This form of contrast is an effective way to quantify contrast for periodic functions "f"("x") and is also known as the modulation "mf" of a periodic signal "f". Modulation quantifies the relative amount by which the amplitude (or difference) ("f"max − "f"min)/2 of "f" stands out from the average value (or background) ("f"max + "f"min)/2. In general, "mf" refers to the contrast of the periodic signal "f" relative to its average value. If "mf" = 0, then "f" has no contrast. If two periodic functions "f" and "g" have the same average value, then "f" has more contrast than "g" if "mf" > "mg".