageMagick suite of command line tools. This article aims to wrap up the loose ends left by the last two articles, and assumes that you have read those articles. It also discusses functionality which builds on that discussed in those articles. I recommend that if you haven't read them yet, now is a good time to check them out. TODOURLSTOOLDARTICLES

One of the cool things you can do with ImageMagick is build montages of images. This has come in handy for the writing of both this article and the previous one in this series. I also find it useful to generate "contact sheets" for images from my digital cameras for instance.

You create montages with ImageMagick using the montage command. Here's a quick example invocation, as well as what it produces:

montage waterfall-* montage.jpg

Produces the picture:

Figure TODO. The default montage output

You can exhert more control over the montage than this however. If you use the -geometry parameter, then you can specify the maximum size of the individual images in the montage, as well as the spacing between the images.

montage -geometry 80x80+5+5 waterfall-* montage.jpg

Which specifies that each image should be no more than 80 by 80 pixels, and have a horizontal and vertical spacing of 5 pixels. It gives us:

Figure TODO. Geometrized montage output

You can also specify the number of images that appear in a given row. For example:

montage -geometry 80x80+5+5 -tile 2x3 waterfall-* montage.jpg

Which specifies that there should be three rows of two images each.

Figure TODO. Tiled montage output

There are many other montage options available, including changing the background color and drawing frames around the images. I recommend that you checkout the montage man page for more information.

As mentioned in TODOTITLE, ImageMagick easily copes with file formats which can store more than one image. When you think about it, one of the most interesting uses of such a format is animations. For example, animated GIFs are just a series of GIF images which are displayed at 25 frames per second or so.

ImageMagick does quite well at handling these multiple image formats, including the animated ones. For example, consider a scenario in which we have several images which are the frames of an animation. These might have been produced in your favourite graphics editor, or generated in some other way which is outside the scope of this article. For these various animation examples, we'll use some simple TIFF images which I produced programmatically.

My program produced a bunch of images named output-<number>.tif, which are just a simple line moving around a rectangle. Here's a couple of snapshots that I put into a single image file using the montage command as described above.

Figure TODO. A few frames from the animation

How do we convert these many image files into an animated single image? Well, if the destination file for the convert command is one which supports multiple images in one file, then all will be well.

Here's an example:

convert output-* ani.gif

Which gives us this silly little animation:

Figure TODO. Our animation

Using the -combine command line arguement to convert will have the same effect as the example above, while being a little bit more explicit. Using the -loop command line arguement will add the Netscape looping extensions to your GIF animations, which will be handy for creating those annoying never ending advertisements for your website.

Now that we've got this image file which is made up of lots of frames, let's now explore how to extract frames from the file. convert lets you sepcify a sub image with the following syntax:

convert ani.gif[42] frame42.gif

You might need to use quotes around the input image and frame number specification to stop your shell from trying to interpret the frame specification as some sort of shell construct. This will vary based on the shell that you're using at the time.

Being able to extract sub images is actually a really useful feature. For example, it makes it really easy to extract single frames from all those home movies that you have floating around, assuming that you have the right delegate installed for the image format that you're trying to extract the frames from.