RMagick mask Paperclip image attachment

Question

I am using Paperclip to attach image files to an object and all is well but I'd also like to mask (preferably only one of the image style – main_feature) the image upon upload. I'm able to save my masked image using masked_image.write('result.png') but can't seem to get it to update the image attribute for Paperclip. With the following code I get this error:

No handler found for /var/folders/q1/xf59whv514lgw_xr10hb208m0000gn/T/gaither-nomask20130312-80235-yaj1s7.png PNG 1020x470 1020x470+0+0 DirectClass 8-bit

has_attached_file :image,
  :styles =>{
  :main_feature => "1020x470",
  :top_feature => "345x159",
  :smallest => "229x131"
},
:storage => :s3,
:s3_credentials => "#{Rails.root}/config/s3.yml",
:path => ":attachment/:id/:style.:extension",
:url => "/:id/:style/:basename.:extension",
:bucket => "bucketname"

after_image_post_process :post_process_image

def post_process_image
  require 'rubygems'
  require 'RMagick'
  require "open-uri"


  mask = Magick::ImageList.new  
  urlimage = open("http://bucket.s3.amazonaws.com/mask.png")
  mask.from_blob(urlimage.read)

  imgfile = Magick::Image.read(image.queued_for_write[:original].path).first

  mask.matte = true
  imgfile.matte = true


  masked_image = imgfile.composite(mask, Magick::CenterGravity, Magick::CopyOpacityCompositeOp)
  self.update_attribute(:image, masked_image)

end

Answer 1

After a little trial and error, I ended up writing a Paperclip processor:

model:

:styles =>{
   :main_feature => {:geometry => "1020x470", :processors => [:masker] },
   :top_feature => "345x159",
   :smallest => "229x131"
}

/lib/paperclip_processors/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  

  src = @file
  dst = Tempfile.new([@basename, @format])
  dst.binmode

  begin
    parameters = []

    parameters << ':source'
    parameters << ':mask'
    parameters << '-alpha'
    parameters << 'on'
    parameters << '-compose'
    parameters << 'CopyOpacity'
    parameters << '-composite'
    parameters << ':dest'

    parameters = parameters.flatten.compact.join(" ").strip.squeeze(" ")

    mask_path = File.expand_path('lib/paperclip_processors/mask.png')
    success = Paperclip.run("convert", parameters, :source => "#{File.expand_path(src.path)}[0]", :mask => "#{mask_path}[0]", :dest => File.expand_path(dst.path))



     rescue PaperclipCommandLineError => e
       raise PaperclipError, "There was an error during the mask for #{@basename}" if @whiny
     end

     dst
   end

  end
 end