http://codebrawl.com/contests/pixelizing-images-with-chunkypng

mw_chunky_pixelizer.rb

require 'chunky_png'
require 'backports' unless RUBY_VERSION >= "1.8.7"
include ChunkyPNG

image = Image.from_file('input.png')

xslices = (0...image.width).each_slice(10).to_a
yslices = (0...image.height).each_slice(10).to_a
xslices.product(yslices) do |xslice, yslice|
  tots = [0, 0, 0, 0]
  xslice.product(yslice).each do |x, y|
    rgba = Color.to_truecolor_alpha_bytes(image[x,y])
    tots.map! { |t| t + rgba.shift }
  end
  weight = xslice.size * yslice.size
  pixelzd_color = Color.rgba(*tots.map { |t| (t.to_f / weight).round })
  image.rect(xslice.first, yslice.first, xslice.last, yslice.last, Color::TRANSPARENT, pixelzd_color)
end

image.save('output.png')