We can obtain some additional interesting effects by taking the absolute value of the noise function. This technique introduces a discontinuity of the derivative because the function folds on itself when it reaches 0. When this folding is done to noise functions at several frequencies and the results are summed, the result is cusps or creases in the texture at various scales. Perlin started referring to this type of noise as turbulence because it is reminiscent of turbulent flow. It shows up in a variety of places in nature, so this type of noise can be used to simulate various things like flames or lava. The two-dimensional appearance of this type of noise is shown in Figure 8.28.