How do I invoke liquid-rescale with the Paperclip gem? - ruby-on-rails-3

Question

I'm using the Paperclip gem with a Rails 3.1.1 app. It's working as advertised and expected. I would like to use the imagemagick -liquid-rescale delegate, however. According to the imagemagick documentation (which I tried in terminal.app), this works:
convert logo_trimmed.jpg -liquid-rescale 75x100%\! logo_lqr.jpg
I tried a variation...
convert my_pic.jpg -liquid-rescale 60x60\! my_new_pic.jpg
That worked as expected, too. I've tried several permutations in my Image model in my rails app, but I cannot get Paperclip to invoke liquid-rescale. My latest attempt was:
has_attached_file :pic, :styles => {:square => "-liquid-rescale 60x60\!" }
This fails without an error message, merely duplicating the original image with a new name.
How do I instruct paperclip to invoke liquid-rescale?

Answer 1

Here's what I did. I had to write a custom processor, which I named liquid.In the model:
has_attached_file :pic, :styles => {:square => {:processors =>[:liquid],:geometry => "60x60>"} }
I'm not sure whether the :geometry option is necessary, but I added it because thumbnail.rb in the paperclip gem says that it's not optional.
I then added a file: /my_app/lib/paperclip_processors/liquid.rb with contents:
module Paperclip
class Liquid < Thumbnail
def transformation_command
"-resize '60x60>' -liquid-rescale '60x60!'"
end
end
end
Finally, I ran the following in the console:
Image.all.each {|i| i.pic.reprocess!}
That did the trick.