Sobel filter with RMagick

Sobel.rb

class Sobel
  require 'RMagick'
  include Magick

  @@kernels = {
    :up => [1, 2, 1, 0, 0, 0, -1, -2, -1],
    :down => [-1, -2, -1, 0, 0, 0, 1, 2, 1],
    :left => [1, 0, -1, 2, 0, -2, 1, 0, -1],
    :right => [-1, 0, 1, -2, 0, 2, -1, 0, 1]
  }

  def self.apply (source, type=:all)
    result = source.dup.quantize(256, GRAYColorspace)

    vertical = lambda { result = result.convolve(3, @@kernels[:up]).convolve(3, @@kernels[:down]) }
    horizental = lambda { result = result.convolve(3, @@kernels[:left]).convolve(3, @@kernels[:right]) }

    case type
    when :all
      vertical.call
      horizental.call
    when :vertical
      vertical.call
    when :horizental
      horizental.call
    end

    result
  end
end