png_quant_processor.rb

class PngQuantProcessor < ::Tilt::Template
  def prepare
    # noop
  end

  def evaluate(scope, locals, &block)
    IO.popen("pngquant -", "rb+") do |process|
      process.write(data)
      process.close_write
      process.read
    end
  end
end

See also https://gist.github.com/dv/7188341