Painting with Constable

ImageMagick annoys me. It annoys me mainly because it’s a pain in the ass to install. As functionality goes, ImageMagick is pretty much the bees knees, but like many Ruby developers, I tend to develop on a Mac, an operating without much of an official package manager. Installing tools that I use every day, especially tools that have complex requirements like ImageMagick, gets to be a bit annoying. I long for the days when I can apt-get install imagemagick but still use all of the the rather lovely hardware that a Mac provides.

Over the past several years I’d had some reasonable experience with virtualisation, and recently Vagrant has brought virtualisation to OS X in a way that made it instantly, easily available. Suddenly it’s trivial to run Ubuntu on my Mac, and with that I get easy access to apt and the packages for ImageMagick. Unfortuantely I can’t use them natively on my Mac because they’re only accessible from inside the VM. Better than nothing though, and so for image manipulation at the command line I’ve been surviving.

During my more recent three or four years of work I’ve tended to work a fair amount with messaging, making services available on the message bus for use by remote clients. It can be really useful not having to worry about the implementation of a service, knowing just that a message in a certain format sent to a certain destination will cause the work you’ve requested to be done, so I tried to provide exactly that for ImageMagick by exposing ImageMagick on my VM as a sevice on the bus. It seems to work fairly well, so I’ve thrown up a project on GitHub and released a RubyGem. The project is called Constable, the README explains why.

Constable is very nearly a drop-in relacement for ImageMagick. After you’ve installed the gem and setup the service you can use the same ImageMagick commands to do a lot of the same stuff that a local install of ImageMagick would let you do, but of course there are come caveats. Output must, at the moment at least, be streamed to STDOUT. ImageMagick supports this by allowing your output filename to the the form of format:- eg jpg:- or png:-. While there are probably other shortcomings, I’ve not run into them, possibly because while I use ImageMagic a lot I don’t use it in a particularly complex manner. If you do come across any problems let me know. If you can submit a patch to fix any problems you encounter I’d be a happy boy.

Setting up an example service is covered in the “Up and runing fast” section of the README so I’ll skip that and run through a very quick operation to show creating a few JPEGs with text in them, pasting one on top of the other, and creating a PNG that’s 50% smaller in dimension that the original images.

First, of course, make sure the service is up as described in the README.

Second, and a little bit of an undocumented easter egg at the moment, install the binstubs for the ImageMagick services.

sudo constable-install

Note that this will overwrite the following files if they exist:

/usr/bin/identify
/usr/bin/convert
/usr/bin/compare
/usr/bin/composite

This step is optional, but does provide the same commands that ImageMagick uses. If you prefer not to perform this step you can simply prefix each command with constable- and immediately after each command add a double dash eg. convert foo.jpg png:- becomes constable-convert -- foo.jpg png:-

Now on to actually using the sevice. Creating text based images is pretty easy using the convert command; here I create two JPEGs, one in blue tones with the text “Anthony” which I stream to anthony.jpg, and ther other in pink tones with the text “Cleopatra”, which I stream to cleopatra.jpg:

convert -background lightblue -fill blue -font Candice -pointsize 72 \
  label:Anthony   jpg:- > anthony.jpg
convert -background pink      -fill red  -font Candice -pointsize 72 \
  label:Cleopatra jpg:- > cleopatra.jpg

They don’t look great, but they’re good enough for a simple demonstration:

To combine them we can use the convert command in a different invocation:

convert anthony.jpg cleopatra.jpg +append jpg:- \
  > anthony_and_cleopatra.jpg

Resulting in this magnificent creation:

Finally we can convert the image to a PNG at 50% of the size of the input image, again using convert:

convert anthony_and_cleopatra.jpg -resize 50% png:- \
  > anthony_and_cleopatra.png

Which outputs the smaller PNG:

This demonstration will work both with the service running in the VM and a local ImageMagick install. I’m rather happy with that, but there’s no reason to stop there. Why not expose this as a real service somewhere and write a plugin for AttachmentFu or Paperclip that uses this service to properly offline image processing? Get that crazy stuff out of the request response cycle!

Anyway. The code is out there, it’s pretty badly written but it works and it’s available for you to use. Let me know if you find it useful, and if you’d like to do something that it can’t yet do, please do submit patches or give me a yell.

written by

Craig

published

2011-10-05