When you want to manipulate images with Ruby (or your Rails application) you'll probably want RMagick installed. This is no easy feat on Mac OS X.
The official guide suggests installing X11 and using darwinports to install everything. This guide shows you how to easily install RMagick on you Mac OS X system. In this case I use Mac OS X 10.4.9.
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Before you jump in, make sure you have <ahref="http://developer.apple.com/tools/xcode/">Xcode</a> installed. You can get it for free from Apple.
I'll also assume you have Ruby and rubygems installed and working already.
You will need to download, compile and install several graphics libraries that RMagick needs. Let's do this now.