gistfile1.rb

require 'RMagick'

class WatermarkMe
  def initialize(app, args)
    @app = app
    @watermark_text, @mime_types = *args
    @mime_types ||= %w[image/jpeg image/png image/gif]
  end
  
  def call(env)
    status, headers, response = @app.call(env)
    
    response = add_watermark(response) if use_watermark?(env, headers)
    
    [status, headers, response]
  end
  
  def use_watermark?(env, headers)
    @mime_types.include?(headers['Content-Type']) &&
    !(env['HTTP_REFERER'] && env['HTTP_REFERER'].match(/^https?:\/\/#{env['HTTP_HOST']}/))
  end
  
  def add_watermark(response)
    s = ''
    response.each { |l| s << l }
    
    image = Magick::Image.from_blob(s).first
    mark  = Magick::Image.new(image.columns, image.rows)

    gc             = Magick::Draw.new
    gc.gravity     = Magick::CenterGravity
    gc.pointsize   = 32
    gc.font_family = "Helvetica"
    gc.font_weight = Magick::BoldWeight
    gc.stroke      = 'none'
    gc.annotate(mark, 0, 0, 0, 0, @watermark_text)

    mark = mark.shade(true, 310, 30)

    image.composite!(mark, Magick::CenterGravity, Magick::HardLightCompositeOp)

    return image.to_blob
  end
end