Masker: Custom image processor for Paperclip (requires masking image)

masker.rb

module Paperclip
  class Masker < Processor
    def initialize file, options = {}, attachment = nil
      super
      @format = File.extname(@file.path)
      @basename = File.basename(@file.path, @format)
    end

    def make
      source      = @file
      destination = Tempfile.new([@basename, '.png'])
      destination.binmode

      parameters = [
        ':source', ':mask',
        '-alpha', 'off',
        '-gravity', 'center',
        '-compose', 'CopyOpacity',
        '-composite', '-trim',
        ':dest'
      ].join(" ")

      begin
        mask_path = File.expand_path('lib/paperclip_processors/mask.gif')
        Paperclip.run("convert", parameters, :source => File.expand_path(source.path), :mask => mask_path, :dest => File.expand_path(destination.path))
      rescue PaperclipCommandLineError
        raise PaperclipError, "There was an error during the mask for #{@basename}" if @whiny
      end

      destination
    end
  end
end