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Here is where you supply a signal, either a texture or procedural pattern, to create a bump to the surface to "fake" surface details like small bumps or scratches. This means an artist doesn't have to model these tedious and often repetitious parts of a model.

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Example
The above image is created with the following RenderMan network.

A Lamadiffuse is used to create the base material and a LamaSheen is layered over the top using a LamaLayer node.  The LayerMix is lowered to 0.6 and the sheen is broken up with a fingerprint and smudge grunge map.

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