rmagick composite watermark

  # 0 = opaque (Magick::OpaqueOpacity) 1= transparent (Magick::TransparentOpacity)
  def watermark(opacity = 0.99, size = 'm')
    manipulate! do |img|
      logo = Magick::Image.read("#{Rails.root}/app/assets/images/watermark#{size}.png").first
      logo.alpha(Magick::ActivateAlphaChannel)

      pixels = logo.get_pixels(0, 0, logo.columns,logo.rows)

      for pixel in pixels
          pixel.opacity = opacity
      end

      logo_opacity = Magick::Image.new(logo.columns, logo.rows)
      logo_opacity.store_pixels(0, 0, logo.columns, logo.rows, pixels)

      #logo.opacity = (1 - opacity.to_f) * Magick::QuantumRange
      img.alpha(Magick::ActivateAlphaChannel)
      #img = img.dissolve(logo, 1.0, opacity, Magick::NorthWestGravity, 0, 0)
      img = img.composite(logo_opacity, Magick::NorthWestGravity, 0, 0, Magick::OverCompositeOp)
    end
  end