jekyll-responsive-image/lib/jekyll/responsive_image.rb

require 'jekyll'
require 'rmagick'

module Jekyll
  class ResponsiveImage
    class Tag < Liquid::Tag
      DEFAULT_QUALITY = 85

      def initialize(tag_name, markup, tokens)
        super

        @attributes = {}
        @markup = markup

        @markup.scan(::Liquid::TagAttributes) do |key, value|
          # Strip quotes from around attribute values
          @attributes[key] = value.gsub(/^['"]|['"]$/, '')
        end
      end

      def resize_image(img, config)
        output_dir = config['output_dir']
        ensure_output_dir_exists!(output_dir)

        resized = []

        config['sizes'].each do |size|
          width = size['width']
          ratio = width.to_f / img.columns.to_f
          height = (img.rows.to_f * ratio).round

          filename = resized_filename(img.filename, width, height)
          filepath = "#{output_dir}/#{filename}"

          next unless needs_resizing?(img, width)

          resized.push(image_hash(filepath, width, height))

          # Don't resize images more than once
          next if File.exists?(filepath)

          Jekyll.logger.info "Generating #{filepath}"

          i = img.scale(ratio)
          i.write(filepath) do |f|
            f.quality = size['quality'] || DEFAULT_QUALITY
          end

          i.destroy!
        end

        resized
      end

      # Insert resize information into a file path
      #
      #   resized_filename(/foo/bar/file.name.jpg, 500, 300)
      #     => /foo/bar/file.name-500x300.jpg
      #
      def resized_filename(path, width, height)
        File.basename(path).sub(/\.([^.]+)$/, "-#{width}x#{height}.\\1")
      end

      def needs_resizing?(img, width)
        img.columns > width
      end

      def ensure_output_dir_exists!(dir)
        unless Dir.exists?(dir)
          Jekyll.logger.info "Creating output directory #{dir}"
          Dir.mkdir(dir)
        end
      end

      # Build a hash containing image information
      def image_hash(path, width, height)
        {
          'path'   => path,
          'width'  => width,
          'height' => height,
        }
      end

      def render(context)
        config = context.registers[:site].config['responsive_image']
        config['output_dir'] ||= 'assets/resized'
        config['sizes'] ||= []

        img = Magick::Image::read(@attributes['path']).first
        @attributes['original'] = image_hash(@attributes['path'], img.columns, img.rows)
        @attributes['resized'] = resize_image(img, config)
        @attributes['template'] ||= config['template']

        partial = File.read(@attributes['template'])
        template = Liquid::Template.parse(partial)

        template.render!(@attributes)
      end
    end
  end
end

Liquid::Template.register_tag('responsive_image', Jekyll::ResponsiveImage::Tag)