gray1 = @rgb.inject(0) {|sum,e| sum+e}
gray2 = pixel.rgb.inject(0) {|sum,e| sum+e}
gray1 < gray2 or raise PixelError.new(self,"is not less bright than",pixel)
gray1 = @rgb.inject(0) {|sum,e| sum+e}
gray2 = pixel.rgb.inject(0) {|sum,e| sum+e}
gray1 < gray2 or raise PixelError.new(self,"is not less bright than",pixel)