diff --git a/classes/Filter.rb b/classes/Filter.rb
new file mode 100644
index 0000000..8b6673a
--- /dev/null
+++ b/classes/Filter.rb
@@ -0,0 +1,100 @@
+require 'singleton'
+
+class Filter
+  include Singleton
+  @config = {}
+  @cleanup = []
+
+  attr_accessor :config, :cleanup
+  
+  def initialize
+    @config = {}
+    @cleanup = []
+  end
+
+  def cleanup
+    @cleanup.each do |f| 
+      if File.exists? f; File.unlink(f); end
+    end
+  end
+
+  def recalc_pixels
+    if @config['print']
+      if !@config['dpi']; raise "DPI not defined"; end
+      if (@config['dpi'].to_i.to_s) != @config['dpi'].to_s; raise "DPI not integer"; end
+      if @config['width_inches'] && (@config['width_inches'].to_f != 0)
+        @config['width'] = (@config['width_inches'].to_f * @config['dpi'].to_f).to_i
+      else
+        @config.delete('width')
+      end
+      if @config['height_inches'] && (@config['height_inches'].to_f != 0)
+        @config['height'] = (@config['height_inches'].to_f * @config['dpi'].to_f).to_i
+      else
+        @config.delete('height')
+      end 
+      
+      if (!@config['width'] && !@config['height'])
+        raise "No dimensions defined!"
+      end
+    end
+  end
+
+  #
+  # Set the config
+  #
+  def config=(c)
+    @config = c
+    
+    recalc_pixels
+  end
+
+  #
+  # Build a temporary PNG from an SVG file
+  #
+  def inkscape(input, target)
+    params = []
+
+    width = @config['width']
+    height = @config['height']
+    inkscape_target = target
+    
+    if @config['rotate']
+      case @config['rotate']
+        when 90, -90
+          t = width; width = height; height = t
+          inkscape_target = target + "-pre.png"
+      end
+    end
+
+    if width && (width.to_i != 0); params << "-w #{width} "; end
+    if height && (height.to_i != 0); params << "-h #{height} "; end
+    
+    system("inkscape -e \"#{inkscape_target}\" -y 1.0 #{params.join(" ")} \"#{input}\"")    
+    
+    if @config['rotate']
+      command = [
+        "\"#{inkscape_target}\"",
+        "-rotate #{@config['rotate']}",
+        "\"#{target}\""
+      ]
+      
+      convert(command)
+      File.unlink(inkscape_target)
+    end
+  end
+
+  def convert(command, verbose = false)
+    system("convert " + (verbose ? "-verbose " : "" ) + [ command ].flatten.join(" "))
+  end
+
+  #
+  # Get the dimensions of a file
+  #
+  def get_dimensions(input)
+    dimensions = nil
+    IO.popen("identify -format '%w,%h' \"#{input}\"") do |fh|
+      dimensions = fh.readlines.first.split(",").collect { |d| d.to_i }
+    end
+    dimensions
+  end
+end
diff --git a/minicomic-backend.rb b/minicomic-backend.rb
index df95356..af28fb9 100755
--- a/minicomic-backend.rb
+++ b/minicomic-backend.rb
@@ -2,101 +2,8 @@
 
 require 'yaml'
 require 'time'
-require 'singleton'
 
-class Filter
-  include Singleton
-  @config = {}
-  @cleanup = []
-
-  attr_accessor :config, :cleanup
-  
-  def initialize
-    @config = {}
-    @cleanup = []
-  end
-
-  def cleanup
-    @cleanup.each do |f| 
-      if File.exists? f; File.unlink(f); end
-    end
-  end
-
-  def recalc_pixels
-    if @config['print']
-      if !@config['dpi']; raise "DPI not defined"; end
-      if @config['width_inches'] && (@config['width_inches'].to_f != 0)
-        @config['width'] = (@config['width_inches'].to_f * @config['dpi'].to_f).to_i
-      else
-        @config.delete('width')
-      end
-      if @config['height_inches'] && (@config['height_inches'].to_f != 0)
-        @config['height'] = (@config['height_inches'].to_f * @config['dpi'].to_f).to_i
-      else
-        @config.delete('height')
-      end  
-    end
-  end
-
-  #
-  # Set the config
-  #
-  def config=(c)
-    @config = c
-    
-    recalc_pixels
-  end
-
-  #
-  # Build a temporary PNG from an SVG file
-  #
-  def inkscape(input, target)
-    params = []
-
-    width = @config['width']
-    height = @config['height']
-    inkscape_target = target
-    
-    if @config['rotate']
-      case @config['rotate']
-        when 90, -90
-          t = width; width = height; height = t
-          inkscape_target = target + "-pre.png"
-      end
-    end
-
-    if width && (width.to_i != 0); params << "-w #{width} "; end
-    if height && (height.to_i != 0); params << "-h #{height} "; end
-    
-    system("inkscape -e \"#{inkscape_target}\" -y 1.0 #{params.join(" ")} \"#{input}\"")    
-    
-    if @config['rotate']
-      command = [
-        "\"#{inkscape_target}\"",
-        "-rotate #{@config['rotate']}",
-        "\"#{target}\""
-      ]
-      
-      convert(command)
-      File.unlink(inkscape_target)
-    end
-  end
-
-  def convert(command, verbose = false)
-    system("convert " + (verbose ? "-verbose " : "" ) + [ command ].flatten.join(" "))
-  end
-
-  #
-  # Get the dimensions of a file
-  #
-  def get_dimensions(input)
-    dimensions = nil
-    IO.popen("identify -format '%w,%h' \"#{input}\"") do |fh|
-      dimensions = fh.readlines.first.split(",").collect { |d| d.to_i }
-    end
-    dimensions
-  end
-end
+require File.dirname(__FILE__) + '/classes/Filter.rb'
 
 class InputFilter < Filter
   OutputFilename = "tmp.png"
@@ -378,7 +285,7 @@ class TempBitmapToPaginatedPrint < OutputFilter
   end
 
   def targets(info)
-    (@config['spread'] == true) ? [ filename(info) + "-left.png", "tmp-right.png" ] : filename(info)
+    (@config['spread'] == true) ? [ filename(info) + "-left.png", filename(info) + "-right.png" ] : filename(info)
   end
 
   def process_pagination(output, face, total_faces, left, right)
@@ -574,6 +481,8 @@ files.each do |filename|
           any_rebuilt = true
 
           puts "Rebuilding #{filename_display} (#{type})..."
+          puts "  Using #{filename} as a source"
+          puts "  and writing to #{targets.inspect}"
 
           tmp_files = input_obj.build(filename)
 
@@ -588,7 +497,7 @@ files.each do |filename|
                 output_files << targets[i]
               end
           end
-
+          
           input_obj.cleanup
         end
         if info['is_paginated']
diff --git a/tests/TestFilter.rb b/tests/TestFilter.rb
new file mode 100644
index 0000000..7b756a4
--- /dev/null
+++ b/tests/TestFilter.rb
@@ -0,0 +1,47 @@
+#!/usr/bin/ruby
+
+require 'rubygems'
+require 'test/unit'
+require 'mockfs/override'
+require File.dirname(__FILE__) + '/../classes/Filter.rb'
+
+class TestFilter < Test::Unit::TestCase
+  def setup
+    @filter = Filter.instance
+  end
+
+  def test_recalc_pixels
+    @filter.config = {}
+
+    assert_raise RuntimeError do
+      @filter.config = {'print' => true}
+    end
+
+    assert_raise RuntimeError do
+      @filter.config = {'print' => true,
+                        'dpi' => 'test'}
+    end
+
+    assert_raise RuntimeError do
+      @filter.config = {
+        'print' => true,
+        'dpi' => 10
+      }
+    end
+    
+    [ 'width', 'height' ].each do |dim|
+      assert_raise RuntimeError do
+        @filter.config = {
+          'print' => true,
+          'dpi' => 10,
+          'width' => 10
+        }
+      end
+
+      @filter.config = {
+        'width' => 10
+      }
+      assert_equal(10, @filter.config['width'])
+    end
+  end
+end
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