# File lib/rvg/rvg.rb, line 192
    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