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This is a sample application that shows how to implement image cropping using the popular Paperclip Rails plugin and the Jcrop jQuery plugin for selecting the cropping area.

Paperclip project page: Jcrop home page:

Most of the inspiration for this application was taken from this thread: and especially this helpful piece of code:

Downloading and testing the application

git clone git://
cd rjcrop
rake db:migrate
git submodule init
git submodule update

After that you can point your browser to localhost:3000, upload an image and see how easy is to create a cropped image that can be used as an avatar.

How does it work?

The tricky part creating this application was to find an simple way of interacting with Paperclip in order to create the cropped images after the model was saved. In fact it was necessary to reprocess the attachments after they were initially loaded by the plugin.

The model

class User < ActiveRecord::Base

  has_attached_file :avatar,
        :styles => { :normal => "240x240>",
                     :small = > "55x55#" },
        :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}"

  # helper method used by the cropper view to get the real image geometry
  def avatar_geometry(style = :original)
    @geometry ||= {}
    @geometry[style] ||= Paperclip::Geometry.from_file avatar.path(style)


The model includes the has_attached_file as usual but it specifies a custom processor jcropper that is slightly different from the original thumbnail.rb processor provided by Paperclip.

The process of creating the avatar works like this:

  • The user uploads an image and Paperclip creates the :normal and the :small version and because none of the :crop_x, :crop_y, :crop_w, :crop_h are defined crop_str returns an empty string. This way the jcropper processor works just like the original thumbnail processor.

  • Then the user is presented with the normal thumbnail that is used to choose the cropping rectangle.

  • Finally the Paperclip reprocess! method is invoked in the controller:

The controller:

# POST /users/1/crop
def crop
  @user = User.find(params[:id])

  if @user.update_attributes(params[:user])
  • Because now the cropping rectangle parameters are provided by the view, then the crop_str specifies how to crop the original image. The string looks like: “-crop 500x500+125+450” that in the ImageMagick convert command means “crop the original image at offset 125,450 with a size of 500x500”. Finally the images are resized using :normal => “240x240>” and :small = > “55x55” parameters.

The lib/papercli_processors/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 < Thumbnail

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

    def crop_string

    def crop_string?
      not crop_string.blank?



The jcropper processor inherits from the original Thumbnail processor but redefines the transformation_command in order to get the crop_string from the model if it is defined. Otherwise it works just like the original thumbnail processor.

The Jcrop and jQuery magic

All the magic happens in cropping.html.erb view and it should be easy to understand by reading the Jcrop documentation. One important thing to remark is how the ratio between the original image and the normal thumbnail is computed using the avatar_geometry helper method from the model:

function showPreview(coords)
  var ratio = <%= @user.avatar_geometry(:original).width %> / <%= @user.avatar_geometry(:normal).width %>; 


Juan Schwindt juan(at)