Even though gd, gd-devel and the ruby GD and CAPTCHA gems were installed on my Red Hat Enterprise Linux 4 server, Ruby’s CAPTCHA gem seemed to blow-up with the error:

undefined method `StringFT'

Hmmm. I googled around and found that you need a library to bind the GD calls to ruby. This makes perfect sense, but documentation for the issue is scarce.

Eventually, I found the following rpm and installed it.

wget http://ftp.at.gnucash.org/opsys/linux/redhat.com/contrib/libc5/i686/ruby-GD-1.0_97111
rpm -ivh ruby-GD-0.7.4-0vl1.i386.rpm

Unfortunately, the RPM assumed that ruby would be installed under /local, but on RHEL everything is under /usr. So, I found where the RPM stored the library and relocated it to the proper directory.

$ rpm -qpl ruby-GD-0.7.4-0vl1.i386.rpm
/local/lib/site_ruby/1.8/i386-linux/GD.so
sudo mv /local/lib/site_ruby/1.8/i386-linux/GD.so /usr/lib/site_ruby/1.8/i386-linux/

Lo and behold, Ruby’s CAPTCHA gem worked like the clappers after this was sorted. Unfortunately, it took me over an hour to sort out the problem. It would seem that something as mundane as CAPTCHA would have just worked under Rails. The major problem seems to be that the ruby-gd maintainer’s site http://tam.0xfa.com is out of commission.

Under Mac OS X, it’s a bit easier because you can use Darwin Ports to install rb-gd. However, that install appeared to bomb for me because I’m using the latest ruby compiled via the Hivelogic instructions. I attempted to symbolically link the ruby binaries to /opt/local/bin, but that didn’t resolve the issue. I’m still looking for a workaround to this.