class ImageUploader < CarrierWave::Uploader::Base

  include CarrierWave::MiniMagick
  ...
  version :t do
    process :cropper
    process :resize_to_fill => [75, 75]
  end
  ...
  def cropper
    manipulate! do |img| 
      img = img.crop "#{model.crop_x}x#{model.crop_y}+#{model.crop_w}+#{model.crop_h}"
      img
    end 
  end

end
	
process :cropper

def cropper
  manipulate! do |img|
    if model.crop_x.blank?
      image = MiniMagick::Image.open(current_path)
      model.crop_w = (image[:width] * 0.8).to_i
      model.crop_y = (image[:height] * 0.8).to_i
      model.crop_x = (image[:width] * 0.1).to_i
      model.crop_y = (image[:height] * 0.1).to_i
    end
    img.crop "#{model.crop_w}x#{model.crop_h}+#{model.crop_x}+#{model.crop_y}"
  end 
end