optimize outfit image generation - 4x speed boost on my box

Use the ImageMagick flatten command to generate the output all at
once instead of compositing each layer individually, and download
the layers in parallel. On my box, saving roopal27 five times took
a total of 30 seconds before, whereas now it takes 7 seconds. I
expect it to be even better on the production box, where latency
is even lower.

Gemfile

@@ -46,6 +46,8 @@ gem "mini_magick", "~> 3.4"
 gem "fog", "~> 1.1.2"
 gem "carrierwave", "~> 0.5.8"
 
+gem "parallel", "~> 0.5.17"
+
 group :development_async do
   # async wrappers
   gem 'eventmachine',     :git => 'git://github.com/eventmachine/eventmachine.git'

Gemfile.lock

@@ -149,6 +149,7 @@ GEM
     open4 (1.3.0)
     openneo-auth-signatory (0.1.0)
       ruby-hmac
+    parallel (0.5.17)
     polyglot (0.3.3)
     rack (1.2.5)
     rack-fiber_pool (0.9.2)
@@ -256,6 +257,7 @@ DEPENDENCIES
   newrelic_rpm
   nokogiri (~> 1.5.2)
   openneo-auth-signatory (~> 0.1.0)
+  parallel (~> 0.5.17)
   rack-fiber_pool
   rails (= 3.0.5)
   rdiscount (~> 1.6.5)

app/models/outfit.rb

@@ -138,25 +138,28 @@ class Outfit < ActiveRecord::Base
   # file.
   def create_image!(output)
     unless image_layers.empty?
-      base_layer = image_layers.first
-      above_layers = image_layers[1..-1]
-      write_temp_swf_asset_image!(base_layer, output)
-      output.close
-
-      unless above_layers.empty?
-        Tempfile.open(['outfit_overlay', '.png']) do |overlay|
-          above_layers.each do |layer|
-            overlay.open
-            write_temp_swf_asset_image! layer, overlay
-            overlay.close
-            
-            previous_image = MiniMagick::Image.open(output.path)
-            overlay_image = MiniMagick::Image.open(overlay.path)
-            output_image = previous_image.composite(overlay_image)
-            output_image.write output.path
-          end
-        end
+      temp_image_files = Parallel.map(image_layers, :in_threads => 8) do |swf_asset|
+        image_file = Tempfile.open(['outfit_layer', '.png'])
+        write_temp_swf_asset_image!(swf_asset, image_file)
+        image_file.close
+        image_file
       end
+      
+      # Here we do some awkwardness to get the exact ImageMagick command we
+      # want, though it's still less awkward than handling the command
+      # ourselves. Give all of the temporary images as input, flatten them and
+      # write them to the output path.
+      command = MiniMagick::CommandBuilder.new('convert')
+      temp_image_files.each { |image_file| command.push image_file.path }
+      command.layers 'flatten'
+      command.push output.path
+      
+      # Though the above command really is sufficient, we still need a dummy
+      # image to handle execution.
+      output_image = MiniMagick::Image.new(output.path)
+      output_image.run(command)
+      
+      temp_image_files.each(&:unlink)
     else
       output.close
     end