gistfile1.txt

compile '/images/screenshots/*/', rep: :thumbnail do
  filter :thumbnailize, factor: 0.25
end

route '/images/screenshots/*/', rep: :thumbnail do
  item.identifier.chop + '-thumbnail.' + item[:extension]
end

-------------------
# layouts/screenshot.html.haml

- path = "/images/screenshots/#{@image}"
- screenshot = @items.find {|i| i.identifier == "#{path}/" }

# Fails here, unless the full-size screenshots exist in output, unless I use 'screenshot.compiled_content', which spits out a big error, of course. 
- thumbnail = screenshot.rep_named(:thumbnail)

- image = ChunkyPNG::Image.from_file("output#{path}-thumbnail.png")

- @title ||= @alt
= link_to screenshot.path, rel: "lightbox-gallery" do
  %img{ class: "thumb", src: thumbnail.path, alt: @alt, title: @title, width: image.width, height: image.height }

class Thumbnailize < Nanoc3::Filter
  identifier :thumbnailize
  type       :binary

  def run(filename, params={})
    factor = params[:factor]

    image = ChunkyPNG::Image.from_file(filename)
    image.resample_nearest_neighbor! (image.width * factor).round, (image.height * factor).round

    image.save output_filename
  end
end