The Facing Ratio Utility shader.
The colour seen at grazing angles (on the edge of the objects).
→ Center Colour
The colour seen at the center of the colour, where the normal is usually more facing the viewer.
This is a value in the [ 0..1 ] range used in a Bias Function to affect how the output is interpolated between the center and the edge. A bias function is similar to a Gamma function but with the range set between 0 and 1. A value of 0.5 interpolates linearly between center and edge.
Applies a contrast to the output.
The normal to use then computing the facing ratio. If nothing is connected to this plug, the surface normal will be used.