Carrierwave PDF to Image using MiniMagick

My requirement is to convert pdf to images if a pdf file is being uploaded. So far This is what I have done .

class ImageUploader < CarrierWave::Uploader::Base

  # Include RMagick or MiniMagick support:
  # include CarrierWave::RMagick
  include CarrierWave::MiniMagick
  include CarrierWave::MimeTypes

  # Choose what kind of storage to use for this uploader:
  storage :file
  # storage :fog

  # Override the directory where uploaded files will be stored.
  # This is a sensible default for uploaders that are meant to be mounted:
  def store_dir
    "#{PRIVATE_UPLOADS_PATH}/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  # Provide a default URL as a default if there hasn't been a file uploaded:
  # def default_url
  #   # For Rails 3.1+ asset pipeline compatibility:
  #   # ActionController::Base.helpers.asset_path("fallback/" + [version_name, "default.png"].compact.join('_'))
  #
  #   "/images/fallback/" + [version_name, "default.png"].compact.join('_')
  # end

  process :set_content_type
  process :set_model_ext_attributes

  # Create different versions of your uploaded files:
  version :large, if: :image? do
    process :resize_and_pad => [800, 600]
  end
  version :thumb, if: :image? do
    process :resize_and_pad => [100, 100]
  end

  version :normal, if: :pdf? do
    process :efficient_conversion => [640, 960]
  end

  def efficient_conversion(width, height)
    manipulate! do |img|
      img.format("png") do |c|
        c.fuzz        "3%"
        c.trim
        c.resize      "#{width}x#{height}>"
        c.resize      "#{width}x#{height}<"
      end
      img
    end
  end


  # Add a white list of extensions which are allowed to be uploaded.
  # For images you might use something like this:
  def extension_white_list
     %w(jpg jpeg gif png pdf)
  end

  def set_model_ext_attributes
    model.display_filename ||= file.filename
    model.content_type_cd = CONTENT_TYPES.rassoc(file.content_type).first if file.content_type
    model.file_size = file.size
  end

  def content_type
    CONTENT_TYPES.assoc(model.content_type_cd).last
  end

  # Override the filename of the uploaded files:
  # Avoid using model.id or version_name here, see uploader/store.rb for details.
  # def filename
  #   "something.jpg" if original_filename
  # end

  protected
    def image?(new_file)
      new_file.content_type.start_with? 'image/'
    end

    def pdf?(new_file)
      new_file.content_type.end_with? '/pdf'
    end

end

But I keep getting an error saying

MiniMagick::Error: Unable to format to png
from /Users/rkamat/.rvm/gems/ruby-1.9.3-p547/gems/mini_magick-3.7.0/lib/mini_magick/image.rb:266:in `format'

at this below line

 img.format("png") do |c|

Any help around this would be appreciated as I have spends hours trying to debus this.

I have veen referring the documentation in the carrierwave upload to get this working http://ift.tt/1N3lq9z