Since I migrated away from Windows on my development machine a couple of months back, I’ve not looked back. One of the main benefits I’ve found is being able to build software packages locally, and Homebrew makes that easy. Now I can easily grab the latest version of various dependencies such as MySQL and ImageMagick and install them in seconds, just as if I was using yum or apt-get.
However, one problem has plagued me since I started using the Homebrew-installed version of ImageMagick in conjunction with my local Alfresco installs, with errors such as this in the logs

 2012-08-02 10:20:10,547  DEBUG [transform.magick.AbstractImageMagickContentTransformerWorker] [main] org.alfresco.service.cmr.repository.ContentIOException: 07020000 Failed to perform ImageMagick transformation:
Execution result:
   os:         Mac OS X
   command:    [/usr/local/bin/convert, /Users/wabson/Development/projects/share-extras-2/software/tomcat/temp/Alfresco/ImageMagickContentTransformerWorker_init_source_4067880520213204197.gif[0], /Users/wabson/Development/projects/share-extras-2/software/tomcat/temp/Alfresco/ImageMagickContentTransformerWorker_init_target_7684811633126657147.png]
   succeeded:  true
   exit code:  133
   out:        
   err:        dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ApplicationServices.framework/Versions/A/Frameworks/ImageIO.framework/Versions/A/ImageIO
  Expected in: /usr/local/lib/libjpeg.8.dylib
 in /System/Libra

This frustrated me for a while. It happened every time Alfresco tried to call convert in order to effect an image transform – for instance generating doclib thumbnails – but running the exact same command at a bash prompt worked fine.
Various internet posts all pointed to the environment in which ImageMagick is run as being the cause for such problems. It seemed the environment variable DYLD_LIBRARY_PATH in particular was known to cause problems on OS X, but checking /etc/profile, /etc/bashrc and the custom alfresco.sh script I use to start up Alfresco (included in my Tomcat packages) did not yield any trace of such a variable.
Finally I found a comment in ALF-13452, which led me to check the Spring config within the thirdparty subsystem, responsible for starting up ImageMagick.

   <bean id="transformer.worker.ImageMagick">
      <property name="mimetypeService">
         <ref bean="mimetypeService" />
      </property>
      <property name="executer">
         <bean name="transformer.ImageMagick.Command">
            <property name="commandsAndArguments">
               <map>
                  <entry key=".*">
                     <list>
                        <value>${img.exe}</value>
                        <value>${source}</value>
                        <value>SPLIT:${options}</value>
                        <value>${target}</value>
                     </list>
                  </entry>
               </map>
            </property>
            <property name="processProperties">
               <map>
                  <entry key="MAGICK_HOME">
                     <value>${img.root}</value>
                  </entry>
                  <entry key="DYLD_LIBRARY_PATH">
                     <value>${img.dyn}</value>
                  </entry>
                  <entry key="LD_LIBRARY_PATH">
                     <value>${img.dyn}</value>
                  </entry>
               </map>
            </property>
            <property name="defaultProperties">
               <props>
                  <prop key="options"></prop>
               </props>
            </property>
         </bean>
      </property>
     ...

There it was. The subsystem itself was setting up a few environment variables before firing up the external process, and these were defined in processProperties. One was the DYLD_LIBRARY_PATH variable.
The solution was trivial. I copied the Spring config in tomcat/webapps/alfresco/WEB-INF/classes/alfresco/subsystems/thirdparty/default/imagemagick-transform-context.xml into tomcat/shared/classes/alfresco/extension/subsystems/thirdparty/default/default/imagemagick-transform-context.xml as per the subsystems docs on overriding beans, and commented out the offending <entry> element.

mkdir -p tomcat/shared/classes/alfresco/extension/subsystems/thirdparty/default/default/
cp tomcat/webapps/alfresco/WEB-INF/classes/alfresco/subsystems/thirdparty/default/imagemagick-transform-context.xml tomcat/shared/classes/alfresco/extension/subsystems/thirdparty/default/default/
vim tomcat/shared/classes/alfresco/extension/subsystems/thirdparty/default/default/imagemagick-transform-context.xml

The result – image conversions now work perfectly in my local dev environment using the Homebrew-installed version of ImageMagick.