class Magick::RVG

def background_fill_opacity=(opacity)

1.0 (opaque). The default is 1.0 when the background_fill= attribute has been set.
Opacity of the background fill color, a number between 0.0 (transparent) and 
def background_fill_opacity=(opacity)
  warn 'background_fill_opacity= has no effect in nested RVG objects' if @nested
  begin
    @background_fill_opacity = Float(opacity)
  rescue ArgumentError
    raise ArgumentError, "background_fill_opacity must be a number between 0 and 1 (#{opacity} given)"
  end
end