JCrop + Carrierwave

gistfile1.rb

# https://github.com/gzigzigzeo/carrierwave-meta
# Integrating CarrierWave with JCrop
# Let implement the behavior like at this demo: deepliquid.com/projects/Jcrop/demos.php?demo=thumbnail
# The uploader:

class CropUploader < SobakaUploader
  include CarrierWave::Meta

  # Crop source is a source image converted from original which could be bigger than source area (left image in the example).
  version :crop_source do
    process :resize_to_fit => [300, 300]
    process :store_meta

    # This is the cropped version of parent image. Let crop to 50x50 square.
    version :crop do
      process :crop_to => [50, 50]
    end
  end

  # Defines crop area dimensions.
  # This should be assigned before #store! and #cache! called and should be saved in the model's instance. 
  # Otherwise cropped image would be lost after #recreate_versions! is called.
  # If crop area dimensions are'nt assigned, uploader calculates crop area dimensions inside the
  # parent image and creates the default image.
  model_delegate_attribute :x
  model_delegate_attribute :y
  model_delegate_attribute :w
  model_delegate_attribute :h

  # Crop processor
  def crop_to(width, height)
    # Checks that crop area is defined and crop should be done.
    if ((crop_args[0] == crop_args[2]) || (crop_args[1] == crop_args[3]))
      # If not creates default image and saves it's dimensions.
      resize_to_fill_and_save_dimensions(width, height)
    else
      args = crop_args + [width, height]
      crop_and_resize(*args)
    end
  end

  def crop_and_resize(x, y, width, height, new_width, new_height)
    manipulate! do |img|
      cropped_img = img.crop(x, y, width, height)
      new_img = cropped_img.resize_to_fill(new_width, new_height)
      destroy_image(cropped_img)
      destroy_image(img)      
      new_img
    end
  end

  # Creates the default crop image.
  # Here the original crop area dimensions are restored and assigned to the model's instance.  
  def resize_to_fill_and_save_dimensions(new_width, new_height)
    manipulate! do |img|
      width, height = img.columns, img.rows
      new_img = img.resize_to_fill(new_width, new_height)
      destroy_image(img)

      w_ratio = width.to_f / new_width.to_f
      h_ratio = height.to_f / new_height.to_f

      ratio = [w_ratio, h_ratio].min

      self.w = ratio * new_width
      self.h = ratio * new_height
      self.x = (width - self.w) / 2
      self.y = (height - self.h) / 2

      new_img
    end
  end

  private
  def crop_args
    %w(x y w h).map { |accessor| send(accessor).to_i }
  end    
end

# Post should have :crop_source_version_x, :crop_source_version_y, :crop_source_version_h, :crop_source_version_w columns
class Post < ActiveRecord::Base
  mount_uploader CropUploader, :image
end

# Let's upload an image
post = Post.new
post.image = params[:image] # Let the uploaded file is 800x600 JPEG
post.save!

post.image.crop_source.width  # 300
post.image.crop_source.height # 200  
post.image.crop_source.crop.width  # 50
post.image.crop_source.crop.height # 50

# Default crop area coordinates within the limits of big image dimensions: square at the center of an image
post.image.crop_source.crop.x # 50
post.image.crop_source.crop.y # 50
post.image.crop_source.crop.w # 200 
post.image.crop_source.crop.h # 200

# Let user change the crop area with JCrop script. Pass new crop area parameters to the model.
post.crop_source_crop_x = 100
post.crop_source_crop_y = 100
post.crop_source_crop_w = 100
post.crop_source_crop_h = 100

post.save! # Crop image is reprocessed

post.image.crop_source.crop.width  # 50
post.image.crop_source.crop.height # 50

CarrierWave::MiniMagick shouldn't be included in this example?