Roughness increases the number and strength of the layers in a fractal texture resulting in a rougher looking texture.

All components that have a Roughness control available are fractal textures. Fractal textures consist of multiple layers of a self-similar patterns scaled to different sizes and then combined using different methods. Roughness increases the number of fractal layers in the texture while also increasing the strength of the smaller layers.  The end result is that you get a rougher looking texture.  Figure F1 below shows the Fractal component with 0%, 25%, 50%, 75%, and 100% Roughness settings.

Note: when you have the Roughness of a component set to a low value, try scaling out to see more of the texture. Using Roughness and Lacunarity you can get quite a few different looks out of fractal textures.

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