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Sheen is a useful view-dependent effect similar to a Fresnel effect in specular lobes of many materials.
Note that it is visible along the edges of objects as a reflective or shiny result. This is useful for creating looks like velvet or lustrous effects on stones and shiny fabrics. You may find this similar to using Edge Color alone in PxrSurface Specular effects.
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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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RenderMan Fundamentals
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