class ImageProcessing::Vips::Processor

def composite(overlay, _mode = nil, mode: "over", gravity: "north-west", offset: nil, **options)
  # if the mode argument is given, call the original Vips::Image#composite
  if _mode
    overlay = [overlay] unless overlay.is_a?(Array)
    overlay = overlay.map { |object| convert_to_image(object, "overlay") }
    return image.composite(overlay, _mode, **options)
  end
  overlay = convert_to_image(overlay, "overlay")
  # add alpha channel so that #gravity can use a transparent background
  overlay = overlay.add_alpha unless overlay.has_alpha?
  # apply offset with correct gravity and make remainder transparent
  if offset
    opposite_gravity = gravity.to_s.gsub(/\w+/, "north"=>"south", "south"=>"north", "east"=>"west", "west"=>"east")
    overlay = overlay.gravity(opposite_gravity, overlay.width + offset.first, overlay.height + offset.last)
  end
  # create image-sized transparent background and apply specified gravity
  overlay = overlay.gravity(gravity, image.width, image.height)
  # apply the composition
  image.composite(overlay, mode, **options)
end