image_resizer.rb

require 'mini_magick'

class ImageResizer

  attr_accessor :height, :width, :padding, :stretch, :grayscale

  def initialize(path)
    @image = MiniMagick::Image.open(path)
  end

  def image_format
    @image[:format]
  end

  def to_s
    process!
    @image.to_blob
  end

  private

  def process!
    # grayscale the image
    @image.colorspace 'gray' if @grayscale

    # bail out if there are no dimensions to play with
    return if @height.nil? && @width.nil?

    # keep default dimensions if not given new ones
    @width = @image[:width] if @width.nil? || @width <= 0
    @height = @image[:height] if @height.nil? || @height <= 0

    # ignore the aspect ratio
    if !!@stretch
      @image.resize "#{@width}x#{@height}!"

    # add padding
    elsif !!@padding
      @image.combine_options do |opts|
        opts.extent "#{@width}x#{@height}"
        opts.gravity 'center'
        opts.background 'white'
      end

    # resize with aspect ratio
    else
      @image.resize "#{@width}x#{@height}>"
    end

  end

end

reglass.rb

require 'sinatra'
require 'haml'
require './image_resizer'

$image_param_hash = {
    :stretch=   => 's',
    :padding=   => 'p',
    :grayscale= => 'g',
    :width=     => /^w(\d+)$/,
    :height=    => /^h(\d+)$/
}

get '/:dir/:image' do
  # split out filename and image params
  image_params = params[:image].split('.')
  filename = "#{image_params.shift}.#{image_params.pop}"
  path = "/www/images/#{params[:dir]}/#{filename}"

  # 404 on missing images
  not_found unless File.exists? path

  # parse the image params and apply changes
  image = ImageResizer.new(path)
  $image_param_hash.each do |meth, param|
    if param.is_a?(String) && image_params.include?(param)
      image.send(meth, true)
    elsif param.is_a?(Regexp)
      image.send(meth, $1.to_i) if image_params.find { |ip| ip =~ param }
    end
  end

  # render the image
  content_type "image/#{image.image_format}"
  image.to_s
end

get '/' do
  haml :index
end

__END__

@@ index
!!!
%html
  %head
    %title RE-glass
  :css
    h1 {
      margin: 200px auto 0;
      width: 200px;
      text-align: center;
    }
  %body
    %h1 RE-glass