controller_for_image_model.rb

class ImageModelsController < ApplicationController
  before_filter :find_image_model, :only => [:update]

  def update
    if @image_model.update_attributes(params[:image_model])
      @image_model.image.reprocess!
    end
  end

protected
  def find_image_model
    @image_model = ImageModel.find params[:id]
  end
end

cropper_setup.js

$(function() {
  function setCropMarks(c) {
    $('#crop_x').val(c.x);
    $('#crop_y').val(c.y);
    $('#crop_w').val(c.w);
    $('#crop_h').val(c.h);
  }
  $.globalEval('var cropSettings = '+$('#crop_settings').val())
  $('.crop img').Jcrop({
    setSelect: cropSettings,
    onChange: setCropMarks,
    onSelect: setCropMarks
  });
});

edit.html.erb

<% form_for @image_model do |f| %>
  <div class='crop'><%= image_tag(@image_model.image.url) %></div>
  <%= f.hidden_field :crop_x, :id => 'crop_x' %>
  <%= f.hidden_field :crop_y, :id => 'crop_y' %>
  <%= f.hidden_field :crop_w, :id => 'crop_w' %>
  <%= f.hidden_field :crop_h, :id => 'crop_h' %>
  <%= hidden_field_tag :crop_settings, @image_model.crop_settings.to_json %>
<% end %>

jcropper.rb

# Jcropper paperclip processor
#
# This processor very slightly changes the default thumbnail processor in order to work properly with Jcrop
# the jQuery cropper plugin.

module Paperclip
  # Handles thumbnailing images that are uploaded.
  class Jcropper < Processor

    attr_accessor :current_geometry, :target_geometry, :format, :whiny, :convert_options

    # Creates a Thumbnail object set to work on the +file+ given. It
    # will attempt to transform the image into one defined by +target_geometry+
    # which is a "WxH"-style string. +format+ will be inferred from the +file+
    # unless specified. Thumbnail creation will raise no errors unless
    # +whiny+ is true (which it is, by default. If +convert_options+ is
    # set, the options will be appended to the convert command upon image conversion 
    def initialize file, options = {}
      super
      geometry          = options[:geometry]
      @file             = file
      @crop             = geometry[-1,1] == '#'
      @target_geometry  = Geometry.parse geometry
      @current_geometry = Geometry.from_file @file
      @convert_options  = options[:convert_options]
      @whiny            = options[:whiny].nil? ? true : options[:whiny]
      @format           = options[:format]

      @current_format   = File.extname(@file.path)
      @basename         = File.basename(@file.path, @current_format)
    end

    # Returns true if the +target_geometry+ is meant to crop.
    def crop?
      @crop
    end
    
    # Returns true if the image is meant to make use of additional convert options.
    def convert_options?
      not @convert_options.blank?
    end

    # Performs the conversion of the +file+ into a thumbnail. Returns the Tempfile
    # that contains the new image.
    def make
      src = @file
      dst = Tempfile.new([@basename, @format].compact.join("."))
      dst.binmode

      command = <<-end_command
        "#{ File.expand_path(src.path) }[0]"
        #{ transformation_command }
        "#{ File.expand_path(dst.path) }"
      end_command

      begin
        success = Paperclip.run("convert", command.gsub(/\s+/, " "))
      rescue PaperclipCommandLineError
        raise PaperclipError, "There was an error processing the thumbnail for #{@basename}" if @whiny
      end

      dst
    end

    # Returns the command ImageMagick's +convert+ needs to transform the image
    # into the thumbnail.
    def transformation_command
      scale, crop = @current_geometry.transformation_to(@target_geometry, crop?)
      trans = ''
      trans << " #{convert_options}" if convert_options?
      trans << " -resize \"#{scale}\""
      trans << " -crop \"#{crop}\" +repage" if crop
      trans
    end
  end
end

model_with_attached_image.rb

# Example usage of Jcropper

# == Schema Information
# Table name: image_models
#
#  id                 :integer(4)      not null, primary key
#  image_file_name    :string(255)
#  image_content_type :string(255)
#  image_file_size    :integer(4)
#  image_updated_at   :datetime
#  crop_x             :integer(4)      default(0)
#  crop_y             :integer(4)      default(0)
#  crop_h             :integer(4)      default(0)
#  crop_w             :integer(4)      default(0)
#

class ImageModel < ActiveRecord::Base
  has_attached_file :image,
    :styles => {
      :resized => '120x41>',
      :cropped => '100%x100%'
    },
    :processors => [:jcropper],
    :convert_options => { :all =>  proc { |m| "-crop #{m.crop_w}x#{m.crop_h}+#{m.crop_x}+#{m.crop_y}" } }

  def crop_settings
    [crop_x, crop_y, (crop_x+crop_w), (crop_y+crop_h)]
  end
end