foo.rb

class Foo < ActiveRecord::Base
 has_attached_file :photo,
  :styles => {
    :icon => "32x32#",
    :thumb => "160x130", 
    :area => "590x480" },
    :processors => [:jcropper]

    attr_accessor :crop_x, :crop_y, :crop_w, :crop_h

    def crop_str
      if !crop_x.blank? && !crop_y.blank? && !crop_w.blank? && !crop_h.blank?
        "-crop #{crop_w}x#{crop_h}+#{crop_x}+#{crop_y}"
      else
        ""
      end
    end
end

jcropper.rb

module Paperclip
    class Jcropper < Thumbnail

      def transformation_command
        scale, crop = @current_geometry.transformation_to(@target_geometry, crop?)
        trans = ''
        if crop_string?
          trans << " #{crop_string}"
          trans << " -resize \"#{scale}\""
        else
          trans << " -resize \"#{scale}\""
          trans << " -crop \"#{crop}\" +repage" if crop
        end
        trans
      end

      def crop_string
        @attachment.instance.crop_str
      end

      def crop_string?
        not crop_string.blank?
      end
    end
  end