LuxRender Materials Metal2
From LuxRender Wiki
The Metal2 material is an upgraded version of the old metal material. Rather than accepting simply presets or measured data, metal2 simply has a generic fresnel input that allows simple loading of both measured and arbitrary NK values. The fresnelcolor texture can help by generating an NK profile to give a particular color, allowing simple custom colors and reflection color texturing.
Defines the NK curve used by the metal. This can be a constant value, although it is more useful to supply one of the fresnel textures. The fresnelcolor texture can be used to give custom colors to the metal. The fresnelname texture can load presets and measured data files in the Sopra or Luxpop formats. Some exporters have an option to auto-generate a fresnelcolor or fresnelname texture to simplify the configuration of measured data and custom colors.
This determines how shiny the material is by varying the roughness of microfacets. If your exporter uses the exponent to control roughness, higher values are shinier, with 0 being matte. If your exporter uses the direct roughness control, lower values are shinier, with 0 being a perfect reflector and .80 being matte. Values between .80 and 1 are an unrealistic "super-matte" and should be avoided.