Added sobel edge algorhytm.

bin/magick_sobel

@@ -0,0 +1,54 @@
+#!/usr/bin/env ruby
+# Encoding: utf-8
+
+require 'optparse'
+
+lib = File.expand_path(File.dirname(__FILE__) + '/../lib')
+$LOAD_PATH.unshift(lib) if File.directory?(lib) && !$LOAD_PATH.include?(lib)
+
+require 'rmagick/screwdrivers'
+
+# options = {
+#   :thoroughly   => false,
+#   :logger       => nil
+# }.merge(options)
+
+options = {}
+
+OptionParser.new do |opts|
+  opts.banner = "Usage: #{$0} [options] FILE"
+
+  # Quality
+  opts.on("-q", "--quality [1-100]", Integer, "Quality of resulting JPEG (default: 90)") do |quality|
+    options[:quality] = quality
+  end
+
+  # Thoroughly
+  opts.on("-r", "--roughly", "If set, the image will not be grayscaled before processing (requires less resources, default: false)") do |roughly|
+    options[:roughly] = !!roughly
+  end
+
+  # Verbose?
+  opts.on("-v", "--verbose", "Run verbosely (default: false)") do |v|
+    if v
+      require 'logger'
+      options[:logger] = Logger.new STDOUT
+    end
+  end
+
+  # No argument, shows at tail.  This will print an options summary.
+  opts.on_tail("-h", "--help", "Show this message") do
+    puts opts
+    exit
+  end
+
+end.parse!
+
+raise "Run `#{$0} --help` for execution examples. Exiting…" if ARGV.size < 1
+
+file = ARGV.shift
+
+outfile = File.basename(file).sub(/(\.\w+)$/, "-sobel\\1")
+Magick::Screwdrivers::sobel(file, options).write(File.join File.dirname(file), outfile) { 
+  self.quality = options[:quality] || 90
+}

lib/rmagick/screwdrivers.rb

@@ -3,6 +3,7 @@
 require "rmagick/screwdrivers/poster"
 require "rmagick/screwdrivers/collage"
 require "rmagick/screwdrivers/scale"
+require "rmagick/screwdrivers/sobel"
 
 module Magick
   module Screwdrivers

lib/rmagick/screwdrivers/sobel.rb

@@ -0,0 +1,74 @@
+# encoding: utf-8
+
+require 'RMagick'
+
+module Magick
+  class Image # monkeypatch
+    def at x, y
+      px = self.pixel_color(x, y)
+      (px.red + px.green + px.blue) / 3
+    end
+  end
+
+  module Screwdrivers
+    # based on http://blog.saush.com/2011/04/20/edge-detection-with-the-sobel-operator-in-ruby/
+    def self.sobel file, options={}
+      options = {
+        :roughly       => false,
+        :logger        => nil
+      }.merge(options)
+
+      sobel_x = [[-1,0,1], [-2,0,2], [-1,0,1]]
+      sobel_y = [[-1,-2,-1], [0,0,0], [1,2,1]]
+
+      begin
+        orig = img_from_file(file)
+        orig = orig.quantize 256, Magick::GRAYColorspace unless options[:roughly]
+      rescue
+        warn(options[:logger], "Skipping invalid file #{file}…")
+        return nil
+      end
+
+      #                 500×375
+      scale = Math.sqrt(187_500.0 / (orig.columns * orig.rows))
+
+      info(options[:logger], "Original is [#{orig.columns}×#{orig.rows}] image")
+      info(options[:logger], "Scale factor: [#{scale}]")
+      img = scale < 1 ? orig.scale(scale) : orig
+
+      info(options[:logger], "Will process [#{img.columns}×#{img.rows}] image")
+
+      edge = Image.new(img.columns, img.rows) { 
+        self.background_color = 'transparent' 
+      }
+
+      for x in 1..img.columns-2
+        for y in 1..img.rows-2
+          pixel_x = (sobel_x[0][0] * img.at(x-1,y-1)) + (sobel_x[0][1] * img.at(x,y-1)) + (sobel_x[0][2] * img.at(x+1,y-1)) +
+                    (sobel_x[1][0] * img.at(x-1,y))   + (sobel_x[1][1] * img.at(x,y))   + (sobel_x[1][2] * img.at(x+1,y)) +
+                    (sobel_x[2][0] * img.at(x-1,y+1)) + (sobel_x[2][1] * img.at(x,y+1)) + (sobel_x[2][2] * img.at(x+1,y+1))
+
+          pixel_y = (sobel_y[0][0] * img.at(x-1,y-1)) + (sobel_y[0][1] * img.at(x,y-1)) + (sobel_y[0][2] * img.at(x+1,y-1)) +
+                    (sobel_y[1][0] * img.at(x-1,y))   + (sobel_y[1][1] * img.at(x,y))   + (sobel_y[1][2] * img.at(x+1,y)) +
+                    (sobel_y[2][0] * img.at(x-1,y+1)) + (sobel_y[2][1] * img.at(x,y+1)) + (sobel_y[2][2] * img.at(x+1,y+1))
+
+          val = Math.sqrt((pixel_x * pixel_x) + (pixel_y * pixel_y)).ceil
+          edge.pixel_color x, y, Pixel.new(val, val, val)
+        end
+      end
+
+      edge = edge.scale(orig.columns, orig.rows) if scale < 1
+
+      case orig.orientation 
+      when Magick::RightTopOrientation
+        edge.rotate!(90)
+      when Magick::BottomRightOrientation
+        edge.rotate!(180)
+      when Magick::LeftBottomOrientation
+        edge.rotate!(-90)
+      end
+
+      edge
+    end
+  end
+end

rmagick-screwdrivers.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |s|
   s.files         = `git ls-files`.split("\n")
   s.test_files    = `git ls-files -- {test,spec,features}/*`.split("\n")
   s.bindir        = 'bin'
-  s.executables   = ['magick_collage', 'magick_poster', 'magick_scale']
+  s.executables   = ['magick_collage', 'magick_poster', 'magick_scale', 'magick_sobel']
   s.require_paths = ['lib']
 
   s.add_development_dependency 'rspec'